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DIETARY FIBER AND DIETARY FIBER RICH FOODS
Chapter 3 DIETARY FIBER AND DIETARY FIBER RICH FOODS Introduction Dietary fiber (DF) has been consumed for centuries and most food labels in the supermarket now list dietary fiber. Even though fiber is not considered a nutrient, health professionals and nutritionists agree that fiber is required in sufficient amounts for the proper functioning of the gastrointestinal tract. DF is the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine. The reductions in LDL-cholesterol, attenuating glycemic and insulin response, increasing stool bulk, and improving laxation have been associated with DF intake through the consumption of foods rich in this dietary component, such as vegetables, fruits, whole grains, and nuts. DF consumption has established the basis for associating high-fiber diets in epidemiological studies with reduced risk of most of the major dietary problems in the U.S.A.; namely, obesity, coronary disease, diabetes, gastrointestinal disorders, including constipation, inflammatory bowel diseases like diverticulitis and ulcerative colitis, and colon cancer (Jones, 2000). Despite the understanding of health benefits of DF and its association with reduced risk of many diseases, the intake remains low in many parts of the world, in particular in the U.S.A. One of the reasons for this may be the difficult challenge to increase fiber consumption in the diet. The fiber sources usually used in foods have not made high-fiber foods with acceptable sensory properties. A product development technologist who makes foods, using high fiber ingredients needs to realize that a product not only supply fiber, but also provide enhanced functional properties to make high-fiber foods taste better, thus encouraging continued intake of this type of product.
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Why is fiber important? What does fiber do? This chapter will answer these questions in detail. It is the purpose of this chapter to provide an overview of important oligosaccharides and polysaccharides that function as DF, to explain in detail their occurrence and structures and their various physiological effects and health implications, and also to describe the role high fiber ingredients play in food development. Definition Establishing a definition for dietary fiber has a long history. The term ‘dietary fiber’ was coined by Hipsley in 1953 and since then its definition has undergone several revisions. The history of the definition of DF is presented in Figure 3.1. While defining dietary fiber, it was intended to balance between nutritional knowledge and analytical method capabilities. While the physiologically based definitions most widely accepted have generally been accurate in defining the dietary fiber in foods, scientists and regulators have tended, in fact, to rely on analytical procedures as the definitional basis. As a result, incompatibility between theory and practice has resulted in confusion regarding the components that make up dietary fiber. In November 1998, the president of American Association of Cereal Chemists (AACC) International appointed a scientific review committee and assigned the task of reviewing, and if necessary, updating the definition of dietary fiber. The updated definition includes the same food components as the historical working definition used for almost 30 years. But the updated definition more clearly describes the makeup of DF and its physiological functionality. This definition typically includes the fiber components; nonstarch polysaccharides (NSP) and resistant oligosaccharides (RO), lignin, substances associated with the NSP and lignin complex in plants, and other analogous carbohydrates, such as resistant starch (RS) and dextrins, and synthesized carbohydrate compounds, like polydextrose (Tungland and Meyer, 2002). Finally, dietary fiber is defined as the edible parts of the plant and analogous carbohydrate that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine. It includes polysaccharides, lignin and associated plant substances. Dietary fiber exhibits one or more of the following: laxation (fecal bulking and softening; increased frequency; and/or regularity), blood cholesterol attenuation, and/or blood sugar regulation.
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FIGURE 3.1 — History Of Definition Of Dietary Fiber Over The Years Organization
Year
Definition
Hipsley
1953
Coined term “dietary fiber” as a shorthand term for nondigestable constituents making up the plant cell wall.
Trowell and others
1972-1976
Used Hipsley term in conjunction with a dietary fiber hypothesis related to health observations. The term was defined as: “consisting of the plant polysaccharides and lignin which are resistant to hydrolysis by digestive enzymes of man.”
Asp, Schweizer, Furda, Theander, Bakker, Soutgate and others
1976-1981
Developed methods directed at quantifying food components meeting definition
Prosky
1979
Began process of developing worldwide consensus on fiber definition and methodology for dietary fiber
Canadian Association of Official Analytical Chemists Workshop
1981
Consensus on fiber definition and analytical approach
Prosky, Asp, Furda, Schweizer, DeVries and Harland
1981 -1985
Validate consensus methodology in multinational collaborative studies
AOAC
1985
Official Method of Analysis 985.29, Total dietary Fiber in Foods-Enzymatic-Gravimetric Method, adopted, becoming de facto working definition for dietary fiber
Health and Welfare Canada
1985
Defined dietary fiber as: “the endogenous components of plant material in the diet which are resistant to digestion by enzymes produced by humans. They are predominately nonstarch polysaccharides and lignin and may include, in addition, associated substances.
Scientific community
1985-1988
Developed methodology and collaboratively studied these for various types of fiber.
US-FDA
1987
Defined dietary fiber as the material isolated by AOAC method 985.29
Life Sciences Research Office (LSRO)
1987
Defined dietary fiber as: the endogenous components of plant materials in the diet that are resistant to digestion by enzymes produced by humans
Health Canada
1988
Defined (dietary fiber) as: being the endogenous components of plant material in the diet which are resistant to digestion by enzymes produced by man: they are predominately nonstarch polysaccharides and lignin. The composition varies with the origin of the fiber, and includes soluble and insoluble substances. Defined (novel fiber or novel source) as: (1) a food that has been manufactured to be a source of dietary fiber, and has not traditionally been used for human consumption to any significant extent, or (2) had been chemically processed (oxidized), or (3) had been highly concentrated from its plant source.
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FIGURE 3.1 — History Of Definition Of Dietary Fiber Over The Years - Continued Germany
1989
Defined fiber as: substances of plant origin, that cannot be broken down to resorbable components by the body’s own enzymes in the small intestine. Included are essentially soluble and insoluble nonstarch polysaccharides (cellulose, pectin, hydrocolloids) and lignin and resistant starch. Substances like some sugar substitutes, organic acids, chitin and so on, which either are not or are incompletely absorbed in the small intestine, are not included.
Lee, Mongeau, Li, Theander and others
1988-1994
Various fiber methodologies fitting definition of dietary fiber developed, validated and brought to an Official Method status
Japan
1990
Dietary fiber defined as: material isolated by a modified method of AOAC 985.29
AOAC
1991
Official Method of Analysis 991.42, Insoluble Dietary Fiber in Foods and Food Products, Enzymatic-Gravimetric Method-Phosphate Buffer, adopted.
International Fiber Survey
1992
Reaffirms consensus on physiological dietary fiber definition.
Belgium
1992
Defined dietary fiber as: the components of food that are not normally broken down by the body’s own enzymes of humans
International Fiber fiber Survey
1993
Reaffirmed consensus on physiological dietary
Italy
1993
Defined dietary fiber as: the edible substance of vegetable origin which normally is not hydrolyzed by enzymes secreted by the human digestive system
AOAC International
1995
Workshop on definition of complex carbohydrates and dietary fiber reaffirms consensus on physiological dietary fiber definition and inclusion components
FAO/WHO
1995
(Codex Alimertarius Commission) Defined dietary fiber as: the edible plant or animal material not hydrolyzed by the endogenous enzymes of the human digestive tract as determined by the agreed upon method. Approved AOAC methods 985.29 & 991.43.
China
1995
Defined dietary fiber as: the sum of food components that are not digested by intestinal enzymes and absorbed into the body
Denmark
1995
Defined dietary fiber as: the material isolated by AOAC methods 985.29 and 997.08 (fructan method)
Committee on Medical Aspects (UK)
1998
Defined dietary fiber as: nonstarch polysaccharide as measured by the Englyst method of Foods [Committee on Medical Aspects of Food and Nutrition Policy (COMA)]
Finland
1998
Defined dietary fiber as: part of the carbohydrate obtained using AOAC Methods 985.29 and AOAC 997.08.
definition and reaffirms inclusive components
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FIGURE 3.1 — History Of Definition Of Dietary Fiber Over The Years - Continued Norway
1998
Defined dietary fiber as: material isolated by AOAC Method 985.29 and inulin and oligofructose
AACC
1998
Assigns Scientific Committee to review and develop definition of Dietary Fiber
Sweden
1999
Defined dietary fiber as: edible material that cannot be broken down by human endogenous enzymes and determined with AOAC Methods 985.29 and/or 997.08 (fructan method)
Food Standards Agency (U.K.)
1999
Defined dietary fiber as: material isolated by AOAC methods 985.29 and 997.08 (fructan method)
AACC
2000
Defined dietary fiber as: the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine. Dietary fiber includes polysaccharides, oligosaccharides, lignin, and associated plant substances. Dietary fibers promote beneficial physiological effects including laxation, and/or blood cholesterol attenuation, and/or blood glucose attenuation.
Australia New Zealand Food Authority (ANZFA)
2001
Following the lines of the AACC definition, defined dietary fiber as: that fraction of the edible part of plants or their extracts, or analogous carbohydrates, that are resistant to digestion and absorption in the human small intestine, usually with complete or partial fermentation in the large intestine. The term includes polysaccharides, oligosaccharides (DP > 2), and lignins. Dietary fiber promotes one or more of these beneficial physiological effects: laxation, reduction in blood cholesterol, and/or modulation of blood glucose. They accepted by use of AOAC methods 985.29 and 997.08 (fructan method) for labeling.
National Academy of Science (NAS)
2002
2002 Panel on the Definition of Dietary Fiber defined the dietary fiber complex to include dietary fiber consisting of nondigestible carbohydrates and lignin that are intrinsic and intact in plants, functional fiber consisting of isolated, nondigestible carbohydrates which have beneficial physiological effects in humans, and total fiber as the sum of dietary fiber and functional fiber.
Chemistry Of Dietary Fiber The physical properties of dietary fiber are predominated by the shape (conformation) of the individual chains, and the way in which they interact with one another. Each dietary fiber molecule typically contains several thousand monosaccharide units which are often arranged in a linear sequence, like a very long string of beads, although more complex branched arrangements also occur. In contrast to globular
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proteins, polysaccharides normally have structures based on regular repeating sequences. The simplest arrangement is where all the monosaccharides are the same, and are linked together in the same way along the chain. Disaccharide repeating consequences (-A-B-A-B-) are also common, and larger repeating units (up to octasaccharide) can occur, particularly in polysaccharides produced by bacteria. The constituent monosaccharides have a ring structure, which can be either five-membered or six-membered, and are linked together by ‘glycosidic bonds’ with a shared oxygen atom between adjacent sugars. The polysaccharides of greatest practical importance, both as commercial hydrocolloids and as constituents of dietary fiber, are built up from six membered (pyranose) rings consisting of one oxygen atom and five carbon atoms, which are numbered sequentially from the ring oxygen as C-1 to C-5, and with a sixth carbon atom, numbered as C-6, lying outside the ring. As a consequence of the tetrahedral bonding arrangement of carbon, and the requirement to avoid steric clashes between adjacent groups, the pyranose ring is locked in a fixed, chairlike geometry, and the overall shape of the polysaccharide molecule is dictated by the torsional angles characterizing the relative orientation of neighboring sugars. These angles may be either fixed at the same values for equivalent linkages along the polymer chain, giving regular, ordered chain geometry, or constantly fluctuating, to give the disordered ‘random coil’ geometry typical of polysaccharide solutions. The chemical structures of different dietary fibers are given in Figures 3.2 and 3.3. Physical Properties Of Dietary Fiber When considering the action of cooking on cell wall structure and comparing cooked and raw plant foods, the different solubility characteristics of cell wall polysaccharides should be considered. Cell wall structures are degradable to varying degrees, depending on the structure and the conditions used. An important function of insoluble fibers is to increase lumenal viscosity in the intestine. It is not yet clear whether the soluble fibers in food have the same effect. Other polymeric components of the diet (proteins, gelatinized starch) and mucus glycoproteins liberated from the epithelia contribute to viscosity. Particulate materials present in chyme, such as insoluble fiber or hydrated plant tissues, also contribute to a lesser extent to overall viscosity. Digesta viscosity is highly sensitive to changes in ionic concentration that are due to intestinal secretion or absorption of aqueous fluids. Raw apples undergo little damage of cells upon ingestion
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FIGURE 3.2 — Chemical Structures Of Starch And Other Polysaccharides
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FIGURE 3.3 — Chemical Structures Of Polyfructans
and mastication. Gastric hydrochloric acid only solubilizes a small proportion of the pectin. Cooking the apples results in cell damage, and hence significant proportions of the middle lamellae pectic polysaccharides are solubilized. These make the digesta more viscous. Vegetables undergo structural change during cooking and mastication, e.g., cellular disintegration. The cells in the intact carrot are each bounded by an intact cell wall; after cooking most, if not all, the cell walls have been ruptured and the cell contents lost. The grinding of
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foods before cooking and ingestion may also have pronounced effects on fiber action. Cell walls may be disrupted, and the reduced particle size of some fiber preparations such as wheat bran may be less biologically effective. The effects of other cooking processes, e.g., Maillard reactions, are not known. Controlled drying of a heated starch gel can produce any of the different X-ray diffraction patterns, depending on the temperature. On cooling, gelatinized starchy foods will retrograde. During retrogradation, solubility of the starch molecule decreases and so does its susceptibility to hydrolysis by acid and enzymes. Chain length and linearity are important factors affecting retrogradation. The longer the starch chains, the greater the number of interchain hydrogen bonds formed (Dobbing, 1989). Classification Of Dietary Fiber Several different classification systems have been used to classify the components of dietary fiber: based on their role in the plant, based on the type of polysaccharide, based on their simulated gastrointestinal solubility, based on the site of digestion, and based on products of digestion and physiological classification. However, none is entirely satisfactory, as the limits can not be absolutely defined. The most widely used classification for dietary fiber has been to differentiate dietary components on their solubility in a buffer at a defined pH, and/or their fermentability in an in vitro system using an aqueous enzyme solution representative of human alimentary enzymes. However, there is still debate regarding the most appropriate means to classify dietary fiber. Since most fiber types are at least partially fermented, it is suggested that it may be most appropriate to refer to them as partially or poorly fermented and well fermented. Classification Based On Solubility Based on solubility, dietary fiber is classified into two types – soluble and insoluble. Soluble fiber dissolves in water. This includes gums, mucilages, pectin and some hemicelluloses. These fibers are found in all types of peas and beans like lentils, split peas, pinto beans, black beans, kidney beans, garbanzo beans, and lima beans, as well as oats, barley, and some fruits and vegetables like apples, oranges, and carrots. Fiber from psyllium seed is also in this group. For people with diabetes, eating foods that contain soluble fiber can help control or lower the level of sugar in their blood and decrease insulin needs; and, studies have shown that including one or two servings of beans, oats, psyllium, or other sources of soluble fiber help
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lower fasting blood sugar levels. It may also help lower blood cholesterol levels, especially LDL-cholesterol or the “bad” cholesterol. Fiber decreases blood cholesterol by binding to bile acids, which are made of cholesterol, in the gastrointestinal tract and carrying them out of the body as waste. Researchers have found that soluble fibers in beans, psyllium fiber, oats, and oat bran help lower blood cholesterol levels in many groups of people. Insoluble fiber does not dissolve in water. Cellulose, lignin, and the rest of the hemicelluloses, are all insoluble fibers. These fibers provide structure to plants. Whole grains, wheat and corn fiber, and many vegetables like cauliflower, green beans, and whole potatoes are good sources of insoluble fiber. The skins of fruits and vegetables are also good sources of insoluble fiber. And, wheat bran is a good source of insoluble fiber, which is why it is added to many dry breakfast cereals. Insoluble fiber, also known as “roughage”, aids digestion by trapping water in the colon. The water that is trapped by insoluble fiber keeps the stool soft and bulky. This promotes regularity and prevents constipation. Wheat bran, for example is high in insoluble fiber, and also helps prevent two kinds of intestinal diseases, diverticulosis and hemorrhoids. Classification Based On Fermentability Fibers that are well fermented include pectin, guar gum, acacia (gum arabic), inulin, polydextrose, and oligosaccharides. The less wellfermented types include cellulose, wheat bran, corn bran, oat hull fiber, and some resistant starches. The fiber types based on fermentability are listed in Figure 3.4. Generally, well fermented fibers are soluble in water, while partially or poorly fermented fibers are insoluble. Classification Based On The Way The Monomeric Units Present Indigestible polysaccharides (fiber components) consist of all nonstarchy polysaccharides (NSP) resistant to digestion in the small intestine and fermentable in the large intestine. These polysaccharides are typically long polymeric carbohydrate chains containing up to several hundred thousand monomeric units. The polysaccharides differ by the number and type of monomeric units linked together, the order in the chain, the types of linkages between the various monomers, the presence of branch points in the backbone of the molecule, and those having acidic groups present (for example, uronic acids in pectins). Examples of these NSP compounds are cellulose with beta-glycosidic bonds, nonglucose sugars (hemicelluloses such as arabinoxylans and arabinogalactans), sugar acids (pectins), gums, and mucilages. Resistant
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FIGURE 3.4 — Classification Of Fiber Components Based On Fermentability Characteristic
Fiber component
Main food source
Partial or low fermentation
Cellulose
Plants (vegetables, sugar beet, various brans) Cereal grains Woody plants Plant Fibers Fungi, yeasts, invertebrates Plants (corn, potatoes, grains, legumes, bananas) Bacterial fermentation
Hemicellulose Lignin Cutin/suberin/other plant waxes Chitin and chitosan, collagen Resistant starches Curdlan Well fermented
β-glucans Pectins Gums
Inulin Oligosaccharides/analogues
Animal origin
Grains (oat, barley, rye) Fruits, vegetables, legumes, sugar beet, potato Leguminous seed plants (guar, locust bean), seaweed extracts (carrageenan, alginates), plant extracts (gum acacia, gum karaya, gum tragacanth), microbial gums (xanthan, gellan) Chicory, Jerusalem artichoke, onions, wheat Various plants and synthetically produced (polydextrose, resistant maltodextrin, fructooligosaccharides, galactooligosaccharides, lactulose) Chondroitin
oligosaccharides, such as the fructans [inulin and fructooligosaccharides (FOS)] (Figure 3.4) are characterized as carbohydrates with a relatively low degree of polymerization (DP), as compared to the NSP. FOS differ from fructopolysaccharides (inulin) only in chain length. The strict definition of an oligosaccharide is a chain of monomeric units with a DP of 3-10. Lignin is a phenylpropane polymer, and not a carbohydrate that is covalently bound to the fibrous polysaccharides (cellulose) of plant cell walls. Lignin has a heterogeneous composition ranging from 1 or 2 units to many phenyl propanes that are cyclically linked. It is likely these two characteristics have established the basis for it being included as a dietary fiber. Another group of compounds, found in several physiological definitions, the analogous carbohydrate(s), refer to compounds that are
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analogous to those of naturally-occurring dietary fibers. These compounds demonstrate the physiological properties of the respective materials for which they are analogous to, but are not obtained by eating the whole or part of the native originating plant, such as fruits, vegetables, grains, legumes, and nuts. They can be produced during food processing by chemical and/or physical processes, or by purposeful synthesis or isolation as a concentrated form from the native plant. These “analogous” carbohydrates can include, but are not limited to, those isolated from Crustacea and single-cell organisms, polydextrose, resistant maltodextrins and starch, and the modified celluloses. Resistant starch (RS) is defined as the sum of starch and starch products of starch degradation that is not broken down by human enzymes in the small intestine of healthy individuals. A classification of these starches based on the origin of their resistance to digestion has been proposed by Englyst et al (1992). Resistant starch is not a homogenous entity, but rather the resistance is dependent on a number of natural or processing phenomena which make up the subcategories RS1, 2, 3, and 4. RS1 relates to resistance conferred due to physical entrapment of starch, as found in partly milled grains or chewed cereals, seeds, or legumes. RS2 includes starch granules that are highly resistant to digestion by alpha-amylase until gelatinized. This form is typically found in raw or uncooked potato, banana (particularly when green), and high amylose maize starch. RS3 relates to the retrograded starch polymers from food processing of the above mentioned sources. RS4 includes chemically modified, commercially produced resistant starches that are likely degraded by amylases to alcohol soluble fractions and are used in many baby food applications. RS may have the similar health benefits as dietary fiber. Also included in the fiber component list are the associated plant substances, such as waxes and cutin. These components are found as waxy layers at the surface of the cell walls, made up of highly hydrophobic, long chain hydroxy aliphatic fatty acids. Suberin, another one of these associated substances, even though not fully characterized, is speculated to be a highly branched, crosslinked molecule containing polyfunctional phenolics, polyfunctional hydroxyacids, and dicarboxylic acids, having ester linkages to the plant cell walls. Analysis Of Dietary Fiber Adoption of the proposed definition for regulatory, research, and nutrition purposes will result in little change of analytical methodology, food labels, or food databases from the current situation. While several
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methods have been developed for analyzing dietary fiber, two primary methods are now used for content labeling: enzymatic gravimetric methods (for example, the AOAC procedure), and enzymatic chemical methods (for example, the Englyst and Southgate procedures). The AOAC procedures primarily measure NSP, lignin, and a portion of RS, as does the Southgate method, while RS and lignin are not measured by the Englyst method. Due to method limitations of these primary methods, other, more specific, methods must be used to measure other components of dietary fiber, such as inulin, FOS, RS, and lignin. Current methodologies will continue to accurately quantitate the amount of fiber in the majority of foods, the exception being those foods containing a significant amount of dietary fiber which is soluble in a solvent mixture of 4 parts alcohol and 1 part water. This exceptionally soluble dietary fiber has heretofore been excluded from the quantity of dietary fiber reported on food labels and entered into database(s) for analytical, as opposed to definitional, reasons. Additional methods, or adjustments to current methods, which assure inclusion of the exceptionally soluble dietary fiber, will increase the reported dietary fiber level of a few foods, particularly foods high in fructans such as onions and leeks. Methods accurately fitting the definition will minimize regulatory confusion and result in accurate nutrition labeling of food products. Method Requirements Adoption of the definition for dietary fiber, i.e. “Dietary fiber is the remnants of the edible part of plants and analogous carbohydrates that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the human large intestine. It includes polysaccharides, oligosaccharides, lignin and associated plant substances. Dietary fibers exhibit one or more of either laxation (fecal bulking and softening; increased frequency; and/or regularity), blood cholesterol attenuation, and/or blood glucose attenuation,” will result in relatively few method changes or changes in food labels or food databases. Analytically inclusive components fitting this definition include cellulose, hemicellulose, lignin, gums, mucilages, oligosaccharides, pectins, waxes, cutin, and suberin. Analytical methodology useful for food labeling needs to effectively quantitate all of these components, while excluding all other food components. The analytical method also must quantitate the dietary fiber using a set of standardized conditions which will convert the food to the state of the food as it is most likely to be consumed. That is, the method should not
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quantitate “resistant starch” as dietary fiber merely because the starch is resistant to digestion because it is ungelatinized as it is found in the food product as labeled and sold, when there is a chance it will be cooked prior to consumption. Thus, a starch gelatinization step is necessary in any method developed for dietary fiber analysis as is a sample digestion step with enzymes that simulate the human digestion system to the closest extent possible in the laboratory. Applicable Methods In the 1981 definition, “Dietary Fiber consists of the remnants of edible plant cells, polysaccharides, lignin and associated substances resistant to (hydrolysis) digestion by the alimentary enzymes of humans” as in the proposed definition, dietary fiber is the remnants of the edible parts of plants resistant to digestion in the human small intestine. This resistance to digestion was, and remains, the key focus of the analytical method requirements. The first Official Method of Analysis developed based on the 1981 consensus definition was AOAC 985.29. This method is based on the premise of resistance to digestion. Human digestive enzymes are known to digest fats, proteins, and starch. Using 985.29, the food samples are defatted, then heated to gelatinize the starch (the primary form of starch in foods as consumed), then subjected to enzymatic digestion by protease, amylase, and amyloglucosidase (glucoamylase) to remove the digestible components of the food. The residues are quantitated, and adjusted for protein and ash to assure against a protein contribution from the enzymes, and assure that inorganic materials present in the sample are not quantitated as dietary fiber. The enzymes utilized for starch and protein digestion are required to completely digest representative starch and proteins. The method and the enzymes must also pass a purity of activity test to assure against extraneous enzymatic activity, i.e. to assure that the method does not destroy, and the enzymes do not digest any of the dietary fiber components listed above. Substrates to use to assure against extraneous enzymatic activity are listed in the referenced table and section. Other AOAC Official Methods of Analysis and AACC Approved Methods of Analysis adopted since that time have the same or similar method performance requirements, and are listed in Figure 3.5. Additional Methods Requirements Since the time of the adoption of the consensus definition in 1981, and the adoption of Official Method of Analysis 985.29 in 1985, dietary fiber research has expanded dramatically. This expanded knowledge
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FIGURE 3.5 — Official And Approved Methods For Dietary Fiber Analysis AOAC Official Method of Analysis
AACC Approved Method of Analysis
Designation
Title
Designation
Title
AOAC 985.29
Total Dietary Fiber in Foods Enzymatic-Gravimetric Method
AACC 32-05
Total Dietary Fiber
AOAC 991.42
Insoluble Dietary Fiber in Foods and Food Products Enzymatic-Gravimetric Method, Phosphate Buffer
AACC 32-20
Insoluble Dietary Fiber
AOAC 991.43
Total, Soluble, and Insoluble Dietary Fiber in Foods Enzymatic-Gravimetric Method, MES-Tris Buffer
AACC 32-07
Determination of Soluble, Insoluble and Total Dietary Fiber in Foods and Food Products
AOAC 992.16
Total Dietary Fiber, Enzymatic-Gravimetric Method
AACC 32-06
Total Dietary Fiber Rapid Gravimetric Method
AOAC 993.19
Soluble Dietary Fiber in Food and Food Products, Enzymatic-Gravimetric Method (Phosphate Buffer)
AOAC 993.21
Total Dietary Fiber in Foods and Food Products with <2% Starch, Nonenzymatic Gravimetric Method
AOAC 994.13
Total Dietary Fiber (Determined as Neutral Sugar Residues, Uronic Acid Residues, and Klason Lignin) Gas Chromatographic Colorimetric-Gravimetric Method (Uppsala Method)
AACC 32-25
Total Dietary Fiber Determined as Neutral Sugar Residues, Uronic Acid Residues, and Klason Lignin (Uppsala Method)
AACC 32-21 Insoluble and Soluble Dietary Fiber in Oat Products Enzymatic Gravimetric Method
includes the discovery of “resistant starch,” expanded knowledge of the physiological and chemical properties of fructans, including inulin, and the technical capabilities to produce edible carbohydrate-based polymers that are analogous to dietary fiber in their digestive and fermentative behaviors. The analytical methodology adopted in 1985 depends upon the fiber fraction isolated being insoluble in a mixture of 4 parts alcohol and 1 part water. This 4:1 solvent mixture is a traditional chemical means of separating simple sugars and other compounds from the more complex starches and proteins in the samples prior to the analysis of
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the simpler compounds. In the early 1980s, the 4-part alcohol, 1-part water solvent mixture was believed adequate for precipitating and isolating the dietary fiber from the enzyme digestion media. It is now evident that this mixture is not sufficient for the isolation of all dietary fibers, and additional methods need to be used in conjunction with AACC 32-05 (AOAC 985.29) or their equivalents to address those fibers not precipitated. Fructan(s), because of the conformation of the molecule(s), are nearly 100% soluble in the 4-part alcohol, 1-part water mixture. As a result, they will not be isolated as part of the precipitate using 985.29 or equivalent methods. Fructans are part of the “remnants of the edible part of plants that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the human large intestine.” Because fructans are not isolated as part of contemporary methodology, the recently adopted AOAC Official Method of Analysis 997.08, Fructans in Food Products, Ion Exchange Chromatographic Method (AACC Proposed Method of Analysis 32-21), or AOAC 999.03, Measurement of Total Fructan in Foods, Enzymatic/Spectrophotometric Method (AACC Proposed Method 32-32), must be used. In addition, a small amount of inulinase enzyme must be added during the enzymatic digestion steps of the contemporary methods to digest the small amount of fructan that co-precipitates with the rest of the fiber to avoid duplicate quantization. Fructans are nonexistent, or occur in small quantities in most foods such as whole grains, fruits, and vegetables which are consumed in significant quantity. It is likely that there will be little impact on the food labels of these foods on a per-serving basis. A few foods, such as onions and leeks, contain high levels of fructans, so the food label of these products may need to be adjusted slightly on a per-serving basis when the inulin content is added to the fiber quantized by contemporary methodology. Polydextrose, like fructans, is also nearly 100% soluble in the 4-part alcohol, 1-part water mixture, due to the highly branched nature and relatively low molecular weight of the molecule. No significant amount of polydextrose is measured as dietary fiber by AOAC Official Method 985.29 or equivalent, therefore AOAC Official Method of Analysis 2000.11, Polydextrose in Foods by Ion Chromatography, has recently been approved. For foods that contain polydextrose, this method can be used as an adjunct to AOAC 985.29 or equivalent methods in order to determine polydextrose as dietary fiber. Advancing technical capabilities now allow the production of edible
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carbohydrate-based polymers that are analogous to dietary fiber in their digestive and fermentative behaviors. Since it is impossible to completely predict the analytical behavior of these analogous carbohydrates relative to the behavior of naturally occurring dietary fibers, methods for the analysis of other, currently available, analogous carbohydrate materials, or for those that may be developed in the future cannot currently be prescribed. It will be good if those involved with the research and production of such materials who are best equipped with the knowledge and resources develop appropriate analytical methods for their respective materials when used as an ingredient, and quantized when used as part of a food product. Subsequent to the adoption of Official Method of Analysis 985.29, researchers discovered that, in some foods, primarily processed grain products, a small percentage of the starch becomes resistant to the enzymatic digestion procedure of the method. This starch is truly resistant to digestion, resisting digestion in the human intestine (actually, additional quantities of starch typically pass into the large intestine with the resistant starch) and during the analytical processes for quantizing dietary fiber. In addition, starch in other foods also resists digestion, either because it is in a granular form, or because it has retrograded into a digestion-resistant crystalline domain. For labeling purposes, it is not clear what portion of this starch, if any, should be considered as dietary fiber. In some cases, the resistant starch is a component of a not fully ripened plant material. In other cases it is the result of incomplete cooking, or of heating and cooling the food product. In any of these cases, there is no consistent means of producing data for labeling purposes. Less than ripe plant materials can be ripened, or can be at various stages of ripeness when consumed. Less than fully cooked, or heated and cooled products can be cooked, or reheated or at various stages of cooling and crystallization, and the quantity of resistant starch changed before consumption. Therefore, for labeling purposes, utilizing the standardized methodology of AOAC 985.29 or equivalent provides the most reliable and accurate assessment of the quantity of digestively resistant starch that can consistently be delivered to the consumer at the time of consumption. For research purposes, other definitions for resistant starch and other methods for the quantization of the resistant starch thus defined may be suitable. But for labeling purposes, the starch that is truly resistant to digestion in a method that standardizes the treatment of the sample to simulate the likely state of the food at the time of consumption and digests the sample with enzymes that simulate the human small intestine is in order.
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DIETARY FIBER METABOLISM IN GASTROINTESTINAL TRACT Physicochemical Characteristics And Physiological Effects The gastrointestinal tract is the primary area of action of dietary fiber, most notably in the large intestine. The physiological effects of dietary fiber depend on a myriad of variables, but generally they depend on the type (partially fermentable or highly fermentable), the dose of a specific fiber consumed, the composition of the entire fiber-containing meal, and the individual physiological profile of the subject consuming the fiber-containing meal. However, the major physiological effects of dietary fiber originate from the interactions with colonic content throughout its fermentation. Through its varying physicochemical properties, dietary fiber intake influences several metabolic processes, including the absorption of nutrients, carbohydrate and fat metabolism, and cholesterol metabolism. It further has influence on colonic fermentation and affects the production of stools. In the large intestine, dietary fiber influences the colonic structure and barrier function, and as the large intestine encompasses a significant body of the human immune system, it is also likely to have influence on elements of immune function. Dietary fibers, as mentioned previously, differ in their water solubility and can have rheological effects; many well fermented fibers form viscous solutions in the gut (for example, guar gum), but notable exceptions are gum arabic and inulin. Some form gels (pectins), while others have a high water holding capacity (WHC) (for example, cellulose). A changed rheology of the intestinal contents can also have physiological effects. A high viscosity is generally connected with a delayed gastric emptying and increased small intestinal transit time. Fibers with a high WHC (that is, partially fermented fibers) can directly influence the volume and bulk of the intestinal content. The solubility of a fiber is related to its fermentability. Almost all of the fiber sources are fermented to some degree by the resident microorganisms present in the colon. The most notable exceptions are cellulose derivatives, such as carboxymethyl cellulose (CMC), which is soluble but almost nonfermentable by the human colon flora. The physical properties of fiber having primary influence on physiology are its dispersibility in water or WHC; its intestinal bulk due to nondigestibility; its ability to increase viscosity (rheology change) as associated with the more fermentable fiber component; its ability to adsorb or bind bile acids; and its fermentability by microorganisms in the gut.
DIETARY FIBER
81
Colonic Fermentation And Its Consequences The large intestine plays a role in managing and conserving water and electrolytes, to further the digestion of residual material passing from the small intestine, and provides a route for residual, nondigestible material and toxins to pass. The large intestine is the most heavily colonized region of the digestive tract, with up to 1011 to 1012 anaerobic bacteria for every gram of intestinal content (Gibson and Roberfroid 1995). These bacteria produce enzymes that further the digestion of proteins and carbohydrates (fiber) passing undigested from the small intestine. Many variables can influence the extent of the fiber fermentation and, consequently, the nature and amount of the various end products produced from the fermentation, including gases (methane, hydrogen, carbon dioxide), short chain fatty acids (SCFA) (C2-C4 organic acids), and an increased bacterial mass. The extent of fermentation typically ranges from completely fermented (many watersoluble fibers) to little fermentation, for example, cellulose particles. However, of the many factors influencing the extent of fermentation, the primary influence is the physicochemical nature of the fiber. Green et al (1998), while working with human fecal slurries and different fiber sources, observed significantly different levels of SCFA and gas produced from the fiber sources. Botham et al (1998) further noted that the degree of fermentation and concentration of the various end products, particularly the SCFA, change due to the chemical structure and nature of the fiber source. Cellulose composed of β-1,4-linked Dglucose is hardly fermented, whereas starch, which has -1,4 and -1,6 linked D-glucosyl residues, is much more susceptible. It is also clear that amylose film shows limited fermentation, due to the limited accessibility to fermentative enzymes, whereas an amylose gel having better access for enzymes is fermented to a greater extent. Increases in microbial mass from fiber fermentation contribute directly to stool bulk, which is a large part of the stool weight. Bacteria are about 80% water and have the ability to resist dehydration, and thus contribute to water-holding in fecal material. Gas production from colonic fermentation can also have some influence on stool bulk. Trapping of gas can contribute to increased volume and a decrease in fecal transit time. WHC of dietary fiber was originally thought to be an important element for maintaining water content of stool. However, water content of stool is relatively constant at about 25%, and the WHC of dietary fiber likely does not have a direct influence on stool bulk. Bourquin et al (1996) reported that the dispersibility and WHC of fiber determine the ability of microorganisms to penetrate undigested food
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FUNCTIONAL FOODS
and degrade the fiber for growth. As such, WHC has an indirect relationship to stool bulk through its influence on the fermentation of fiber. Hence, fiber sources having high WHC, such as gums or pectin, generally tend to be more readily fermented than those with lower WHC, such as wheat bran, that contain higher levels of insoluble cellulose material. Poorly fermented fibers, such as cellulose, exert an indirect stool bulking effect that causes shorter fecal transit times, a greater fecal mass, and laxation effects. Several researchers working with various fiber sources containing both poorly fermented and well fermented fiber, such as wheat and barley bran, cellulose, soy fiber, and inulin, have shown stool bulking/laxation related effects (Causey et al., 2000). In addition, different fiber types can yield different fermentation products. Poorly fermented cellulose produces very little acid during its fermentation, most of which is only acetic acid; by contrast, in the case of more fermentable fibers, large quantities of SCFA are formed, including propionic, butyric, and acetic acids, in varying proportions. Thus, within a given gut microbiota environment, it may be possible to manipulate several of the key variables of the fermentation, notably the feeding of different fiber sources and combinations of fiber, to manipulate the specific types and amount of the SCFA (Silk et al., 2001). The potential to modify the amounts and distribution of the SCFA and the site of their production in the colon may be important, as each has different physiological influence, with varying implications on human health. In addition, as various gut microorganisms have specificity for fibers of different chemical structure, utilizing fiber sources, such as inulin, that promote bifidobacteria more selectively can provide a means to modify colonic microbiota to a more healthy balance, thereby potentially increasing host health. The metabolic end products of fermentation including the gases, SCFA, and increased microbiota, play a pivotal role in the physiological effects of fiber and implications for local effects in the colon and systemic effects. The gases produced from fiber fermentation by strict anaerobic species, such as bacteriodes, some nonpathogenic species of clostridia and yeasts, anaerobic cocci, and some species of lactobacilli, are mostly released as flatulence or are absorbed and subsequently lost from the body through the lungs. However, some of the hydrogen and carbon dioxide produced from these microorganisms may be further metabolized to methane (CH4) by methanogenic bacteria, thus reducing intestinal gas pressure. Of these anaerobic microorganisms, the clostridia, eubacteria, and anaerobic cocci are the most gas producing, while the bifidobacteria are the only group of the
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83
common gut microbiota that do not produce any gases. The SCFA resulting from the fermentation process provide a certain amount of energy from their metabolism in the liver. The energy content of a fiber is, from a scientific standpoint, dependent on the degree of fermentation. Fibers that are not fermented to any extent have a caloric content approaching 0 kcal/gram, while data from caloric studies indicate that the average energy yield from dietary fiber fermentation in monogastric species is in the range of 1.5 to 2.5 kcal/g (Smith et al., 1998). It should be noted that legal values for nutritional labeling can be different. The primary SCFA generated by fermentation are acetate, propionate and butyrate, accounting for 83 to 95% of the total SCFA concentration in the large intestine, which ranges from about 60 to 150 mmol/L. The concentrations of these acids are highest where concentrations of microbiota are also highest, namely in the cecum and transverse colon. Corresponding to these higher acid levels, the pH is also typically lowest in the transverse colon (5.4 to 5.9) and gradually increases through the distal colon to 6.6 to 6.9. At the colonic level, the fermentation of fiber increases the concentrations of these health-promoting SCFA and endogenous microbiota, exerting potential health effects, such as inhibiting the growth of pathogens, increasing mineral absorption, or producing vitamins. The SCFA absorbed into the portal blood system and reaching the liver and kidneys can further influence metabolism. This can lead to systemic effects, such as changes in glycemia, lipidemia, uremia, and overall nitrogen balance. Influence on lipids is an example of a potential health effect; a high serum lipid level is connected with a increased risk of cardiovascular disease. This risk may be lowered by the consumption of fermentable fibers. PHYSIOLOGICAL FUNCTIONS OF DIETARY FIBER Dietary Fiber And Cancer Colon cancer is one of the leading causes of cancer morbidity and mortality among both men and women in the Western countries, including the U.S.A. Historical observational and epidemiological studies from around the world have long supported that increased consumption of fruits and vegetables and high fiber intake provide a protective relationship between dietary fiber intake and colon cancer incidence (Byers, 2000). Of particular interest is the utilization of fermentable fiber by the colonic microbiota that can result in changes to the numbers and types
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FUNCTIONAL FOODS
of bacteria and, more importantly, changes to their metabolic activities in terms of the formation of genotoxins, carcinogens, and tumor promoters. Reddy (1999) emphasized synergistic effect when both probiotics and prebiotics are used together for possible mechanistic effects for cancer inhibition. Selective prebiotic fiber sources, such as inulin, resistant starches, and some oligosaccharides, act as selective substrate for bacteria that produce specific SCFA and can lower the intestinal pH. The SCFA butyrate has been shown to increase apoptosis in human colonic tumor cell lines (Scheppach, 1998). Possible mechanisms for the anticarcinogenic and antitumorigenic effect of highly fermentable fibers are not completely understood and require further research. However, it is likely that some or all are involved in a metabolic chain reaction for the inhibitory effect to occur. The primary mechanisms involved with these effects are proposed to be: a reduction in the production of carcinogenic substances by decreasing the amount of pathogenic bacteria in the colon; and/or lowering the colonic pH to affect pH-dependent enzymatic reactions; for example, secondary bile acid formation; and/or reducing the amount of carcinogenic substances available to colonic mucosa by adsorption of the substances to the cell wall of the microorganisms, by speeding up the intestinal transit time and by increasing colonic contents and thus diluting all components; and/or exerting inhibiting effects on initiation and promotion stages in colon cancer formation in which SCFA, particularly butyric acid, may play a key role. Dietary Fiber And Carbohydrate Metabolism An association between insufficient dietary fiber intake and increased risk of diabetes has been postulated since 1970s. While a direct linkage between insufficient dietary fiber intake and diabetes has not been established, evidence that indicates decreased risk of the disease with increased dietary fiber consumption continues to grow (Chandalia et al., 2000). Well fermented viscous fibers, either as part of a food or as a supplement that is well mixed with food, appear to offer the greatest potential benefit to reduce glycemic response and to increase insulin sensitivity. Glycemic Index And Glycemic Load The glycemic index (GI) is a numerical system of measuring how much of a rise in circulating blood sugar a carbohydrate triggers—the higher the number, the greater the blood sugar response. So a low GI food will cause a small rise, while a high GI food will trigger a dramatic spike. A GI of 70 or more is high, a GI of 56 to 69 inclusive is medium,
DIETARY FIBER
85
and a GI of 55 or less is low. The glycemic load (GL) is a relatively new way to assess the impact of carbohydrate consumption that takes the glycemic index into account, but gives a fuller picture than does glycemic index alone. A GI value indicates only how rapidly a particular carbohydrate turns into sugar. It doesn’t show how much of that carbohydrate is in a serving of a particular food. It is necessary to know both to understand a food’s effect on blood sugar. That is where glycemic load comes in. The carbohydrate in watermelon, for example, has a high GI. But there isn’t a lot of it, so watermelon’s glycemic load is relatively low. A GL of 20 or more is high, a GL of 11 to 19 inclusive is medium, and a GL of 10 or less is low. Epidemiologic evidence suggests that a diet with a high glycemic load or glycemic index may increase the risk of type 2 diabetes. The beneficial physiological effects of viscous fiber sources on blood glucose concentrations have been consistently demonstrated over the last 2 decades. The mechanisms explaining the influence fibers have on reducing postprandial glycemia and fiber’s potential for enhancing carbohydrate metabolism over a longer term remain unclear. However, most likely causes for these influences are those related to small intestinal viscosity and nutrient absorption, and systemic effects from colonic-derived SCFA. Relative to postprandial glycemia, fibers that provide high viscosity in the small intestine (for example, guar gum, pectin) generally offer greater effect. By contrast, the SCFA, produced in the colon from well fermented fiber (for example, inulin), likely influence hepatic cholesterol synthesis, the production of glucose and its utilization later in the day (Luo et al., 1996). As the small intestinal transit times for mixed meals is relatively long (about 6 h), the colonicderived SCFA likely do not explain the acute effects of slowing small intestinal carbohydrate absorption typical of the postprandial effects following the intake of viscous well fermented fibers. The SCFA, namely, acetate, produced from fiber fermentation and absorbed into the peripheral blood may influence systemic metabolic functions. Acetate may influence serum fatty acid levels, which may directly inhibit adipose tissue lipolysis. Studies involving acetate, whether derived from fiber fermentation during chronic feeding studies, or given orally, or given by rectal infusion have all shown acetate to reduce serum fatty acid levels. But none has shown acetate to improve carbohydrate tolerance. However, propionate is gluconeogenic in the liver, potentially increasing blood glucose levels. Even though a clear mechanism for fiber effects on carbohydrate metabolism is yet undefined, the beneficial effects in support of high fiber intake for type 2 diabetes mellitus continue to
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FUNCTIONAL FOODS
increase. While the reduced small intestinal absorption may play a greater role in this effect than the colonic effects, further research is needed in the area of specific fiber sources and carbohydrate tolerance to more fully elucidate potential synergistic effects from SCFA. Dietary Fiber, Lipid Metabolism, And Cardiovascular Disease Total serum cholesterol and low-density-lipoprotein (LDL) cholesterol levels are generally accepted as biomarkers, indicative of potential risk for developing the disease. As such, research has primarily focused on their reduction as a means to reduce the risk of developing cardiovascular disease (CVD). Substantial experimental data support that blood cholesterol can be lowered using well fermented fiber types that produce relatively high viscosity, and epidemiological evidence supports the relationship between higher dietary fiber intake and reducing the risk of cardiovascular disease (Anderson et al., 2000). They concluded that high-fiber diets may protect against obesity and CVD by lowering insulin levels. A meta-analysis of 67 controlled studies using viscous well fermented fibers (oat-25, psyllium-17, pectin-7, and guar gum-18) showed reduction in serum cholesterol with higher rather than lower fiber levels (Brown et al., 1999). Less viscous fiber sources, like inulin, have generally shown consistent lipid lowering in animal studies, but human studies have shown variable results (Boeckner et al., 2000). When considering the results of human lipid studies, a number of factors need to be addressed, including: individual variation, duration of administration of the fiber source, fermentation rates of the various fiber types and chain fractions, intakes of dietary fat and carbohydrate in the background diet; and prior serum lipid levels. Mechanisms for cholesterol-lowering ability of water-soluble fiber have been suggested, but no consensus has been reached. Studies indicate effects on cholesterol formation due to certain well fermented fibers, such as psyllium, citrus pectin, oat bran, and rolled oats (Anderson et al., 1984), bind bile acids increasing their excretion and decrease cholesterol in the liver. Certain fibers, such as pectins and galactomannans that have high WHC and generate higher viscosity, also have influence on small intestine absorption of nutrients. It has also been suggested that the hypocholesterolemic effect of dietary fibers might also be mediated by the SCFA from fiber fermentation. SCFA are absorbed from the colon; butyrate and propionate are extracted by the colonic mucosa and liver, respectively, whereas acetate reaches peripheral circulation. Propionate primarily is reported to inhibit fatty acid metabolism, which plays a key role in the synthesis of cholesterol (Demigne et al., 1995). Kok et al
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87
(1998) postulated that lower glucose and insulin levels found after feeding inulin at a dose of 10% to rats contributed to reduced hepatic fatty acid and triglyceride synthesis. This would positively influence lipid metabolism by increasing the secretion of intestinal hormones, namely, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). These gut hormones are known to regulate postprandial insulin release and also to have direct insulinlike actions on lipid metabolism. Dietary Fiber, Mineral Bioavailability And Bone Health Certain fiber sources from fruits and vegetables that have cation exchange capacity from unmethylated galacturonic acid residues and phytic acid from cereal fibers, have been found to depress the absorption and retention of several minerals. However, certain highly fermentable fibers have resulted in improved metabolic absorption of certain minerals, such as calcium, magnesium, and iron, even when phytic acid is present at lower concentrations (Lopez et al., 1998). These compounds include pectin, various gums, resistant starches, cellulose, certain oligosaccharides like soy and fructooligosaccharides, inulin, lactulose, and related sugars. Mineral absorption has generally been accepted as stemming from diffusion in the small intestine. However, studies now indicate that highly fermentable fibers, such as inulin and fructooligosaccharides, also promote mineral absorption in the colon. Through their fermentation by colonic microbiota and subsequent SCFA production, these fiber components stimulate the proliferation of epithelial cells in the ceco-colon and reduce the luminal pH (Younes et al., 1996). The SCFA and lower pH may, in turn, dissolve insoluble mineral salts, especially calcium, magnesium, and iron, in the luminal content and increase their diffusive absorption via the paracellular route. In particular, the accumulation of calcium phosphate in the large intestine and the solubilization of minerals by SCFA are likely to play an essential role in the enhanced mineral absorption in the colon. Also, a recent study has demonstrated that fructooligosaccharides (inulin) stimulates the transcellular route of calcium absorption in the large intestine, as indicated by increased concentrations of calbindin-D9k, a calcium binding protein that plays an important role in intestinal calcium transport (Ohta et al., 1998). Dietary Fiber, Nitrogen Utilization And Bile Acid Metabolism Fiber also has influence on nitrogen balance. Fiber acts as substrate for increased microbial mass, which utilizes high fecal nitrogen and
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FUNCTIONAL FOODS
creates a marked enlargement of the cecum. The SCFA produced from fermentation of fiber and their associated lowering of colonic pH provide an added effect by protonating potentially toxic ammonia (NH3) to produce ammonium ion (NH4+), a form that is nondiffusible into the portal blood system (Younes et al., 1995). The consequence of this process is higher nitrogen retention in the cecum, increased fecal nitrogen excretion, lower blood ammonia levels, and decreased uremia. Studies in both animals and humans have shown that fecal nitrogen excretion is increased during consumption of a high soluble fiber diet (Vanhoof and De Schrijver 1996). Nitrogen balance, however, is not compromised due to a concomitant decrease in renal nitrogen excretion, likely due to a strong transfer of urea nitrogen to the intestine, to depress the plasma uremia. This shift does not appear to alter protein bioavailability, and seems more evident when the dietary protein level is moderate. The acidification of the luminal content by the SCFA may also potentially modify the metabolism of bile acids. Of particular interest is reducing the conversion of primary to secondary bile acids, as these are believed to be associated with increased risk of colon cancer. Dietary Fiber, Role In Gut Barrier Function And Gastrointestinal Disorders The health of the large intestinal wall and its microbial ecosystem play a key role in gastrointestinal health. Through its fermentation, dietary fiber and SCFA are important elements for both protecting the health of the large intestinal wall and stimulating repair in a damaged colon. The health of this organ is very important as it, in addition to serving as a primary site for water reabsorption, plays an important role as a major immune organ and functions as a barrier to prevent foreign materials from dietary or microbial origin from crossing into the internal body cavity. The integrity of the intestinal barrier changes with stress, starvation, and in several clinical situations where the gut is damaged; for example, Crohn’s disease, celiac sprue, extensive burn injury, antibiotic therapy, parasites, rheumatoid arthritis and indomethacin-associated enteritis, and intestinal obstruction (Lipman, 1995). Many of these disorders result indirectly from the loss of barrier function and are directly related to bacterial translocation. Various researchers showed that specific well fermented fibers, such as inulin, encourage the growth of healthpromoting bacteria and minimize the growth of pathogens and the
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production of subsequent harmful byproducts of protein degradation (that is, ammonia, phenolic products, amines, and N-nitroso compounds), which have been linked to various types of cancer and ulcerative colitis (Birkett et al., 1996). Further, researchers working with animals and in vitro testing have suggested a role of various fibers on intestinal immune function (Meyer et al., 2000). The SCFA, from fiber fermentation, particularly butyrate, play a key role in the health of the colon. They are suggested to be of influence in both stimulation of cell division and regulation of apoptosis (Wasan and Goodlad, 1996). Unlike the small intestine, which can derive energy from various endogenic sources, such as glutamine from muscle breakdown or ketone bodies from hepatic ketogenesis, during periods of starvation, the colon epithelial cells derive their energy from SCFA, particularly butyrate. Butyrate, is a preferred nutrient for colonocytes, prevents colonic mucosal atrophy, which develops within a day of oral starvation. Intestinal permeability or “leaky gut syndrome” is viewed as an indicator for sub-clinical disease states. As gut barrier function is lost, the formation of chronic bowel inflammation may appear, since exposure of the mucosa to luminal antigens can be responsible for inflammation. In order to keep the colonocytes healthy and minimize potential for chronic bowel inflammation, it is important to maintain a positive balance between crypt cell growth and luminal cell loss by apoptosis or sloughing (Lynn et al., 1994). While different fiber types have different effects on epithelial cell growth, it is generally accepted that more fermentable fibers have greater influence on cell growth, as being promoted via SCFA production. Poorly fermented fibers, such as cellulose, appear to maintain muscle layer, independently from the mucosal layer, while a fermentable substrate is required for mucosal layer growth. Poorly fermented fibers may also influence colonocyte proliferation by direct abrasive action (Folino et al., 1995). The mucus layer is important to the gut lining, provides lubrication and protects the gut from enzymatic acid and toxin attack. It is also food for several microbial species, helps remove microorganisms, and further serves as an antioxidant (Satchithanandan et al., 1996). Highly fermentable fibers appear more likely than poorly fermentable fibers to alter intestinal mucus composition, either through their direct mechanical effects or indirectly by regulating mucosal metabolism via SCFA derived from fiber fermentation.
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PROPERTIES AND PHYSIOLOGICAL EFFECTS OF SELECTED NON DIGESTIBLE POLYSACCHARIDES (NDP) AND NON DIGESTIBLE OLIGOSACCHARIDES (NDO) An overview of many different NDPs, their chemical makeup and occurrence, and a summary of the effects on the human intestine is given in Figure 3.6. A description of the physicochemical structures of selected NDOs and NDPs will provide a further understanding of possible physiological effects of these isolated dietary fiber ingredients based on human studies. Resistant Starch Resistant starch is part of our daily diet or can be added as an ingredient to various food products. The colonic fermentation of resistant starch in humans is well documented (Heijnen et al., 1998). Stool bulking effects for resistant starch were found as well, probably as a consequence of the increased bacterial mass in the feces. Further, consumption of resistant starch (and/or its subset resistant maltodextrin) may also stimulate the growth of specific bacteria purported to provide beneficial health effects, that is, the bifidobacteria and lactobacilli. Potential health benefits of resistant starch, such as blood sugar and cholesterol control, weight control and energy management, gut disorders, and colon cancer were reported by Baghurst et al (1996). Fermentation of resistant starch gives rise to relatively high levels of butyric acid, which, as mentioned earlier, may have implications in tumorigenesis and gut barrier health. Animal model studies show that resistant starch inhibits chemically induced carcinogenesis in the colon of rats (Sakamoto et al., 1996). Pectins Pectins are present in plant cell walls, and are polysaccharides with -1, 4-linked D-galacturonic acid backbone with a variable amount of neutral sugars (arabinose, galactose, xylose) present as side-chains. Rhamnose may also be present in the backbone. The galacturonic acid residues are substituted with acetyl and methyl ester groups. The main applications for pectins as food additives are as gelling and thickening agents in many food products. Most pectins for food use are extracted from citrus or apple. Due to their gelling behavior, these soluble polysaccharides may decrease the rate of gastric emptying and influence small intestinal transit time. This explains their hypoglycemic properties. Various human studies show that pectins are fermented to a large extent in the colon.
By alkaline deacetylation of crustacean chitins. Found in fungi cell walls of Zygomycetes sp. Extracted from plants (wood pulp, bamboo, wheat, cottonseed hulls)
2-amino-2-deoxy-βD-glucose
β-1-4-D-glucose
Chitosan
Microcrystalline Cellulose
Produced by fermentation using Pseudomonas elodea
Produced by fermentation using Xanthomonas campestris
β-1-4-D-glucose (backbone), β-D-glucuronic acid, D-rhamnose
β-1-4-D-glucose (backbone), β-D-glucuronic acid, β-D-rhamnose
Gellan gum
Xanthan gum
Chemical reaction of cellulose with caustic followed by reaction with substituting reagent
Produced by fermentation using Alcaligenes faecalis var. myxogenes
β-1-3-D-glucose
Curdlan (insoluble β-glucan)
Modified cellulose Various functional groups (MC, CMC, MHPC) substituted for OH- of cellulose
Cereal (barley, oats)
Occurrence & Production
β-1-4-D-glucose and β-1-3-D-glucose
Composition
β-glucan
Name
Increases water holding capacity, increased fecal bulk, partially fermented in human gut.
Increases fecal excretion of neutral steroids. Interferes with intestinal absorption of cholesterol.
Increased water holding capacity
Fermented in large intestine, strong butyrate production, blood lipid effects
Human Physiological Effects
Adds viscosity, fermented to short-chain fatty acids in human gut Pastry fillings, sauces Adds viscosity, fermented to and gravies, salad short chain fatty acids in dressings, dairy products, human gut beverages, puddings
Icings, fillings, dessert gels, low-sugar jams and jellies, puddings and confections
Toppings, fillings, icings, Partial fermentation in the breadings, soups, sauces, human gut gravies, baked goods, biofilms
Dressings and sauces, beverages, baked goods, whipped toppings
High viscosity limits food uses. Decreased hepatic cholesterol and triglycerides
Processed meat, poultry, seafood products, noodles and pasta, sauces and dressings desserts, biofilms, jellies
Breakfast cereals, functional food products
Food Applications
FIGURE 3.6 — Commercial Nondigestible Polysaccharides
DIETARY FIBER 91
Not in nature, in Japan by transglucosylation of glucose.
Cyclic molecules of -1-4 linked D-glucose, (-cyclodextrin-hexamer, β-cyclodextrin-hetamer & g-cyclodextrin-octamer)
Mixture of β-1-6 linked D-glucose oligomers
Mixture of -D-glucose
β-Cyclodextrins
Gentiooligosaccharides (GeOS)
Glucooligosaccharides (-GOS)
By transglucosidation using an -glucosidase from Leuconostoc mesenteroides
Not in nature
4-O-β-D-glucopyranosyl-D -glucose [(4-O-β-D-glucopyranosyl)n -D-glucose]
Not in nature, degradation of cellulose
Produced by pyrolysis of corn starch with HCl, further enzymatic hydrolyzed
Cellobiose (CEL) Cellodextrins
Cellobiose and cellodextrins
Mixed and random linkages, -1-4 and 1-6 glucosidic bonds from starch and 1-2 and 1-3 bonds, from transglucosidation
Resistant maltodextrin
By vacuum thermal polymerization of glucose, sorbitol and citric acid
Occurrence & Production
Mixed and random glycosidic linkages, (1,6-bonds)-D-glucose
Composition
Polydextrose
Name
Sauces, salad dressings, ice cream, frozen desserts, partially hydrolyzed used as highly soluble fiber
No information
No information
No information
Nutritional beverages, functional foods, low viscosity fiber source
Low calorie, bulking agent for sugar replacement in foods (confections, desserts, dairy, baked goods, fillings)
Food Applications
Animal studies show influence of intestinal microbiota. No human studies
Scarce data, no conclusion on effects on intestinal microbiota
May have effect based on fermentation, no published studies on effects
No studies on prebiotic effects
Partially digestible, laxation and colonic fermentation, effects on lipid levels
Fermented to produce microbiota, SCFA stool bulking and stool softening in intestine
Human Physiological Effects
FIGURE 3.6 — Commercial Nondigestible Polysaccharides - Continued
92 FUNCTIONAL FOODS
Composition
Dried exudates from stems and branches of African bush Acacia senegal
β-D-galactose (backbone), L-arabinose, L-rhamnose, D-glucuronic acid (highly branched)
-1-4-D-galacturonic acid backbone/neutral sugar side chains, some ester groups
Acacia (gum Arabic)
Pectin
Fruits and vegetables (apples, citrus, sunflowers, sugar beet)
Extracted from red algae, Dairy products, Rhodophyceae (Gelidium sp. confectionery, baked and Gracilaria sp.) products, meat analogues, desserts
β-1-3-D-galactose and 3,6-anhydro-β-L-galactose
Agar
Jams and jellies, low-sugar or sugar free jams and jellies, beverages, milk products, biofilms
Spray dried flavors, confectionery products, jellies, (dry mixed puddings, desserts cake mixes)
Decrease gastric emptying and small intestine transit time (hypoglycemic properties) Fermented in large intestine No effects on stool weight, decrease in serum cholesterol
Fermented in the human gut. Prebiotic
Adds viscosity, fermented in the human gut to short chain fatty acids
Adds viscosity, decreases gastric emptying and small intestine transit time (hypoglycemic properties) Fermented in large intestine to short chain fatty acids
Both animal and human studies indicate influence of intestinal microbiota
Human Physiological Effects
Dairy products, bakery products, dessert gels, processed meat, low-sugar jams and jellies
Extracted from red algae (Rhodophyceae)
Mixture of sulfated polysaccharides made up of -D-galactose & 3, 6-anhydro-D-galactose
Carrageenan
Dairy products, bakery products, dessert gels, processed meat, low-sugar jams and jellies
Food Applications
Linear -1-6 linked glucose residues, some 1-4 linkages, 6-O--D-glucopyranosyl-Dglucose, 6-O--Dglucopyranosyl-(1-6)--Dglucopyranosyl-D-glucose, 6-O--D-glucopyranosyl(1-6)--D-glucopyranosyl(1-4)-D-glucose
Occur in miso, soy sauce, sake and honey. Prepared commercially by transglucosylation of glucose residue. By transglucosidase (-glucosidases)
Occurrence & Production
Isomaltose - IMA Isomaltose - IMT Panose - PAN
Isomalto-oligosaccharids
Name
FIGURE 3.6 — Commercial Nondigestible Polysaccharides - Continued
DIETARY FIBER 93
Human Physiological Effects
O-D-galactopyranosyl(1-4)-O-β-Dgalactopyranosyl-(1-4)D-glucopyranose
Not in nature, produced in Japan by action of Crytococcus laurentii on lactose
Functional foods in Japan
Unaffected by human enzymes. Likely to have effect on composition and metabolic activity in human intestinal microbiota
A study in rats indicates effects on composition and metabolic activity of intestinal flora, no human data are available
4’-galactosyllactose (GLL)
Functional food development in Japan
4-O-β-D[galactopyranosyl]nD-sorbitol
Lactitol oligosaccharides (LTOS)
Not in nature, Transgalactosylation of lactitol using Aspergillus oryzae β-galactosidase
Not in nature, alkaline Drug status in EU, Gas production is relatively isomerization of glucose not approved for food use large, due to fermentation by moiety of lactose to fructose, clostridia, Kl. Pneumoniae. marketed as laxative and Studies suggest effects on gut health aid bacterial composition and activity
Studies indicate intestinal flora effects on composition and activity
4-O-β-D-galactopyranosylD-fructose
Sugar replacement, bulking agent, functional foods, prebiotic
Sugar replacement, May have significant effect on bulking agent, functional composition and activity of foods, prebiotic intestinal flora
Food Applications
Lactulose (LAT)
Extraction from soybeans, legumes
Occurrence & Production
Likely some in nature, enzymatic transgalactosylation of lactose
RAF, O--Dgalactopyranosyl-(1-6)- D-glucopyranosyl-β-Dfructofuranoside
Composition
β-galactoβ-D-galactopyranosyl-(1-6) oligosaccharides or -[β-D-galactopyranosyl]ntransgalactoolig(1-4) -D-glucose osaccharides (TOS)
-galactooligosaccharides (raffinose, stachyose/other soy oligosaccharides)
Name
FIGURE 3.6 — Commercial Nondigestible Polysaccharides - Continued
94 FUNCTIONAL FOODS
Composition
Levan-type
Fructans
Neogalactobiose (NGB) Isogalactobiose (IGB) Galsucrose (GAS) Isolactose I (IL1) Isolactose II (IL2) Isolactose III (IL3) lactose trimer
β-D-(2,6)-fructofura-nosyl)n -D-glycopyranoside
β-D-galactopyranosyl-βD-glucopyranoside β-D-galactopyranosyl-D-glucopyranoside -D-galactopyranosylβ-D-glucopyranoside -D-galactopyranosyl-βD-fructofuranosyl-(2-6)-βD-fructofuranoside
Synthetic galactooligosaccharides
Name
Produced by Bacillus polymyxa on sucrose
Chemically by the Koenigs-Knorr reaction
Occurrence & Production
No primary commercial application
Proposed for functional foods
Food Applications
Data indicate that addition of inulin-type fructans affect the bacterial composition and the metabolic pattern of the intestinal microbiota. Studies have shown production of short chain fatty acids and a relatively high production of propionate and butyrate, necessary for colonic health and systemic influences on blood glucose and lipids
No animal or human data exists. Unknown if these products are digested in the upper GI tract
Human Physiological Effects
FIGURE 3.6 — Commercial Nondigestible Polysaccharides - Continued
DIETARY FIBER 95
Composition
β-1-4-D-mannose (backbone), Guar gum (Cyamopsis Sauces, salad dressings, -1-6-D-galactose tetragonolobus), locust ice cream, frozen desserts, bean gum (Ceratonia siliqua) partially hydrolyzed used as highly soluble fiber
Galactomannan residues
Galactomannan (guar gum, locust bean gum)
Guar gum oligosaccharides
Produced by partial hydrolysis of guar gum
Functional foods
Fermented by colon microbiota. Lipid lowering, plasma glucose lowering
Readily fermented in human gut with bifidogenic effects, improves bowel function, shows hypolipidemic effects reduces postprandial glycemia
Fat/sugar replacement, Data indicate that addition of texture modification. inulin-type fructans affect the Dressings and sauces, bacterial composition and the beverages, baked products, metabolic pattern of the fillings, icings, frozen intestinal microbiota. Studies desserts, dairy products, have shown production of processed meats, low fat short chain fatty acids and a spreads, toppings, relatively high production of extruded products, propionate and butyrate, dietary supplements necessary for colonic health (prebiotic functional foods) and systemic influences on blood glucose and lipids
Mixtures of 1- kestose, nystose & 1f-β-fructofuranosylnystose β-D(2,1)-fructofuranosyl)n β-D-fructofuranoside
Fructooligosaccharides
Produced by transfructosylation of a β-fructosidase of Aspergillus niger on sucrose. By partial enzymatic degradation of native inulin
Human Physiological Effects
Fat/sugar replacement, Data indicate that addition of texture modification. inulin-type fructans affect the Dressings and sauces, bacterial composition and the beverages, baked products, metabolic pattern of the fillings, icings, frozen intestinal microbiota. Studies desserts, dairy products, have shown production of processed meats, low fat short chain fatty acids and a spreads, toppings, relatively high production of extruded products, propionate and butyrate, dietary supplements necessary for colonic health (prebiotic functional foods) and systemic influences on blood glucose and lipids
Food Applications
β-D-(2,1)-fructofuranosyl)n -Dglucopyranoside Naturally occurring in Jerusalem artichokes, chicory, onion, and so on. Produced by extraction from chicory root.
Occurrence & Production
Inulin-type
Fructans - Continued
Name
FIGURE 3.6 — Commercial Nondigestible Polysaccharides - Continued
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Naturally occuring as the husk of the psyllium seed
Extracted from seeds of tamarind tree (Tamarindus indica)
Partial enzymatic hydrolysis Functional foods of polyxylan by xylanase from Trichoderma sp.
Dehydration and purification of konjac tubers (Amorphophallus konjac)
Polymer of arabinoxylans with 1,4 and 1,3 linkages
β-(1-4) linked D-glucose, partially substituted with -D-xylopyranose.
β-D-((1,4)-xylose)n
β-1,4-linked D-glucose and D-mannose (glucomannan)
Psyllium seed husk
Xyloglucan
Xylooligosaccharides
Konjac (flour) mannan
Binder in meat, often used with k-carrageenan and xanthan gum for gelling
Food additives in Japan, sauces, dressing, ice cream, mayonnaise
Functional food development as a fiber source
Functional food development, dietary supplements
Highly branched (β-1-3 & (β-1-6) D-arabinose and D-galactose sub-units) 6:1 ratio
Arabinogalactan
Extracted from the pulp of Western Larch trees
Complex mixture of polymers Dried exudates of Asiatic sp. Salad dressings, pickle of D-galacturonic acid, of Astragalus, Leguminosae relish, pulpy beverages, galactose, arabinose, xylose, (Astragalus gummifer) milkshakes, ice cream traces starch and cellulose
Human Physiological Effects
Ability to reduce serum cholesterol and serum triglyceride levels and influence glucose and insulin responses
Not hydrolyzed by human enzymes, Changes in metabolic pattern of intestinal flora observed in rats. Blood lipid effects
Fermented in the human colon. Adds viscosity in the small intestine
Reduced risk of coronary heart disease (health claim, at least 1.7 g per RA. Reduced cholesterol
Data indicate gut fermentation, increasing Lactobacilli populations. Studies show immunological properties
Adds viscosity, fermented in the human gut
Powdered doughnuts, Adds viscosity, fermented French dressings, ice in the human gut pops, cheese spreads, ground meats, meringues
Food Applications
Gum tragacanth
Dried exudate of the Indian tree Sterculia urens
Occurrence & Production
Acetylated galacturonic acid+rhamnose+galactose
Composition
Gum karaya
Name
FIGURE 3.6 — Commercial Nondigestible Polysaccharides - Continued
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High in mannuronic and glucuronic residues
Mannanoligosaccharide mixture containing less than 50% oligosaccharides
Alginate oligosaccharides
Mannanoligosaccharides
Adapted from Tungland and Meyer, 2002
β-1-4-D-mannuronic acids and -1-4-L-guluronic acid
Composition
Alginate
Name
By Saccharomyces cerevisiae on sucrose
Produced by enzymatic degradation of alginate
Extracted from brown algae (Phaeophyceae)
Occurrence & Production
Human Physiological Effects
Limited data, algal oligosaccharides have an effect on either the metabolism or composition of the intestinal microbiota
Used as growth promoter No data available on in animal feed industry fermentation
Functional foods
Dairy products, bakery Adds viscosity, fermented in products, salad dressings, human gut to short chain dessert puddings, foam fatty acids. stabilization, fabricated foods, dietetic products
Food Applications
FIGURE 3.6 — Commercial Nondigestible Polysaccharides - Continued
98 FUNCTIONAL FOODS
DIETARY FIBER
99
Guar Gum Guar gum is a galactomannan isolated from the seed of Cyamopsis tetragonolobus (guar). In its unmodified form, this food additive is used as a thickener in a large variety of food products. Partial enzymatic hydrolysis results in a product that can be used as a soluble dietary fiber. The physiological effects of this fiber source comply with what might be expected from a soluble fiber. Guar gum is readily fermented by the human fecal microbiota, and it has bifidogenic effects, at least with enteral feeding (Okubo et al., 1994). It improves bowel functioning, reducing diarrhea in enterally fed patients and relieves constipation in patients. It shows a hypolipidemic effect in humans, lowering both serum cholesterol and triglycerides, and it reduces postprandial glycemia. Gum Arabic This is exudate from the acacia tree and is a complex arabinogalactan polysaccharide in admixture with a glycoprotein. It has a high molecular weight and it is used as an additive in many food applications as a stabilizer and emulsifier. The physiological effects from human studies include its complete fermentation in the human colon with indications for a bifidogenic effect and its ability to lower serum triglyceride and cholesterol levels (Michel et al., 1998). Fructans Fructans can be divided in 2 classes. Levans are β-2,6 -linked fructans with variable degrees of β-2,1 -linked side chains that are produced by a large variety of bacteria. Inulins, on the other hand, are composed of β-2,1 -linked fructosyl units, and are produced by many dicotyledonous plants as a reserve carbohydrate. Fructans are discussed in detail in the chapter “Prebiotics, probiotics and symbiotics”. Galactooligosaccharides These oligosaccharides are produced from lactose by the transglycosylating activity of β-galactosidase. They consist of a number of β-1,6-linked galactosyl residues linked to a terminal glucose unit via an -1,4-bond. Galactooligosaccharides are not digested in the human alimentary tract, acting as soluble dietary fiber. Reports show a change in colon flora composition and activity following consumption of these compounds (Alles et al., 1999). Various human studies show that these oligosaccharides may relieve constipation, improve calcium absorption, and retard the development of colon cancer in rat model systems.
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Another type of galactooligosaccharides is natural isolates from soybeans. These -galactooligosaccharides (galactosylsucroseoligosaccharides) include raffinose, stachyose, and verbascose and consist of galactose residues linked -1,6 to the glucose moiety of sucrose. Their physiological effects appear similar to the β-linked galactose oligomers, such as gut pathogen reduction with corresponding effects, as expected from this change in colon microbiota. However, no data are available at present. Lactulose Lactulose is a disaccharide of D-galactose linked β-1,4 to fructose. It is manufactured from lactose, where alkali isomerization is used to convert the glucose moiety in the lactose into a fructose residue. The disaccharide is not digested by humans, and promotes growth of bifidobacteria in the colon (Strohmaier, 1996). Lactulose is mostly used as a pharmaceutical in prevention of constipation and in portosystemic encephalopathy. It is not allowed for food use in Europe, due to its drug status, but seems to find its way into the food supplement market (mainly in The Netherlands). Although its use is clearly for physiological reasons and it has well-established health effects, its legal status limits use in food. The physiological effects of lactulose are exploited in Japan. Other Oligosaccharides As indicated in Figure 3.6, there is a wide variety of oligosaccharides, all of which have properties characteristic of dietary fiber, that have effects on the activity and/or composition of the human colon flora. PROPERTIES OF ISOLATED FIBER IN FOOD APPLICATIONS Dietary fiber components, isolated from their native plants, provide many functional properties that affect the technological function of foods. These functional properties also influence the food product’s properties during its processing and its final product quality and characteristics. The primary properties provided by isolated fiber ingredients for food development are related to their solubility, viscosity and gelation-forming ability, water-binding capacity, oil-binding capacity, and mineral and organic molecule-binding capacity. The solubility of fiber as a technological property refers to its solubility in water. Four primary structural features of the polymeric backbone primarily increase solubility. More branching generally results in greater solubility (for example, gum acacia); the presence of ionizing
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groups also tend to increase solubility (for example, pectin methoxylation); the potential for inter unit positional bonding (for example, β-glucans with mixed β-1-3 and β-1-4 linkages) also increase solubility; and alterations of the monosaccharide units or their molecular form (- or β-form) further increase solubility (for example, gum acacia, arabinogalactan, and xanthan gum). Viscosity is another technological property of fiber that provides rheological change in food systems. Generally, as the molecular weight or chain length of the fiber increases, the viscosity of the fiber in solution increases. However, the concentration of the fiber in solution, the temperature, pH, shear conditions of processing, and ionic strength all substantially depend on the fiber used. Primarily, long chain polymers, such as the gums (for example, guar gum, locust bean gum, tragacanth gum, etc.), bind significant water and exhibit high solution viscosity. These are used as thickening agents in foods at low concentrations. While these fiber sources are typically necessary to make food-based delivery systems functional, they are limited, due to their high water binding capacity, in their ability to be used at high levels that may provide significant benefit as a fermentable food source for colon microorganisms. However, in general, highly soluble fibers, those that are highly branched or are relatively short chain polymers, such as gum arabic, isolated arabinogalactans, inulins, and oligosaccharides have low viscosities. These low viscosity fibers are generally used to modify texture or rheology, manage water migration, influence the colligative properties of the food system, and improve the marketability of the food product as a health-promoting or functional food product. These fiber sources can be used in food products at relatively high levels, as they typically enhance the food product’s taste, mouthfeel, and shelf life without significantly altering the specific application characteristics. For example, sugar-free and fat-free products also have potential for high fiber claims and marketed as supplements. Gelation is an important attribute of some fiber ingredients as a means to add form or structure to various food products. Gelation represents the association of polymer units to form a network of junction zones. The gel formed by this process encapsulates water and other components in solution to form a firm 3-dimensional structure. Gel formation depends on the type of gum, its concentration, temperature, presence of ions (for example, calcium), pH, and the presence of other rheology modifiers in the food system. Gums, including guar gum, gum arabic, karaya, carrageenan, and tragacanth gum, as molecular gel forming polymers, are typically used as rheology modifiers or stabilizers
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in food systems. Particle gel forming polymers, such as starch and inulin, are typically used at much higher concentrations than the molecular gel forming gums, and are used with the gums in systems to influence system rheology and overall texture. Fiber ingredients interact with water differently, dictating how the fiber is used and how it functions in a food system. This interaction is generally described as water uptake, hydration, adsorption, absorption, binding, or holding, with the 2 most common being water-binding capacity (WBC) and water-holding capacity (WHC). While being used interchangeably, the terms are differentiated based on the ability of a fiber to retain water under stress. WHC refers to the amount of water the gel system retains within its structure without pressure or stress, while WBC refers to the amount of water the gel system retains after it is stressed, as following centrifugation. The WBC likely has greater practicality, because food manufacturing/processing typically uses some form of physical stress (for example, extrusion, mixing or kneading, homogenization, and so on). The fiber source does not directly influence the WBC, but rather the source determines the physicochemical properties of the fiber ingredient, such as fiber length, particle size, and porosity. These properties in turn influence the WBC and its use and the conditions of its use in food development. However, other factors in the food system can also influence WBC, such as pH, ionic strength, concentration of the fiber component, and interaction with other waterbinding ingredients (that is, sugar, starches, and so on). By contrast, water interactions with soluble fibers are more greatly influenced by pH and ionic strength. Many dietary fibers are fat and/or oil dispersible, and some also bind oil. Oil binding is in part related to its chemical composition, but is more largely a function of the porosity of the fiber structure rather than the affinity of the fiber molecule for oil. By hydrating a fiber with water, the water occupies the fiber pores, significantly reducing oilbinding. This technique is used successfully with batters and film coatings to reduce the oil uptake during frying operations, and reduces the total fat content of the final food product, enhancing crispiness. Some dietary fibers, fruit and vegetable fiber that have cation exchange capacity (CEC) from unmethylated galacturonic acid residues and phytic acid from cereal fiber, are also able to bind cations such as calcium, cadmium, zinc, and copper. The CEC is influenced by the type of fiber (its makeup), the system pH and ionic strength, and the chemical nature of the cation. Cho et al (1997) reported that some dietary fibers have also been shown to absorb organic molecules (for example, lignin
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binds bile acids, and wheat bran binds certain carcinogens like benzopyrazine). However the effect is pH dependent. These primary functional properties of several isolated fiber sources provide the means to make high-fiber foods with high eating quality, The main technological functions in food of isolated fiber components, such as pectin and guar gum, are as gelling and thickening agents. However, other food (fiber) ingredients are also available to modify or stabilize the texture of food product (for example alginates, carrageenans, cellulose and its derivatives, modified starch). Due to their relatively low level of use in prepared food systems, these polysaccharides are used mainly for their technological properties rather than their physiological significance. Yet they fit the general definition of dietary fiber and are considered as dietary fibers according to their physiology. In contrary to the fiber polysaccharides, NDO, and, more specifically, inulins, are used in food products because of their physiological functionality as prebiotic fiber as well as their technological characteristics. These dietary components function particularly well when used in sugar and fat replacer systems, as having synergy with high-water binding thickening agents to add texturizer, provide bulk, and enhance rheology in food product development. These combined properties provide a means to enhance health promoting, high-fiber food product development, without compromising taste. Worldwide Fiber Recommendations And Intake Most data in literature regarding intake of total dietary fiber (TDF) are dependent on the method(s) used to define their dietary content and are estimates using standard food tables. These data are derived from analysis of foods using official methods for fiber labeling, such as AOAC method 985.29. Typically, there has been little indication of the individual components making up this TDF value. More recent studies have used various analytical methods to determine intakes of various individual segments of the dietary fiber, such as the insoluble and soluble nonstarch polysaccharides. While specific official analytical methods are now available for meeting the requirements for measuring these dietary fiber components, only limited data are available. The National Cancer Institute (NCI) recommended the adult fiber consumption should be increased to 20 to 30 g daily, not to exceed 35 g due to research data suggesting fiber-containing foods provide some protection against colon and rectal cancer (Butrum et al., 1988). It was suggested that if these guidelines were followed, they may help reduce risk of these
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cancers. Reviewing recommendations for healthy populations from other agencies and countries suggest that fiber intakes should be increased, but the recommendations are somewhat unclear as to the amounts and types of fiber being recommended. Beneficial Claims For DF (1) A grain product, fruit, or vegetable that contains dietary fiber; low fat and good source of dietary fiber (without fortification) may be beneficial in preventing some types of cancer. (2) A grain product, fruit, or vegetable that contains dietary fiber: low saturated fat, low cholesterol, and low fat, particularly soluble fiber (0.6 g per Reference Amount [RA] without fortification), may reduce the risk of coronary heart disease. (3) A fruit or vegetable, low in fat with good source of vitamin A, vitamin C or dietary fiber (without fortification) may reduce the risks of some types of cancer. Soluble fiber must be labeled. (4) Soluble fiber from: (1) β-glucan soluble fiber from oat bran, rolled oats (oatmeal) and whole oat flour; and (2) psyllium husks may reduce the risk of heart disease if they are low in fat, saturated fat, cholesterol and include 0.75 g of whole oat soluble fiber or 1.7 g of psyllium husk soluble fiber per RA. Soluble fiber must be labeled. (5) Diets rich in whole grain foods and other plant foods and low in total fat, saturated fat, and cholesterol may reduce the risk of heart disease and some cancers. The food must contain 51% or more whole grain ingredients by weight per serving, and a dietary fiber content of at least 3.0 per RA of 55 g, 2.8 g per RA of 50 g, 2.5 g per RA of 45 g, 1.7 g per RA of 35 g, and be low in fat. OATS Oats (Avena sativa) is a whole kernel cereal consumed as groats milled to yield various products – rolled oats, oatmeal, oat flour. FDA has allowed a health claim for an association between consumption of diets high in oat meal, oat bran, or oat flour and reduced risk of coronary heart disease. This is the first health claim for a specific food under Nutrition Labeling and Education Act (NLEA). Oats based functional foods, oats yogurt, and a symbiotic oats based beverage have been developed in the author’s laboratory. Oat Bran Oat bran is the food which is produced by grinding clean oat groats or rolled oats and separating the resulting oat flour by sieving,
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bolting and/or other suitable means into fractions so that the oat bran fraction is not more than 50% of the starting material, and has total βglucan content of at least 5.5% (dry weight basis) and a total dietary fiber content of at least 16% (dry weight basis), and so that one third of the total dietary fiber is soluble fiber (AACC). Oat bran concentrate Natureal GI from GTC Nutrition, Golden, Colo., is high in β-glucan and said to slow the uptake and release of energy from a meal and exert positive control over healthy blood glucose levels and subsequent inulin response. Tappy et al (1996) found that 5 g of βglucan reduced glycemic response by 50% in a 35 g carbohydrate meal. The soluble fiber in Natureal oat bran concentrate has been associated with benefits for blood sugar control and satiety, and with weight management. The effectiveness of ingredient for control of glycemic response is attributed to the viscous nature of β-glucan. Viscosity is critical to slowing stomach emptying and regulating the uptake of energy from a meal. This effect is important to controlling glycemic response and blunting the after-meal insulin surge, which is thought to have positive effects for healthy weight maintenance. β -glucan β-glucan is distributed throughout the endosperm and located in the endosperm cell walls and constitutes 75%. It is a linear, unbranched polysaccharide composed of 4-O-linked β-D-glucopyranosyl units (70%) and 3-O-linked β-D-glucopyranosyl units (30%), MW=1.5-3.0 X 106. There is 3-11% in barley, 3-7% in oats and <1% in wheat. β -glucan Isolate There is no pure form of oat β-glucan. A new product high in β-glucan called Oatrim by Quaker Oats is used as soluble fiber or a fat replacer. Oatrim is prepared by extraction of oats or oat bran with hot water containing heat soluble -amylase. Oats Based Functional Foods Dietary oats have been shown to confer a number of significant physiological effects in the prevention or alleviation of disease and thus may be considered as a multifunctional food. Soluble fiber favors the growth of probiotic bacteria. Oats contain a high percentage of desirable complex carbohydrates that may reduce the risk of certain cancers and constipation and also promotes a good balance of fatty acids. The key cholesterol lowering ingredient in oats is soluble fiber. It works by binding cholesterol-containing bile acids produced in the liver and speeding their exit from the body. It also helps control diabetes by
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preventing erratic swings in blood sugar levels. Soluble fiber slows down the absorption of sugar from the intestines into the blood. It also increases the cell’s insulin sensitivity and assists the cells in drawing sugar from the blood. Due to these benefits, there is a great interest in increasing the consumption of products based on oats that contain both soluble and insoluble fibers. However, consumption of oat-based products is low mainly due to the lack of acceptable food products based on oats. So oats based symbiotic beverage and yogurt were developed in the author’s laboratory with the idea of preparing products having two healthy components, dietary fiber of oats and probiotic lactic acid bacteria wherein the oats is fermented by probiotic bacteria itself. Symbiotic Oats Beverage Oats flour (3-7%, w/v), sugar (4-8%, w/v), inulin (0.2%, w/v) and whey protein concentrate (0.5%, w/v) were blended in water to make a homogenous slurry. The slurry was slowly heated at the rate of 1°C per min. to boiling and boiled for 3 min. to break down the starch content of oats. The cooked slurry was sterilized at 121°C for 15 min. The slurry was then cooled to 37°C stirring at intervals to avoid formation of a layer on the surface. This was inoculated with probiotic culture and mixed well so that the culture was distributed evenly in the slurry. Fermentation was carried out at 37°C in aerobic environment. Oats flour was the main ingredient in the product. We chose to use 5% because less than that would not provide the required amount of dietary fiber, and it was required to add more of inulin to meet the dietary fiber requirement. But if more was used the product would be more viscous, and it would not be a beverage. 0.2% inulin was used to increase the soluble dietary fiber content to meet the requirement (0.75 g/serving). Inulin acts as a water binder, stabilizer and texturizer in addition to being a prebiotic. Whey protein concentrate was used at the level of 0.5%. This was just enough to make a homogenous product, and more than this would coagulate the slurry when it was autoclaved. Whey protein concentrate acts as a stabilizer that makes the product homogenous. Sugar (4%) was added as energy source for the cultures and to give a balanced taste of sweet and sour to the product. All the ingredients and water were blended in a blender to make a homogenous slurry. The slurry was cooked and autoclaved. The beverage was homogenous, free flowing and had smooth texture compared to the unfermented slurry. The pH, titratable acidity and viscosity of the control sample (without fermentation) were 6.3, 0.032% and 420 mPas, respectively. After fermentation for 12 h, there was a
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significant change in pH which reduced to 3.63, titratable acidity which increased to 0.21% and viscosity that decreased to 222 mPas. The product had a good balance of sweet and sour tastes. The beverage provides 0.80 g of soluble dietary fiber per serving (150 mL) which meets the FDA requirements (0.75 g per serving). Since this product contains only plant origin raw materials and whey protein concentrate, it can be considered as low in saturated fat and cholesterol free. It is also suitable for enrichment with traditional flavors. This oats based beverage is typically a non-dairy vegetarian product containing no milk. It serves as an alternative to both dairy and soy beverages and suits a healthy life-style whether vegetarian-oriented or not. Symbiotic Oats Yogurt Symbiotic oats yogurt was formulated as a multifunctional product that delivered dietary fiber and viable probiotic lactic acid bacteria, in which the bacteria fermented the oats, and prepolymerized whey proteins were utilized to form a gel. Preparation of symbiotic oats yogurt is presented in Figure 3.7. The oats yogurt provides 1.68 g of soluble dietary fiber per serving (240ml) which meets the FDA requirement (0.75 g per serving) for labeling. Since this product contains only raw materials from plant origin and whey protein isolate, it can be considered low in saturated fat and cholesterol free. Other oats-based functional foods are being developed in the author’s laboratory. SUMMARY Dietary fiber, an essential part of a healthy diet, provides many health benefits which have been researched to a large extent. This chapter deals in detail about its role in human health and also its functionality in foods as a functional ingredient and/or food. Dietary fiber also known as roughage or bulk includes all parts of plant foods that our body can’t digest or absorb. The term ‘Dietary fiber’ was coined by Hipsley in 1953 and is defined as the edible parts of the plant and analogous carbohydrate that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine. It includes polysaccharides, lignin and associated plant substances. Dietary fiber exhibits one or more of laxation (fecal bulking and softening; increased frequency; and/or regularity), blood cholesterol attenuation, and/or blood sugar regulation.
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FIGURE 3.7 — Preparation Of Symbiotic Oats Yogurt
Dietary fiber is classified into different types based on solubility, fermentability and the arrangement of monomeric units. There are also different methods of analysis of dietary fiber which can be modified based on which fiber type we want to analyse. The physiological benefit of various dietary fiber types depends on how they are metabolized in the body which in turn depends on their physicochemical characteristics. Dietary fiber has been found to have
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an important role in preventing chronic diseases such as cancer, cardiovascular disorders, diabetes, gastrointestinal disorders and also in improving mineral bioavailability. The primary properties provided by isolated fibers for food product development are solubility, viscosity, and gelation-forming ability, waterbinding capacity, oil-binding capacity, and mineral and organic moleculebinding capacity. Oats have been popularly known as a functional food. The first health claim that FDA has allowed for a specific food under The Nutrition Labeling and Education Act (NLEA) is a health claim for an association between consumption of diets high in oat meal, oat bran, or oat flour and reduced risk of coronary heart disease. The main component that makes oats a functional food is β-glucan, a soluble dietary fiber. Focusing on the physiological health aspect of dietary fiber and the types of fibers meeting a physiological definition will help to make the consumer understand the importance of fiber in their diets. References Alles, M. S., Hartemink, R., Meyboom, S., Harry, J. L., van Laere, K. M. J., van Nagengast, F. M. and Hautvast, J. C. A. J. 1999. Effect of transgalactooligosaccharides on the composition of the human intestinal microflora and on putative risk markers for colon cancer. Am. J. Clin. Nutr. 69:980-991. Anderson, J. W., Story, L., Sieling, B., Chen, W. J. L., Petro, M. S. and Story, I. 1984. Hypocholesterolemic effects of oat-bran or bean intake for hypercholesterolemic men. Am. J. Clin. Nutr. 40:1146-1155. Anderson, J. W., Allgood, L. D., Lawrence, A., Altringer, L. A., Lerdack, G. R., Hengehold, D. A. and Morel, J. G. 2000. Cholesterol-lowering effects of psyllium intake adjunctive to diet therapy in men and women with hypercholesterolemia: meta-analysis of 8 controlled trials. Am. J. Clin. Nutr. 71:472-479. Baghurst, P. A., Baghurst, K. I. and Record, S. J. 1996. Dietary fiber, nonstarch polysaccharides and resistant starch: a review. Food Australia 48(3):1-36S. Birkett, A., Muir, J., Phillips, J., Jones, G. and O’Dea, K. 1996. Resistant starch lowers fecal concentrations of ammonia and phenols in humans. Am. J. Clin. Nutr. 63:766772. Boeckner, L. S., Schnepf, M. I. and Tungland, B. C. 2000. Inulin: A review of nutritional and health implications. Adv. Food Nutr. Res. 43:1-63. Bourquin, L. D., Titgemeyer, E. C., Garleb, K. A. and Fahey, G. C. 1996. Fermentation of various dietary fiber sources by human fecal bacteria. Nutr. Res. 16:1119-1131. Brown, L., Rosner, B., Willett, W. W. and Sacks, F. M. 1999. Cholesterol-lowering effects of dietary fiber: a meta-analysis. Am. J. Clin. Nutr. 69:30-42. Butrum, R. R., Clifford, C. K. and Lanza, E. 1988. Dietary guidelines: Rationale. Am. J. Clin. Nutr. 48:888-895. Byers, T. 2000. Diet, colorectal adenomas, and colorectal cancer. N. Engl. J. Med. 342(16):1206-1207. Causey, J. L., Feirtag, J. M., Gallaher, D. D., Tungland, B. C. and Slavin, J. L. 2000. Effects of dietary inulin on serum lipids, blood glucose and the gastrointestinal environment in hypercholesterolemic men. Nutr. Res. 20(21):191-201.
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Chandalia, M., Garg, A., Lutjohann, D., von Bergmann, K., Grundy, S. M., Brinkley, L. J. 2000. Beneficial effects of high dietary fiber intake in patients with type 2 diabetes mellitus. N. Engl. J. Med. 342:1392-1398. Cho, S., DeVries, J. W., Prosky, L. 1997. Dietary fiber analysis and applications. Maryland: AOAC International. Demigne, C., Morand, C., Levrat, A. M., Besson, C., Moundras, C. and Remesy, C. 1995. Effects of propionate on fatty acid and cholesterol synthesis and on acetate metabolism in isolated rat hepatocytes. Br. J. Nutr. 74:209-219. Dobbing, J. 1989. Dietary Starches and Sugars in Man: A Comparison. London: SpringerVedag. 256 pp. Englyst, H. N., Kingman, S. M. and Cummings, J. H. 1992. Classification and measurement of nutritionally important starch fractions. Eur. J. Clin. Nutr. 46(Sup. 2):S33-S50. Folino, M., McIntyre, A. and Young, C. P. 1995. Dietary fibers differ in their effects on large bowel epithelial proliferation and fecal fermentation-dependent events in rats. J. Nutr. 125:1521-1528. Gibson, G. R. and Roberfroid, M. R. 1995. Dietary modulation of the human colonic microbiota: Introducing the concept of prebiotics. Nutr. 125(61):1401-1412. Green, C. J., Van Hoeij, K. A. and Bindels, J. G. 1998. Short chain fatty acid (SCFA) and gas production of individual fiber sources and a mix typical to a normal diet using an in vitro technique. J. Pediatr. Gastroenterol. Nutr. 26:591. Heijnen, M. L. A., Amelsvoort, J. M. M. van, Deurenberg, P. and Beynen, A. C. 1998. Limited effects of consumption of uncooked (RS2) and retrograded (RS3) resistant starch on putative risk factors for colon cancer in healthy man. Am. J. Clin. Nutr. 67:322-331. Jones, J. M. 2000. Dietary advice in North America: the good, the bad and the unheeded. In: McCleary BV, Prosky L, editors. Proceedings of the 1st International Conference On Dietary Fiber; Dublin, Ireland; May. Oxford, U.K.: Blackwell Science. P 30. Kok, N. N., Morgan, L. M., Williams, C. M., Roberfroid, M. B., Thissen, J. P. and Delzenne, N. M. 1998. Insulin, glucagon-like peptide-1, glucose-dependent insulinotropic polypeptide and insulin-like growth factor I as putative mediators of the hypolipidemic effect of oligofructose in rats. J. Nutr. 128:1099-1103. Lipman, T. O. 1995. Bacterial translocation and enteral nutrition in humans: an outsider looks in. J. Parenter. Enteral Nutr. 19:156-165. Lopez, H. W., Coudray, C., Ballanger, J., Younes, H., Demigne, C. and Remesy, C. 1998. Intestinal fermentation lessens the inhibitory effects of phytic acid on mineral utilization in rats. J. Nutr. 128:1192-1198. Luo, J., Rizkalla, S. W., Alamowitch, C., Boussairi, A., Blayo, A., Barry, J. L. Laffitte, A., Cuyon, F., Bornet, F. R. J. and Slama, G. 1996. Chronic consumption of shortchain fructooligosaccharides by healthy subjects decreased basal hepatic glucose production but had no effect on insulin-stimulated glucose metabolism. Am. J. Clin. Nutr. 63:939-945. Lynn, M. E., Mathers, J. C. and Parker, D. S. 1994. Increasing luminal viscosity stimulates crypt cell proliferation throughout the gut. Proc. Nutr. Soc. 53:227A. Meyer, P. D., Tungland, B. C., Causey, J. L. and Slavin, J. L. 2000. The immune effects of inulin in vitro and in vivo. Agro Food Ind Hi-Tech Nov./Dec.:18-20. Michel, C., Kravtchenko, T. P., David, A., Gueneau, S., Kozlowski, F. and Cherbut, C. 1998. In vitro prebiotic effects of Acacia gums onto the human intestinal microbiota depends on both botanical origin and environmental pH. Anaerobe 4:257-266. Nordgaard, I. and Mortensen, P. B. 1995. Digestive processes in the human colon. Nutr. 11:37-45.
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Ohta, A., Motohashi, Y. and Sakuma K. 1998. Dietary fructooligosaccharides change the concentration of calbindin-D9k differently in the mucosa of the small and large intestine of rats. J. Nutr. 128:934-939. Okubo, T., Ishihara, N., Takahashi, H., Fujisawa, T., Kim, M., Yamamoto, T. and Misuoka T. 1994. Effects of partially hydrolyzed guar gum intake in human intestinal microflora and its metabolism. Biosci. Biotechnol. Biochem. 58:1364-1369. Sakamoto, J., Nakaji, S., Sugawara, K., Iwane, S. and Munakata, A. 1996. Comparison of resistant starch with cellulose on 1,2-dimethylhydrazine-induced colon carcinogenesis in rats. Gastroenterol. 110:116-120. Satchithanandam, S., Vargofcak-Apker, M., Calvert, R. J., Leeds, A. R. and Cassidy, M. M. 1990. Alteration of gastrointestinal mucin by fiber feeding in rats. J. Nutr. 120:1179-1184. Scheppach, W. 1998. Butyrate and the epithelium of the large intestine. In: Guillon, F., Amado, R., Amaral-Collaco, M. T., Andersson, H., Asp, N. G., Bach Knudsen, K. E., Champ, M., Mathers, J., Robertson, J. A., Rowland, I., Van Loo, J. editors. Proceedings of the Profiber Conference: Functional Properties of Nondigestible Carbohydrates. Lisbon, Portugal; Feb. P 215. Silk, D. B. A., Walters, E. R., Duncan, H. D., Green, C. J. 2001. The effect of a polymeric enteral formula supplemented with a mixture of six fibres on normal human bowel function and colonic motility. Clin. Nutr. 20:49-58. Smith, T., Brown, J. C. and Livesey, G. 1998. Energy balance and thermogenesis in rats consuming nonstarch polysaccharides of various fermentabilities. Am. J. Clin. Nutr. 68:802-819. Strohmaier, W. 1996. Lactulose, an innovative food ingredient - physiological aspects. In: Proceedings of FIE Conference; Paris; 12-14 Nov. 1996. Maarssen, Netherlands: Miller Freeman. P 69-72. Tappy, L., Gugolz, E. and Wursch, P. 1994. Effects of breakfast cereals containing various amounts of beta-glucan fibers on plasma glucose and insulin responses in NIDDM subjects. Diabetes Care. 19:831-834. Tungland, B. C. and Meyer, D. 2002. Nondigestible Oligo- and Polysaccharides (Dietary Fiber): Their Physiology and Role in Human Health and Food. Comprehensive Reviews in Food Science and Food Safety. 3:73-92. Vanhoof, K. and De Schrijver, R. 1996. Nitrogen metabolism in rats and pigs fed inulin. Nutr. Res. 16:1035-1039. Wasan, H. S. and Goodlad, R. A. 1996. Fiber-supplemented foods may damage your health. Lancet 348:319-320. Younes, H., Garleb, K., Behr, S., Remsey, C. and Demigne, C. 1995. Fermentable fibers or oligosaccharides reduce urinary nitrogen excretion by increasing urea disposal in the rat caecum. J. Nutr. 125:1010-1016. Younes, H., Demigne, C. and Remesy, C. 1996. Acidic fermentation in the caecum increases absorption of calcium and magnesium in the large intestine of the rat. Br. J. Nutr. 75:301-314. (Guo, M.R., Gokavi, S.)
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Why is fiber important? What does fiber do? This chapter will answer these questions in detail. It is the purpose of this chapter to provide an overview of important oligosaccharides and polysaccharides that function as DF, to explain in detail their occurrence and structures and their various physiological effects and health implications, and also to describe the role high fiber ingredients play in food development. Definition Establishing a definition for dietary fiber has a long history. The term ‘dietary fiber’ was coined by Hipsley in 1953 and since then its definition has undergone several revisions. The history of the definition of DF is presented in Figure 3.1. While defining dietary fiber, it was intended to balance between nutritional knowledge and analytical method capabilities. While the physiologically based definitions most widely accepted have generally been accurate in defining the dietary fiber in foods, scientists and regulators have tended, in fact, to rely on analytical procedures as the definitional basis. As a result, incompatibility between theory and practice has resulted in confusion regarding the components that make up dietary fiber. In November 1998, the president of American Association of Cereal Chemists (AACC) International appointed a scientific review committee and assigned the task of reviewing, and if necessary, updating the definition of dietary fiber. The updated definition includes the same food components as the historical working definition used for almost 30 years. But the updated definition more clearly describes the makeup of DF and its physiological functionality. This definition typically includes the fiber components; nonstarch polysaccharides (NSP) and resistant oligosaccharides (RO), lignin, substances associated with the NSP and lignin complex in plants, and other analogous carbohydrates, such as resistant starch (RS) and dextrins, and synthesized carbohydrate compounds, like polydextrose (Tungland and Meyer, 2002). Finally, dietary fiber is defined as the edible parts of the plant and analogous carbohydrate that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine. It includes polysaccharides, lignin and associated plant substances. Dietary fiber exhibits one or more of the following: laxation (fecal bulking and softening; increased frequency; and/or regularity), blood cholesterol attenuation, and/or blood sugar regulation.
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FIGURE 3.1 — History Of Definition Of Dietary Fiber Over The Years Organization
Year
Definition
Hipsley
1953
Coined term “dietary fiber” as a shorthand term for nondigestable constituents making up the plant cell wall.
Trowell and others
1972-1976
Used Hipsley term in conjunction with a dietary fiber hypothesis related to health observations. The term was defined as: “consisting of the plant polysaccharides and lignin which are resistant to hydrolysis by digestive enzymes of man.”
Asp, Schweizer, Furda, Theander, Bakker, Soutgate and others
1976-1981
Developed methods directed at quantifying food components meeting definition
Prosky
1979
Began process of developing worldwide consensus on fiber definition and methodology for dietary fiber
Canadian Association of Official Analytical Chemists Workshop
1981
Consensus on fiber definition and analytical approach
Prosky, Asp, Furda, Schweizer, DeVries and Harland
1981 -1985
Validate consensus methodology in multinational collaborative studies
AOAC
1985
Official Method of Analysis 985.29, Total dietary Fiber in Foods-Enzymatic-Gravimetric Method, adopted, becoming de facto working definition for dietary fiber
Health and Welfare Canada
1985
Defined dietary fiber as: “the endogenous components of plant material in the diet which are resistant to digestion by enzymes produced by humans. They are predominately nonstarch polysaccharides and lignin and may include, in addition, associated substances.
Scientific community
1985-1988
Developed methodology and collaboratively studied these for various types of fiber.
US-FDA
1987
Defined dietary fiber as the material isolated by AOAC method 985.29
Life Sciences Research Office (LSRO)
1987
Defined dietary fiber as: the endogenous components of plant materials in the diet that are resistant to digestion by enzymes produced by humans
Health Canada
1988
Defined (dietary fiber) as: being the endogenous components of plant material in the diet which are resistant to digestion by enzymes produced by man: they are predominately nonstarch polysaccharides and lignin. The composition varies with the origin of the fiber, and includes soluble and insoluble substances. Defined (novel fiber or novel source) as: (1) a food that has been manufactured to be a source of dietary fiber, and has not traditionally been used for human consumption to any significant extent, or (2) had been chemically processed (oxidized), or (3) had been highly concentrated from its plant source.
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FIGURE 3.1 — History Of Definition Of Dietary Fiber Over The Years - Continued Germany
1989
Defined fiber as: substances of plant origin, that cannot be broken down to resorbable components by the body’s own enzymes in the small intestine. Included are essentially soluble and insoluble nonstarch polysaccharides (cellulose, pectin, hydrocolloids) and lignin and resistant starch. Substances like some sugar substitutes, organic acids, chitin and so on, which either are not or are incompletely absorbed in the small intestine, are not included.
Lee, Mongeau, Li, Theander and others
1988-1994
Various fiber methodologies fitting definition of dietary fiber developed, validated and brought to an Official Method status
Japan
1990
Dietary fiber defined as: material isolated by a modified method of AOAC 985.29
AOAC
1991
Official Method of Analysis 991.42, Insoluble Dietary Fiber in Foods and Food Products, Enzymatic-Gravimetric Method-Phosphate Buffer, adopted.
International Fiber Survey
1992
Reaffirms consensus on physiological dietary fiber definition.
Belgium
1992
Defined dietary fiber as: the components of food that are not normally broken down by the body’s own enzymes of humans
International Fiber fiber Survey
1993
Reaffirmed consensus on physiological dietary
Italy
1993
Defined dietary fiber as: the edible substance of vegetable origin which normally is not hydrolyzed by enzymes secreted by the human digestive system
AOAC International
1995
Workshop on definition of complex carbohydrates and dietary fiber reaffirms consensus on physiological dietary fiber definition and inclusion components
FAO/WHO
1995
(Codex Alimertarius Commission) Defined dietary fiber as: the edible plant or animal material not hydrolyzed by the endogenous enzymes of the human digestive tract as determined by the agreed upon method. Approved AOAC methods 985.29 & 991.43.
China
1995
Defined dietary fiber as: the sum of food components that are not digested by intestinal enzymes and absorbed into the body
Denmark
1995
Defined dietary fiber as: the material isolated by AOAC methods 985.29 and 997.08 (fructan method)
Committee on Medical Aspects (UK)
1998
Defined dietary fiber as: nonstarch polysaccharide as measured by the Englyst method of Foods [Committee on Medical Aspects of Food and Nutrition Policy (COMA)]
Finland
1998
Defined dietary fiber as: part of the carbohydrate obtained using AOAC Methods 985.29 and AOAC 997.08.
definition and reaffirms inclusive components
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FIGURE 3.1 — History Of Definition Of Dietary Fiber Over The Years - Continued Norway
1998
Defined dietary fiber as: material isolated by AOAC Method 985.29 and inulin and oligofructose
AACC
1998
Assigns Scientific Committee to review and develop definition of Dietary Fiber
Sweden
1999
Defined dietary fiber as: edible material that cannot be broken down by human endogenous enzymes and determined with AOAC Methods 985.29 and/or 997.08 (fructan method)
Food Standards Agency (U.K.)
1999
Defined dietary fiber as: material isolated by AOAC methods 985.29 and 997.08 (fructan method)
AACC
2000
Defined dietary fiber as: the edible parts of plants or analogous carbohydrates that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine. Dietary fiber includes polysaccharides, oligosaccharides, lignin, and associated plant substances. Dietary fibers promote beneficial physiological effects including laxation, and/or blood cholesterol attenuation, and/or blood glucose attenuation.
Australia New Zealand Food Authority (ANZFA)
2001
Following the lines of the AACC definition, defined dietary fiber as: that fraction of the edible part of plants or their extracts, or analogous carbohydrates, that are resistant to digestion and absorption in the human small intestine, usually with complete or partial fermentation in the large intestine. The term includes polysaccharides, oligosaccharides (DP > 2), and lignins. Dietary fiber promotes one or more of these beneficial physiological effects: laxation, reduction in blood cholesterol, and/or modulation of blood glucose. They accepted by use of AOAC methods 985.29 and 997.08 (fructan method) for labeling.
National Academy of Science (NAS)
2002
2002 Panel on the Definition of Dietary Fiber defined the dietary fiber complex to include dietary fiber consisting of nondigestible carbohydrates and lignin that are intrinsic and intact in plants, functional fiber consisting of isolated, nondigestible carbohydrates which have beneficial physiological effects in humans, and total fiber as the sum of dietary fiber and functional fiber.
Chemistry Of Dietary Fiber The physical properties of dietary fiber are predominated by the shape (conformation) of the individual chains, and the way in which they interact with one another. Each dietary fiber molecule typically contains several thousand monosaccharide units which are often arranged in a linear sequence, like a very long string of beads, although more complex branched arrangements also occur. In contrast to globular
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proteins, polysaccharides normally have structures based on regular repeating sequences. The simplest arrangement is where all the monosaccharides are the same, and are linked together in the same way along the chain. Disaccharide repeating consequences (-A-B-A-B-) are also common, and larger repeating units (up to octasaccharide) can occur, particularly in polysaccharides produced by bacteria. The constituent monosaccharides have a ring structure, which can be either five-membered or six-membered, and are linked together by ‘glycosidic bonds’ with a shared oxygen atom between adjacent sugars. The polysaccharides of greatest practical importance, both as commercial hydrocolloids and as constituents of dietary fiber, are built up from six membered (pyranose) rings consisting of one oxygen atom and five carbon atoms, which are numbered sequentially from the ring oxygen as C-1 to C-5, and with a sixth carbon atom, numbered as C-6, lying outside the ring. As a consequence of the tetrahedral bonding arrangement of carbon, and the requirement to avoid steric clashes between adjacent groups, the pyranose ring is locked in a fixed, chairlike geometry, and the overall shape of the polysaccharide molecule is dictated by the torsional angles characterizing the relative orientation of neighboring sugars. These angles may be either fixed at the same values for equivalent linkages along the polymer chain, giving regular, ordered chain geometry, or constantly fluctuating, to give the disordered ‘random coil’ geometry typical of polysaccharide solutions. The chemical structures of different dietary fibers are given in Figures 3.2 and 3.3. Physical Properties Of Dietary Fiber When considering the action of cooking on cell wall structure and comparing cooked and raw plant foods, the different solubility characteristics of cell wall polysaccharides should be considered. Cell wall structures are degradable to varying degrees, depending on the structure and the conditions used. An important function of insoluble fibers is to increase lumenal viscosity in the intestine. It is not yet clear whether the soluble fibers in food have the same effect. Other polymeric components of the diet (proteins, gelatinized starch) and mucus glycoproteins liberated from the epithelia contribute to viscosity. Particulate materials present in chyme, such as insoluble fiber or hydrated plant tissues, also contribute to a lesser extent to overall viscosity. Digesta viscosity is highly sensitive to changes in ionic concentration that are due to intestinal secretion or absorption of aqueous fluids. Raw apples undergo little damage of cells upon ingestion
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FIGURE 3.2 — Chemical Structures Of Starch And Other Polysaccharides
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FIGURE 3.3 — Chemical Structures Of Polyfructans
and mastication. Gastric hydrochloric acid only solubilizes a small proportion of the pectin. Cooking the apples results in cell damage, and hence significant proportions of the middle lamellae pectic polysaccharides are solubilized. These make the digesta more viscous. Vegetables undergo structural change during cooking and mastication, e.g., cellular disintegration. The cells in the intact carrot are each bounded by an intact cell wall; after cooking most, if not all, the cell walls have been ruptured and the cell contents lost. The grinding of
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foods before cooking and ingestion may also have pronounced effects on fiber action. Cell walls may be disrupted, and the reduced particle size of some fiber preparations such as wheat bran may be less biologically effective. The effects of other cooking processes, e.g., Maillard reactions, are not known. Controlled drying of a heated starch gel can produce any of the different X-ray diffraction patterns, depending on the temperature. On cooling, gelatinized starchy foods will retrograde. During retrogradation, solubility of the starch molecule decreases and so does its susceptibility to hydrolysis by acid and enzymes. Chain length and linearity are important factors affecting retrogradation. The longer the starch chains, the greater the number of interchain hydrogen bonds formed (Dobbing, 1989). Classification Of Dietary Fiber Several different classification systems have been used to classify the components of dietary fiber: based on their role in the plant, based on the type of polysaccharide, based on their simulated gastrointestinal solubility, based on the site of digestion, and based on products of digestion and physiological classification. However, none is entirely satisfactory, as the limits can not be absolutely defined. The most widely used classification for dietary fiber has been to differentiate dietary components on their solubility in a buffer at a defined pH, and/or their fermentability in an in vitro system using an aqueous enzyme solution representative of human alimentary enzymes. However, there is still debate regarding the most appropriate means to classify dietary fiber. Since most fiber types are at least partially fermented, it is suggested that it may be most appropriate to refer to them as partially or poorly fermented and well fermented. Classification Based On Solubility Based on solubility, dietary fiber is classified into two types – soluble and insoluble. Soluble fiber dissolves in water. This includes gums, mucilages, pectin and some hemicelluloses. These fibers are found in all types of peas and beans like lentils, split peas, pinto beans, black beans, kidney beans, garbanzo beans, and lima beans, as well as oats, barley, and some fruits and vegetables like apples, oranges, and carrots. Fiber from psyllium seed is also in this group. For people with diabetes, eating foods that contain soluble fiber can help control or lower the level of sugar in their blood and decrease insulin needs; and, studies have shown that including one or two servings of beans, oats, psyllium, or other sources of soluble fiber help
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FUNCTIONAL FOODS
lower fasting blood sugar levels. It may also help lower blood cholesterol levels, especially LDL-cholesterol or the “bad” cholesterol. Fiber decreases blood cholesterol by binding to bile acids, which are made of cholesterol, in the gastrointestinal tract and carrying them out of the body as waste. Researchers have found that soluble fibers in beans, psyllium fiber, oats, and oat bran help lower blood cholesterol levels in many groups of people. Insoluble fiber does not dissolve in water. Cellulose, lignin, and the rest of the hemicelluloses, are all insoluble fibers. These fibers provide structure to plants. Whole grains, wheat and corn fiber, and many vegetables like cauliflower, green beans, and whole potatoes are good sources of insoluble fiber. The skins of fruits and vegetables are also good sources of insoluble fiber. And, wheat bran is a good source of insoluble fiber, which is why it is added to many dry breakfast cereals. Insoluble fiber, also known as “roughage”, aids digestion by trapping water in the colon. The water that is trapped by insoluble fiber keeps the stool soft and bulky. This promotes regularity and prevents constipation. Wheat bran, for example is high in insoluble fiber, and also helps prevent two kinds of intestinal diseases, diverticulosis and hemorrhoids. Classification Based On Fermentability Fibers that are well fermented include pectin, guar gum, acacia (gum arabic), inulin, polydextrose, and oligosaccharides. The less wellfermented types include cellulose, wheat bran, corn bran, oat hull fiber, and some resistant starches. The fiber types based on fermentability are listed in Figure 3.4. Generally, well fermented fibers are soluble in water, while partially or poorly fermented fibers are insoluble. Classification Based On The Way The Monomeric Units Present Indigestible polysaccharides (fiber components) consist of all nonstarchy polysaccharides (NSP) resistant to digestion in the small intestine and fermentable in the large intestine. These polysaccharides are typically long polymeric carbohydrate chains containing up to several hundred thousand monomeric units. The polysaccharides differ by the number and type of monomeric units linked together, the order in the chain, the types of linkages between the various monomers, the presence of branch points in the backbone of the molecule, and those having acidic groups present (for example, uronic acids in pectins). Examples of these NSP compounds are cellulose with beta-glycosidic bonds, nonglucose sugars (hemicelluloses such as arabinoxylans and arabinogalactans), sugar acids (pectins), gums, and mucilages. Resistant
DIETARY FIBER
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FIGURE 3.4 — Classification Of Fiber Components Based On Fermentability Characteristic
Fiber component
Main food source
Partial or low fermentation
Cellulose
Plants (vegetables, sugar beet, various brans) Cereal grains Woody plants Plant Fibers Fungi, yeasts, invertebrates Plants (corn, potatoes, grains, legumes, bananas) Bacterial fermentation
Hemicellulose Lignin Cutin/suberin/other plant waxes Chitin and chitosan, collagen Resistant starches Curdlan Well fermented
β-glucans Pectins Gums
Inulin Oligosaccharides/analogues
Animal origin
Grains (oat, barley, rye) Fruits, vegetables, legumes, sugar beet, potato Leguminous seed plants (guar, locust bean), seaweed extracts (carrageenan, alginates), plant extracts (gum acacia, gum karaya, gum tragacanth), microbial gums (xanthan, gellan) Chicory, Jerusalem artichoke, onions, wheat Various plants and synthetically produced (polydextrose, resistant maltodextrin, fructooligosaccharides, galactooligosaccharides, lactulose) Chondroitin
oligosaccharides, such as the fructans [inulin and fructooligosaccharides (FOS)] (Figure 3.4) are characterized as carbohydrates with a relatively low degree of polymerization (DP), as compared to the NSP. FOS differ from fructopolysaccharides (inulin) only in chain length. The strict definition of an oligosaccharide is a chain of monomeric units with a DP of 3-10. Lignin is a phenylpropane polymer, and not a carbohydrate that is covalently bound to the fibrous polysaccharides (cellulose) of plant cell walls. Lignin has a heterogeneous composition ranging from 1 or 2 units to many phenyl propanes that are cyclically linked. It is likely these two characteristics have established the basis for it being included as a dietary fiber. Another group of compounds, found in several physiological definitions, the analogous carbohydrate(s), refer to compounds that are
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FUNCTIONAL FOODS
analogous to those of naturally-occurring dietary fibers. These compounds demonstrate the physiological properties of the respective materials for which they are analogous to, but are not obtained by eating the whole or part of the native originating plant, such as fruits, vegetables, grains, legumes, and nuts. They can be produced during food processing by chemical and/or physical processes, or by purposeful synthesis or isolation as a concentrated form from the native plant. These “analogous” carbohydrates can include, but are not limited to, those isolated from Crustacea and single-cell organisms, polydextrose, resistant maltodextrins and starch, and the modified celluloses. Resistant starch (RS) is defined as the sum of starch and starch products of starch degradation that is not broken down by human enzymes in the small intestine of healthy individuals. A classification of these starches based on the origin of their resistance to digestion has been proposed by Englyst et al (1992). Resistant starch is not a homogenous entity, but rather the resistance is dependent on a number of natural or processing phenomena which make up the subcategories RS1, 2, 3, and 4. RS1 relates to resistance conferred due to physical entrapment of starch, as found in partly milled grains or chewed cereals, seeds, or legumes. RS2 includes starch granules that are highly resistant to digestion by alpha-amylase until gelatinized. This form is typically found in raw or uncooked potato, banana (particularly when green), and high amylose maize starch. RS3 relates to the retrograded starch polymers from food processing of the above mentioned sources. RS4 includes chemically modified, commercially produced resistant starches that are likely degraded by amylases to alcohol soluble fractions and are used in many baby food applications. RS may have the similar health benefits as dietary fiber. Also included in the fiber component list are the associated plant substances, such as waxes and cutin. These components are found as waxy layers at the surface of the cell walls, made up of highly hydrophobic, long chain hydroxy aliphatic fatty acids. Suberin, another one of these associated substances, even though not fully characterized, is speculated to be a highly branched, crosslinked molecule containing polyfunctional phenolics, polyfunctional hydroxyacids, and dicarboxylic acids, having ester linkages to the plant cell walls. Analysis Of Dietary Fiber Adoption of the proposed definition for regulatory, research, and nutrition purposes will result in little change of analytical methodology, food labels, or food databases from the current situation. While several
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methods have been developed for analyzing dietary fiber, two primary methods are now used for content labeling: enzymatic gravimetric methods (for example, the AOAC procedure), and enzymatic chemical methods (for example, the Englyst and Southgate procedures). The AOAC procedures primarily measure NSP, lignin, and a portion of RS, as does the Southgate method, while RS and lignin are not measured by the Englyst method. Due to method limitations of these primary methods, other, more specific, methods must be used to measure other components of dietary fiber, such as inulin, FOS, RS, and lignin. Current methodologies will continue to accurately quantitate the amount of fiber in the majority of foods, the exception being those foods containing a significant amount of dietary fiber which is soluble in a solvent mixture of 4 parts alcohol and 1 part water. This exceptionally soluble dietary fiber has heretofore been excluded from the quantity of dietary fiber reported on food labels and entered into database(s) for analytical, as opposed to definitional, reasons. Additional methods, or adjustments to current methods, which assure inclusion of the exceptionally soluble dietary fiber, will increase the reported dietary fiber level of a few foods, particularly foods high in fructans such as onions and leeks. Methods accurately fitting the definition will minimize regulatory confusion and result in accurate nutrition labeling of food products. Method Requirements Adoption of the definition for dietary fiber, i.e. “Dietary fiber is the remnants of the edible part of plants and analogous carbohydrates that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the human large intestine. It includes polysaccharides, oligosaccharides, lignin and associated plant substances. Dietary fibers exhibit one or more of either laxation (fecal bulking and softening; increased frequency; and/or regularity), blood cholesterol attenuation, and/or blood glucose attenuation,” will result in relatively few method changes or changes in food labels or food databases. Analytically inclusive components fitting this definition include cellulose, hemicellulose, lignin, gums, mucilages, oligosaccharides, pectins, waxes, cutin, and suberin. Analytical methodology useful for food labeling needs to effectively quantitate all of these components, while excluding all other food components. The analytical method also must quantitate the dietary fiber using a set of standardized conditions which will convert the food to the state of the food as it is most likely to be consumed. That is, the method should not
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quantitate “resistant starch” as dietary fiber merely because the starch is resistant to digestion because it is ungelatinized as it is found in the food product as labeled and sold, when there is a chance it will be cooked prior to consumption. Thus, a starch gelatinization step is necessary in any method developed for dietary fiber analysis as is a sample digestion step with enzymes that simulate the human digestion system to the closest extent possible in the laboratory. Applicable Methods In the 1981 definition, “Dietary Fiber consists of the remnants of edible plant cells, polysaccharides, lignin and associated substances resistant to (hydrolysis) digestion by the alimentary enzymes of humans” as in the proposed definition, dietary fiber is the remnants of the edible parts of plants resistant to digestion in the human small intestine. This resistance to digestion was, and remains, the key focus of the analytical method requirements. The first Official Method of Analysis developed based on the 1981 consensus definition was AOAC 985.29. This method is based on the premise of resistance to digestion. Human digestive enzymes are known to digest fats, proteins, and starch. Using 985.29, the food samples are defatted, then heated to gelatinize the starch (the primary form of starch in foods as consumed), then subjected to enzymatic digestion by protease, amylase, and amyloglucosidase (glucoamylase) to remove the digestible components of the food. The residues are quantitated, and adjusted for protein and ash to assure against a protein contribution from the enzymes, and assure that inorganic materials present in the sample are not quantitated as dietary fiber. The enzymes utilized for starch and protein digestion are required to completely digest representative starch and proteins. The method and the enzymes must also pass a purity of activity test to assure against extraneous enzymatic activity, i.e. to assure that the method does not destroy, and the enzymes do not digest any of the dietary fiber components listed above. Substrates to use to assure against extraneous enzymatic activity are listed in the referenced table and section. Other AOAC Official Methods of Analysis and AACC Approved Methods of Analysis adopted since that time have the same or similar method performance requirements, and are listed in Figure 3.5. Additional Methods Requirements Since the time of the adoption of the consensus definition in 1981, and the adoption of Official Method of Analysis 985.29 in 1985, dietary fiber research has expanded dramatically. This expanded knowledge
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FIGURE 3.5 — Official And Approved Methods For Dietary Fiber Analysis AOAC Official Method of Analysis
AACC Approved Method of Analysis
Designation
Title
Designation
Title
AOAC 985.29
Total Dietary Fiber in Foods Enzymatic-Gravimetric Method
AACC 32-05
Total Dietary Fiber
AOAC 991.42
Insoluble Dietary Fiber in Foods and Food Products Enzymatic-Gravimetric Method, Phosphate Buffer
AACC 32-20
Insoluble Dietary Fiber
AOAC 991.43
Total, Soluble, and Insoluble Dietary Fiber in Foods Enzymatic-Gravimetric Method, MES-Tris Buffer
AACC 32-07
Determination of Soluble, Insoluble and Total Dietary Fiber in Foods and Food Products
AOAC 992.16
Total Dietary Fiber, Enzymatic-Gravimetric Method
AACC 32-06
Total Dietary Fiber Rapid Gravimetric Method
AOAC 993.19
Soluble Dietary Fiber in Food and Food Products, Enzymatic-Gravimetric Method (Phosphate Buffer)
AOAC 993.21
Total Dietary Fiber in Foods and Food Products with <2% Starch, Nonenzymatic Gravimetric Method
AOAC 994.13
Total Dietary Fiber (Determined as Neutral Sugar Residues, Uronic Acid Residues, and Klason Lignin) Gas Chromatographic Colorimetric-Gravimetric Method (Uppsala Method)
AACC 32-25
Total Dietary Fiber Determined as Neutral Sugar Residues, Uronic Acid Residues, and Klason Lignin (Uppsala Method)
AACC 32-21 Insoluble and Soluble Dietary Fiber in Oat Products Enzymatic Gravimetric Method
includes the discovery of “resistant starch,” expanded knowledge of the physiological and chemical properties of fructans, including inulin, and the technical capabilities to produce edible carbohydrate-based polymers that are analogous to dietary fiber in their digestive and fermentative behaviors. The analytical methodology adopted in 1985 depends upon the fiber fraction isolated being insoluble in a mixture of 4 parts alcohol and 1 part water. This 4:1 solvent mixture is a traditional chemical means of separating simple sugars and other compounds from the more complex starches and proteins in the samples prior to the analysis of
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the simpler compounds. In the early 1980s, the 4-part alcohol, 1-part water solvent mixture was believed adequate for precipitating and isolating the dietary fiber from the enzyme digestion media. It is now evident that this mixture is not sufficient for the isolation of all dietary fibers, and additional methods need to be used in conjunction with AACC 32-05 (AOAC 985.29) or their equivalents to address those fibers not precipitated. Fructan(s), because of the conformation of the molecule(s), are nearly 100% soluble in the 4-part alcohol, 1-part water mixture. As a result, they will not be isolated as part of the precipitate using 985.29 or equivalent methods. Fructans are part of the “remnants of the edible part of plants that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the human large intestine.” Because fructans are not isolated as part of contemporary methodology, the recently adopted AOAC Official Method of Analysis 997.08, Fructans in Food Products, Ion Exchange Chromatographic Method (AACC Proposed Method of Analysis 32-21), or AOAC 999.03, Measurement of Total Fructan in Foods, Enzymatic/Spectrophotometric Method (AACC Proposed Method 32-32), must be used. In addition, a small amount of inulinase enzyme must be added during the enzymatic digestion steps of the contemporary methods to digest the small amount of fructan that co-precipitates with the rest of the fiber to avoid duplicate quantization. Fructans are nonexistent, or occur in small quantities in most foods such as whole grains, fruits, and vegetables which are consumed in significant quantity. It is likely that there will be little impact on the food labels of these foods on a per-serving basis. A few foods, such as onions and leeks, contain high levels of fructans, so the food label of these products may need to be adjusted slightly on a per-serving basis when the inulin content is added to the fiber quantized by contemporary methodology. Polydextrose, like fructans, is also nearly 100% soluble in the 4-part alcohol, 1-part water mixture, due to the highly branched nature and relatively low molecular weight of the molecule. No significant amount of polydextrose is measured as dietary fiber by AOAC Official Method 985.29 or equivalent, therefore AOAC Official Method of Analysis 2000.11, Polydextrose in Foods by Ion Chromatography, has recently been approved. For foods that contain polydextrose, this method can be used as an adjunct to AOAC 985.29 or equivalent methods in order to determine polydextrose as dietary fiber. Advancing technical capabilities now allow the production of edible
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carbohydrate-based polymers that are analogous to dietary fiber in their digestive and fermentative behaviors. Since it is impossible to completely predict the analytical behavior of these analogous carbohydrates relative to the behavior of naturally occurring dietary fibers, methods for the analysis of other, currently available, analogous carbohydrate materials, or for those that may be developed in the future cannot currently be prescribed. It will be good if those involved with the research and production of such materials who are best equipped with the knowledge and resources develop appropriate analytical methods for their respective materials when used as an ingredient, and quantized when used as part of a food product. Subsequent to the adoption of Official Method of Analysis 985.29, researchers discovered that, in some foods, primarily processed grain products, a small percentage of the starch becomes resistant to the enzymatic digestion procedure of the method. This starch is truly resistant to digestion, resisting digestion in the human intestine (actually, additional quantities of starch typically pass into the large intestine with the resistant starch) and during the analytical processes for quantizing dietary fiber. In addition, starch in other foods also resists digestion, either because it is in a granular form, or because it has retrograded into a digestion-resistant crystalline domain. For labeling purposes, it is not clear what portion of this starch, if any, should be considered as dietary fiber. In some cases, the resistant starch is a component of a not fully ripened plant material. In other cases it is the result of incomplete cooking, or of heating and cooling the food product. In any of these cases, there is no consistent means of producing data for labeling purposes. Less than ripe plant materials can be ripened, or can be at various stages of ripeness when consumed. Less than fully cooked, or heated and cooled products can be cooked, or reheated or at various stages of cooling and crystallization, and the quantity of resistant starch changed before consumption. Therefore, for labeling purposes, utilizing the standardized methodology of AOAC 985.29 or equivalent provides the most reliable and accurate assessment of the quantity of digestively resistant starch that can consistently be delivered to the consumer at the time of consumption. For research purposes, other definitions for resistant starch and other methods for the quantization of the resistant starch thus defined may be suitable. But for labeling purposes, the starch that is truly resistant to digestion in a method that standardizes the treatment of the sample to simulate the likely state of the food at the time of consumption and digests the sample with enzymes that simulate the human small intestine is in order.
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DIETARY FIBER METABOLISM IN GASTROINTESTINAL TRACT Physicochemical Characteristics And Physiological Effects The gastrointestinal tract is the primary area of action of dietary fiber, most notably in the large intestine. The physiological effects of dietary fiber depend on a myriad of variables, but generally they depend on the type (partially fermentable or highly fermentable), the dose of a specific fiber consumed, the composition of the entire fiber-containing meal, and the individual physiological profile of the subject consuming the fiber-containing meal. However, the major physiological effects of dietary fiber originate from the interactions with colonic content throughout its fermentation. Through its varying physicochemical properties, dietary fiber intake influences several metabolic processes, including the absorption of nutrients, carbohydrate and fat metabolism, and cholesterol metabolism. It further has influence on colonic fermentation and affects the production of stools. In the large intestine, dietary fiber influences the colonic structure and barrier function, and as the large intestine encompasses a significant body of the human immune system, it is also likely to have influence on elements of immune function. Dietary fibers, as mentioned previously, differ in their water solubility and can have rheological effects; many well fermented fibers form viscous solutions in the gut (for example, guar gum), but notable exceptions are gum arabic and inulin. Some form gels (pectins), while others have a high water holding capacity (WHC) (for example, cellulose). A changed rheology of the intestinal contents can also have physiological effects. A high viscosity is generally connected with a delayed gastric emptying and increased small intestinal transit time. Fibers with a high WHC (that is, partially fermented fibers) can directly influence the volume and bulk of the intestinal content. The solubility of a fiber is related to its fermentability. Almost all of the fiber sources are fermented to some degree by the resident microorganisms present in the colon. The most notable exceptions are cellulose derivatives, such as carboxymethyl cellulose (CMC), which is soluble but almost nonfermentable by the human colon flora. The physical properties of fiber having primary influence on physiology are its dispersibility in water or WHC; its intestinal bulk due to nondigestibility; its ability to increase viscosity (rheology change) as associated with the more fermentable fiber component; its ability to adsorb or bind bile acids; and its fermentability by microorganisms in the gut.
DIETARY FIBER
81
Colonic Fermentation And Its Consequences The large intestine plays a role in managing and conserving water and electrolytes, to further the digestion of residual material passing from the small intestine, and provides a route for residual, nondigestible material and toxins to pass. The large intestine is the most heavily colonized region of the digestive tract, with up to 1011 to 1012 anaerobic bacteria for every gram of intestinal content (Gibson and Roberfroid 1995). These bacteria produce enzymes that further the digestion of proteins and carbohydrates (fiber) passing undigested from the small intestine. Many variables can influence the extent of the fiber fermentation and, consequently, the nature and amount of the various end products produced from the fermentation, including gases (methane, hydrogen, carbon dioxide), short chain fatty acids (SCFA) (C2-C4 organic acids), and an increased bacterial mass. The extent of fermentation typically ranges from completely fermented (many watersoluble fibers) to little fermentation, for example, cellulose particles. However, of the many factors influencing the extent of fermentation, the primary influence is the physicochemical nature of the fiber. Green et al (1998), while working with human fecal slurries and different fiber sources, observed significantly different levels of SCFA and gas produced from the fiber sources. Botham et al (1998) further noted that the degree of fermentation and concentration of the various end products, particularly the SCFA, change due to the chemical structure and nature of the fiber source. Cellulose composed of β-1,4-linked Dglucose is hardly fermented, whereas starch, which has -1,4 and -1,6 linked D-glucosyl residues, is much more susceptible. It is also clear that amylose film shows limited fermentation, due to the limited accessibility to fermentative enzymes, whereas an amylose gel having better access for enzymes is fermented to a greater extent. Increases in microbial mass from fiber fermentation contribute directly to stool bulk, which is a large part of the stool weight. Bacteria are about 80% water and have the ability to resist dehydration, and thus contribute to water-holding in fecal material. Gas production from colonic fermentation can also have some influence on stool bulk. Trapping of gas can contribute to increased volume and a decrease in fecal transit time. WHC of dietary fiber was originally thought to be an important element for maintaining water content of stool. However, water content of stool is relatively constant at about 25%, and the WHC of dietary fiber likely does not have a direct influence on stool bulk. Bourquin et al (1996) reported that the dispersibility and WHC of fiber determine the ability of microorganisms to penetrate undigested food
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FUNCTIONAL FOODS
and degrade the fiber for growth. As such, WHC has an indirect relationship to stool bulk through its influence on the fermentation of fiber. Hence, fiber sources having high WHC, such as gums or pectin, generally tend to be more readily fermented than those with lower WHC, such as wheat bran, that contain higher levels of insoluble cellulose material. Poorly fermented fibers, such as cellulose, exert an indirect stool bulking effect that causes shorter fecal transit times, a greater fecal mass, and laxation effects. Several researchers working with various fiber sources containing both poorly fermented and well fermented fiber, such as wheat and barley bran, cellulose, soy fiber, and inulin, have shown stool bulking/laxation related effects (Causey et al., 2000). In addition, different fiber types can yield different fermentation products. Poorly fermented cellulose produces very little acid during its fermentation, most of which is only acetic acid; by contrast, in the case of more fermentable fibers, large quantities of SCFA are formed, including propionic, butyric, and acetic acids, in varying proportions. Thus, within a given gut microbiota environment, it may be possible to manipulate several of the key variables of the fermentation, notably the feeding of different fiber sources and combinations of fiber, to manipulate the specific types and amount of the SCFA (Silk et al., 2001). The potential to modify the amounts and distribution of the SCFA and the site of their production in the colon may be important, as each has different physiological influence, with varying implications on human health. In addition, as various gut microorganisms have specificity for fibers of different chemical structure, utilizing fiber sources, such as inulin, that promote bifidobacteria more selectively can provide a means to modify colonic microbiota to a more healthy balance, thereby potentially increasing host health. The metabolic end products of fermentation including the gases, SCFA, and increased microbiota, play a pivotal role in the physiological effects of fiber and implications for local effects in the colon and systemic effects. The gases produced from fiber fermentation by strict anaerobic species, such as bacteriodes, some nonpathogenic species of clostridia and yeasts, anaerobic cocci, and some species of lactobacilli, are mostly released as flatulence or are absorbed and subsequently lost from the body through the lungs. However, some of the hydrogen and carbon dioxide produced from these microorganisms may be further metabolized to methane (CH4) by methanogenic bacteria, thus reducing intestinal gas pressure. Of these anaerobic microorganisms, the clostridia, eubacteria, and anaerobic cocci are the most gas producing, while the bifidobacteria are the only group of the
DIETARY FIBER
83
common gut microbiota that do not produce any gases. The SCFA resulting from the fermentation process provide a certain amount of energy from their metabolism in the liver. The energy content of a fiber is, from a scientific standpoint, dependent on the degree of fermentation. Fibers that are not fermented to any extent have a caloric content approaching 0 kcal/gram, while data from caloric studies indicate that the average energy yield from dietary fiber fermentation in monogastric species is in the range of 1.5 to 2.5 kcal/g (Smith et al., 1998). It should be noted that legal values for nutritional labeling can be different. The primary SCFA generated by fermentation are acetate, propionate and butyrate, accounting for 83 to 95% of the total SCFA concentration in the large intestine, which ranges from about 60 to 150 mmol/L. The concentrations of these acids are highest where concentrations of microbiota are also highest, namely in the cecum and transverse colon. Corresponding to these higher acid levels, the pH is also typically lowest in the transverse colon (5.4 to 5.9) and gradually increases through the distal colon to 6.6 to 6.9. At the colonic level, the fermentation of fiber increases the concentrations of these health-promoting SCFA and endogenous microbiota, exerting potential health effects, such as inhibiting the growth of pathogens, increasing mineral absorption, or producing vitamins. The SCFA absorbed into the portal blood system and reaching the liver and kidneys can further influence metabolism. This can lead to systemic effects, such as changes in glycemia, lipidemia, uremia, and overall nitrogen balance. Influence on lipids is an example of a potential health effect; a high serum lipid level is connected with a increased risk of cardiovascular disease. This risk may be lowered by the consumption of fermentable fibers. PHYSIOLOGICAL FUNCTIONS OF DIETARY FIBER Dietary Fiber And Cancer Colon cancer is one of the leading causes of cancer morbidity and mortality among both men and women in the Western countries, including the U.S.A. Historical observational and epidemiological studies from around the world have long supported that increased consumption of fruits and vegetables and high fiber intake provide a protective relationship between dietary fiber intake and colon cancer incidence (Byers, 2000). Of particular interest is the utilization of fermentable fiber by the colonic microbiota that can result in changes to the numbers and types
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FUNCTIONAL FOODS
of bacteria and, more importantly, changes to their metabolic activities in terms of the formation of genotoxins, carcinogens, and tumor promoters. Reddy (1999) emphasized synergistic effect when both probiotics and prebiotics are used together for possible mechanistic effects for cancer inhibition. Selective prebiotic fiber sources, such as inulin, resistant starches, and some oligosaccharides, act as selective substrate for bacteria that produce specific SCFA and can lower the intestinal pH. The SCFA butyrate has been shown to increase apoptosis in human colonic tumor cell lines (Scheppach, 1998). Possible mechanisms for the anticarcinogenic and antitumorigenic effect of highly fermentable fibers are not completely understood and require further research. However, it is likely that some or all are involved in a metabolic chain reaction for the inhibitory effect to occur. The primary mechanisms involved with these effects are proposed to be: a reduction in the production of carcinogenic substances by decreasing the amount of pathogenic bacteria in the colon; and/or lowering the colonic pH to affect pH-dependent enzymatic reactions; for example, secondary bile acid formation; and/or reducing the amount of carcinogenic substances available to colonic mucosa by adsorption of the substances to the cell wall of the microorganisms, by speeding up the intestinal transit time and by increasing colonic contents and thus diluting all components; and/or exerting inhibiting effects on initiation and promotion stages in colon cancer formation in which SCFA, particularly butyric acid, may play a key role. Dietary Fiber And Carbohydrate Metabolism An association between insufficient dietary fiber intake and increased risk of diabetes has been postulated since 1970s. While a direct linkage between insufficient dietary fiber intake and diabetes has not been established, evidence that indicates decreased risk of the disease with increased dietary fiber consumption continues to grow (Chandalia et al., 2000). Well fermented viscous fibers, either as part of a food or as a supplement that is well mixed with food, appear to offer the greatest potential benefit to reduce glycemic response and to increase insulin sensitivity. Glycemic Index And Glycemic Load The glycemic index (GI) is a numerical system of measuring how much of a rise in circulating blood sugar a carbohydrate triggers—the higher the number, the greater the blood sugar response. So a low GI food will cause a small rise, while a high GI food will trigger a dramatic spike. A GI of 70 or more is high, a GI of 56 to 69 inclusive is medium,
DIETARY FIBER
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and a GI of 55 or less is low. The glycemic load (GL) is a relatively new way to assess the impact of carbohydrate consumption that takes the glycemic index into account, but gives a fuller picture than does glycemic index alone. A GI value indicates only how rapidly a particular carbohydrate turns into sugar. It doesn’t show how much of that carbohydrate is in a serving of a particular food. It is necessary to know both to understand a food’s effect on blood sugar. That is where glycemic load comes in. The carbohydrate in watermelon, for example, has a high GI. But there isn’t a lot of it, so watermelon’s glycemic load is relatively low. A GL of 20 or more is high, a GL of 11 to 19 inclusive is medium, and a GL of 10 or less is low. Epidemiologic evidence suggests that a diet with a high glycemic load or glycemic index may increase the risk of type 2 diabetes. The beneficial physiological effects of viscous fiber sources on blood glucose concentrations have been consistently demonstrated over the last 2 decades. The mechanisms explaining the influence fibers have on reducing postprandial glycemia and fiber’s potential for enhancing carbohydrate metabolism over a longer term remain unclear. However, most likely causes for these influences are those related to small intestinal viscosity and nutrient absorption, and systemic effects from colonic-derived SCFA. Relative to postprandial glycemia, fibers that provide high viscosity in the small intestine (for example, guar gum, pectin) generally offer greater effect. By contrast, the SCFA, produced in the colon from well fermented fiber (for example, inulin), likely influence hepatic cholesterol synthesis, the production of glucose and its utilization later in the day (Luo et al., 1996). As the small intestinal transit times for mixed meals is relatively long (about 6 h), the colonicderived SCFA likely do not explain the acute effects of slowing small intestinal carbohydrate absorption typical of the postprandial effects following the intake of viscous well fermented fibers. The SCFA, namely, acetate, produced from fiber fermentation and absorbed into the peripheral blood may influence systemic metabolic functions. Acetate may influence serum fatty acid levels, which may directly inhibit adipose tissue lipolysis. Studies involving acetate, whether derived from fiber fermentation during chronic feeding studies, or given orally, or given by rectal infusion have all shown acetate to reduce serum fatty acid levels. But none has shown acetate to improve carbohydrate tolerance. However, propionate is gluconeogenic in the liver, potentially increasing blood glucose levels. Even though a clear mechanism for fiber effects on carbohydrate metabolism is yet undefined, the beneficial effects in support of high fiber intake for type 2 diabetes mellitus continue to
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increase. While the reduced small intestinal absorption may play a greater role in this effect than the colonic effects, further research is needed in the area of specific fiber sources and carbohydrate tolerance to more fully elucidate potential synergistic effects from SCFA. Dietary Fiber, Lipid Metabolism, And Cardiovascular Disease Total serum cholesterol and low-density-lipoprotein (LDL) cholesterol levels are generally accepted as biomarkers, indicative of potential risk for developing the disease. As such, research has primarily focused on their reduction as a means to reduce the risk of developing cardiovascular disease (CVD). Substantial experimental data support that blood cholesterol can be lowered using well fermented fiber types that produce relatively high viscosity, and epidemiological evidence supports the relationship between higher dietary fiber intake and reducing the risk of cardiovascular disease (Anderson et al., 2000). They concluded that high-fiber diets may protect against obesity and CVD by lowering insulin levels. A meta-analysis of 67 controlled studies using viscous well fermented fibers (oat-25, psyllium-17, pectin-7, and guar gum-18) showed reduction in serum cholesterol with higher rather than lower fiber levels (Brown et al., 1999). Less viscous fiber sources, like inulin, have generally shown consistent lipid lowering in animal studies, but human studies have shown variable results (Boeckner et al., 2000). When considering the results of human lipid studies, a number of factors need to be addressed, including: individual variation, duration of administration of the fiber source, fermentation rates of the various fiber types and chain fractions, intakes of dietary fat and carbohydrate in the background diet; and prior serum lipid levels. Mechanisms for cholesterol-lowering ability of water-soluble fiber have been suggested, but no consensus has been reached. Studies indicate effects on cholesterol formation due to certain well fermented fibers, such as psyllium, citrus pectin, oat bran, and rolled oats (Anderson et al., 1984), bind bile acids increasing their excretion and decrease cholesterol in the liver. Certain fibers, such as pectins and galactomannans that have high WHC and generate higher viscosity, also have influence on small intestine absorption of nutrients. It has also been suggested that the hypocholesterolemic effect of dietary fibers might also be mediated by the SCFA from fiber fermentation. SCFA are absorbed from the colon; butyrate and propionate are extracted by the colonic mucosa and liver, respectively, whereas acetate reaches peripheral circulation. Propionate primarily is reported to inhibit fatty acid metabolism, which plays a key role in the synthesis of cholesterol (Demigne et al., 1995). Kok et al
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(1998) postulated that lower glucose and insulin levels found after feeding inulin at a dose of 10% to rats contributed to reduced hepatic fatty acid and triglyceride synthesis. This would positively influence lipid metabolism by increasing the secretion of intestinal hormones, namely, glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1). These gut hormones are known to regulate postprandial insulin release and also to have direct insulinlike actions on lipid metabolism. Dietary Fiber, Mineral Bioavailability And Bone Health Certain fiber sources from fruits and vegetables that have cation exchange capacity from unmethylated galacturonic acid residues and phytic acid from cereal fibers, have been found to depress the absorption and retention of several minerals. However, certain highly fermentable fibers have resulted in improved metabolic absorption of certain minerals, such as calcium, magnesium, and iron, even when phytic acid is present at lower concentrations (Lopez et al., 1998). These compounds include pectin, various gums, resistant starches, cellulose, certain oligosaccharides like soy and fructooligosaccharides, inulin, lactulose, and related sugars. Mineral absorption has generally been accepted as stemming from diffusion in the small intestine. However, studies now indicate that highly fermentable fibers, such as inulin and fructooligosaccharides, also promote mineral absorption in the colon. Through their fermentation by colonic microbiota and subsequent SCFA production, these fiber components stimulate the proliferation of epithelial cells in the ceco-colon and reduce the luminal pH (Younes et al., 1996). The SCFA and lower pH may, in turn, dissolve insoluble mineral salts, especially calcium, magnesium, and iron, in the luminal content and increase their diffusive absorption via the paracellular route. In particular, the accumulation of calcium phosphate in the large intestine and the solubilization of minerals by SCFA are likely to play an essential role in the enhanced mineral absorption in the colon. Also, a recent study has demonstrated that fructooligosaccharides (inulin) stimulates the transcellular route of calcium absorption in the large intestine, as indicated by increased concentrations of calbindin-D9k, a calcium binding protein that plays an important role in intestinal calcium transport (Ohta et al., 1998). Dietary Fiber, Nitrogen Utilization And Bile Acid Metabolism Fiber also has influence on nitrogen balance. Fiber acts as substrate for increased microbial mass, which utilizes high fecal nitrogen and
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creates a marked enlargement of the cecum. The SCFA produced from fermentation of fiber and their associated lowering of colonic pH provide an added effect by protonating potentially toxic ammonia (NH3) to produce ammonium ion (NH4+), a form that is nondiffusible into the portal blood system (Younes et al., 1995). The consequence of this process is higher nitrogen retention in the cecum, increased fecal nitrogen excretion, lower blood ammonia levels, and decreased uremia. Studies in both animals and humans have shown that fecal nitrogen excretion is increased during consumption of a high soluble fiber diet (Vanhoof and De Schrijver 1996). Nitrogen balance, however, is not compromised due to a concomitant decrease in renal nitrogen excretion, likely due to a strong transfer of urea nitrogen to the intestine, to depress the plasma uremia. This shift does not appear to alter protein bioavailability, and seems more evident when the dietary protein level is moderate. The acidification of the luminal content by the SCFA may also potentially modify the metabolism of bile acids. Of particular interest is reducing the conversion of primary to secondary bile acids, as these are believed to be associated with increased risk of colon cancer. Dietary Fiber, Role In Gut Barrier Function And Gastrointestinal Disorders The health of the large intestinal wall and its microbial ecosystem play a key role in gastrointestinal health. Through its fermentation, dietary fiber and SCFA are important elements for both protecting the health of the large intestinal wall and stimulating repair in a damaged colon. The health of this organ is very important as it, in addition to serving as a primary site for water reabsorption, plays an important role as a major immune organ and functions as a barrier to prevent foreign materials from dietary or microbial origin from crossing into the internal body cavity. The integrity of the intestinal barrier changes with stress, starvation, and in several clinical situations where the gut is damaged; for example, Crohn’s disease, celiac sprue, extensive burn injury, antibiotic therapy, parasites, rheumatoid arthritis and indomethacin-associated enteritis, and intestinal obstruction (Lipman, 1995). Many of these disorders result indirectly from the loss of barrier function and are directly related to bacterial translocation. Various researchers showed that specific well fermented fibers, such as inulin, encourage the growth of healthpromoting bacteria and minimize the growth of pathogens and the
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production of subsequent harmful byproducts of protein degradation (that is, ammonia, phenolic products, amines, and N-nitroso compounds), which have been linked to various types of cancer and ulcerative colitis (Birkett et al., 1996). Further, researchers working with animals and in vitro testing have suggested a role of various fibers on intestinal immune function (Meyer et al., 2000). The SCFA, from fiber fermentation, particularly butyrate, play a key role in the health of the colon. They are suggested to be of influence in both stimulation of cell division and regulation of apoptosis (Wasan and Goodlad, 1996). Unlike the small intestine, which can derive energy from various endogenic sources, such as glutamine from muscle breakdown or ketone bodies from hepatic ketogenesis, during periods of starvation, the colon epithelial cells derive their energy from SCFA, particularly butyrate. Butyrate, is a preferred nutrient for colonocytes, prevents colonic mucosal atrophy, which develops within a day of oral starvation. Intestinal permeability or “leaky gut syndrome” is viewed as an indicator for sub-clinical disease states. As gut barrier function is lost, the formation of chronic bowel inflammation may appear, since exposure of the mucosa to luminal antigens can be responsible for inflammation. In order to keep the colonocytes healthy and minimize potential for chronic bowel inflammation, it is important to maintain a positive balance between crypt cell growth and luminal cell loss by apoptosis or sloughing (Lynn et al., 1994). While different fiber types have different effects on epithelial cell growth, it is generally accepted that more fermentable fibers have greater influence on cell growth, as being promoted via SCFA production. Poorly fermented fibers, such as cellulose, appear to maintain muscle layer, independently from the mucosal layer, while a fermentable substrate is required for mucosal layer growth. Poorly fermented fibers may also influence colonocyte proliferation by direct abrasive action (Folino et al., 1995). The mucus layer is important to the gut lining, provides lubrication and protects the gut from enzymatic acid and toxin attack. It is also food for several microbial species, helps remove microorganisms, and further serves as an antioxidant (Satchithanandan et al., 1996). Highly fermentable fibers appear more likely than poorly fermentable fibers to alter intestinal mucus composition, either through their direct mechanical effects or indirectly by regulating mucosal metabolism via SCFA derived from fiber fermentation.
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PROPERTIES AND PHYSIOLOGICAL EFFECTS OF SELECTED NON DIGESTIBLE POLYSACCHARIDES (NDP) AND NON DIGESTIBLE OLIGOSACCHARIDES (NDO) An overview of many different NDPs, their chemical makeup and occurrence, and a summary of the effects on the human intestine is given in Figure 3.6. A description of the physicochemical structures of selected NDOs and NDPs will provide a further understanding of possible physiological effects of these isolated dietary fiber ingredients based on human studies. Resistant Starch Resistant starch is part of our daily diet or can be added as an ingredient to various food products. The colonic fermentation of resistant starch in humans is well documented (Heijnen et al., 1998). Stool bulking effects for resistant starch were found as well, probably as a consequence of the increased bacterial mass in the feces. Further, consumption of resistant starch (and/or its subset resistant maltodextrin) may also stimulate the growth of specific bacteria purported to provide beneficial health effects, that is, the bifidobacteria and lactobacilli. Potential health benefits of resistant starch, such as blood sugar and cholesterol control, weight control and energy management, gut disorders, and colon cancer were reported by Baghurst et al (1996). Fermentation of resistant starch gives rise to relatively high levels of butyric acid, which, as mentioned earlier, may have implications in tumorigenesis and gut barrier health. Animal model studies show that resistant starch inhibits chemically induced carcinogenesis in the colon of rats (Sakamoto et al., 1996). Pectins Pectins are present in plant cell walls, and are polysaccharides with -1, 4-linked D-galacturonic acid backbone with a variable amount of neutral sugars (arabinose, galactose, xylose) present as side-chains. Rhamnose may also be present in the backbone. The galacturonic acid residues are substituted with acetyl and methyl ester groups. The main applications for pectins as food additives are as gelling and thickening agents in many food products. Most pectins for food use are extracted from citrus or apple. Due to their gelling behavior, these soluble polysaccharides may decrease the rate of gastric emptying and influence small intestinal transit time. This explains their hypoglycemic properties. Various human studies show that pectins are fermented to a large extent in the colon.
By alkaline deacetylation of crustacean chitins. Found in fungi cell walls of Zygomycetes sp. Extracted from plants (wood pulp, bamboo, wheat, cottonseed hulls)
2-amino-2-deoxy-βD-glucose
β-1-4-D-glucose
Chitosan
Microcrystalline Cellulose
Produced by fermentation using Pseudomonas elodea
Produced by fermentation using Xanthomonas campestris
β-1-4-D-glucose (backbone), β-D-glucuronic acid, D-rhamnose
β-1-4-D-glucose (backbone), β-D-glucuronic acid, β-D-rhamnose
Gellan gum
Xanthan gum
Chemical reaction of cellulose with caustic followed by reaction with substituting reagent
Produced by fermentation using Alcaligenes faecalis var. myxogenes
β-1-3-D-glucose
Curdlan (insoluble β-glucan)
Modified cellulose Various functional groups (MC, CMC, MHPC) substituted for OH- of cellulose
Cereal (barley, oats)
Occurrence & Production
β-1-4-D-glucose and β-1-3-D-glucose
Composition
β-glucan
Name
Increases water holding capacity, increased fecal bulk, partially fermented in human gut.
Increases fecal excretion of neutral steroids. Interferes with intestinal absorption of cholesterol.
Increased water holding capacity
Fermented in large intestine, strong butyrate production, blood lipid effects
Human Physiological Effects
Adds viscosity, fermented to short-chain fatty acids in human gut Pastry fillings, sauces Adds viscosity, fermented to and gravies, salad short chain fatty acids in dressings, dairy products, human gut beverages, puddings
Icings, fillings, dessert gels, low-sugar jams and jellies, puddings and confections
Toppings, fillings, icings, Partial fermentation in the breadings, soups, sauces, human gut gravies, baked goods, biofilms
Dressings and sauces, beverages, baked goods, whipped toppings
High viscosity limits food uses. Decreased hepatic cholesterol and triglycerides
Processed meat, poultry, seafood products, noodles and pasta, sauces and dressings desserts, biofilms, jellies
Breakfast cereals, functional food products
Food Applications
FIGURE 3.6 — Commercial Nondigestible Polysaccharides
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Not in nature, in Japan by transglucosylation of glucose.
Cyclic molecules of -1-4 linked D-glucose, (-cyclodextrin-hexamer, β-cyclodextrin-hetamer & g-cyclodextrin-octamer)
Mixture of β-1-6 linked D-glucose oligomers
Mixture of -D-glucose
β-Cyclodextrins
Gentiooligosaccharides (GeOS)
Glucooligosaccharides (-GOS)
By transglucosidation using an -glucosidase from Leuconostoc mesenteroides
Not in nature
4-O-β-D-glucopyranosyl-D -glucose [(4-O-β-D-glucopyranosyl)n -D-glucose]
Not in nature, degradation of cellulose
Produced by pyrolysis of corn starch with HCl, further enzymatic hydrolyzed
Cellobiose (CEL) Cellodextrins
Cellobiose and cellodextrins
Mixed and random linkages, -1-4 and 1-6 glucosidic bonds from starch and 1-2 and 1-3 bonds, from transglucosidation
Resistant maltodextrin
By vacuum thermal polymerization of glucose, sorbitol and citric acid
Occurrence & Production
Mixed and random glycosidic linkages, (1,6-bonds)-D-glucose
Composition
Polydextrose
Name
Sauces, salad dressings, ice cream, frozen desserts, partially hydrolyzed used as highly soluble fiber
No information
No information
No information
Nutritional beverages, functional foods, low viscosity fiber source
Low calorie, bulking agent for sugar replacement in foods (confections, desserts, dairy, baked goods, fillings)
Food Applications
Animal studies show influence of intestinal microbiota. No human studies
Scarce data, no conclusion on effects on intestinal microbiota
May have effect based on fermentation, no published studies on effects
No studies on prebiotic effects
Partially digestible, laxation and colonic fermentation, effects on lipid levels
Fermented to produce microbiota, SCFA stool bulking and stool softening in intestine
Human Physiological Effects
FIGURE 3.6 — Commercial Nondigestible Polysaccharides - Continued
92 FUNCTIONAL FOODS
Composition
Dried exudates from stems and branches of African bush Acacia senegal
β-D-galactose (backbone), L-arabinose, L-rhamnose, D-glucuronic acid (highly branched)
-1-4-D-galacturonic acid backbone/neutral sugar side chains, some ester groups
Acacia (gum Arabic)
Pectin
Fruits and vegetables (apples, citrus, sunflowers, sugar beet)
Extracted from red algae, Dairy products, Rhodophyceae (Gelidium sp. confectionery, baked and Gracilaria sp.) products, meat analogues, desserts
β-1-3-D-galactose and 3,6-anhydro-β-L-galactose
Agar
Jams and jellies, low-sugar or sugar free jams and jellies, beverages, milk products, biofilms
Spray dried flavors, confectionery products, jellies, (dry mixed puddings, desserts cake mixes)
Decrease gastric emptying and small intestine transit time (hypoglycemic properties) Fermented in large intestine No effects on stool weight, decrease in serum cholesterol
Fermented in the human gut. Prebiotic
Adds viscosity, fermented in the human gut to short chain fatty acids
Adds viscosity, decreases gastric emptying and small intestine transit time (hypoglycemic properties) Fermented in large intestine to short chain fatty acids
Both animal and human studies indicate influence of intestinal microbiota
Human Physiological Effects
Dairy products, bakery products, dessert gels, processed meat, low-sugar jams and jellies
Extracted from red algae (Rhodophyceae)
Mixture of sulfated polysaccharides made up of -D-galactose & 3, 6-anhydro-D-galactose
Carrageenan
Dairy products, bakery products, dessert gels, processed meat, low-sugar jams and jellies
Food Applications
Linear -1-6 linked glucose residues, some 1-4 linkages, 6-O--D-glucopyranosyl-Dglucose, 6-O--Dglucopyranosyl-(1-6)--Dglucopyranosyl-D-glucose, 6-O--D-glucopyranosyl(1-6)--D-glucopyranosyl(1-4)-D-glucose
Occur in miso, soy sauce, sake and honey. Prepared commercially by transglucosylation of glucose residue. By transglucosidase (-glucosidases)
Occurrence & Production
Isomaltose - IMA Isomaltose - IMT Panose - PAN
Isomalto-oligosaccharids
Name
FIGURE 3.6 — Commercial Nondigestible Polysaccharides - Continued
DIETARY FIBER 93
Human Physiological Effects
O-D-galactopyranosyl(1-4)-O-β-Dgalactopyranosyl-(1-4)D-glucopyranose
Not in nature, produced in Japan by action of Crytococcus laurentii on lactose
Functional foods in Japan
Unaffected by human enzymes. Likely to have effect on composition and metabolic activity in human intestinal microbiota
A study in rats indicates effects on composition and metabolic activity of intestinal flora, no human data are available
4’-galactosyllactose (GLL)
Functional food development in Japan
4-O-β-D[galactopyranosyl]nD-sorbitol
Lactitol oligosaccharides (LTOS)
Not in nature, Transgalactosylation of lactitol using Aspergillus oryzae β-galactosidase
Not in nature, alkaline Drug status in EU, Gas production is relatively isomerization of glucose not approved for food use large, due to fermentation by moiety of lactose to fructose, clostridia, Kl. Pneumoniae. marketed as laxative and Studies suggest effects on gut health aid bacterial composition and activity
Studies indicate intestinal flora effects on composition and activity
4-O-β-D-galactopyranosylD-fructose
Sugar replacement, bulking agent, functional foods, prebiotic
Sugar replacement, May have significant effect on bulking agent, functional composition and activity of foods, prebiotic intestinal flora
Food Applications
Lactulose (LAT)
Extraction from soybeans, legumes
Occurrence & Production
Likely some in nature, enzymatic transgalactosylation of lactose
RAF, O--Dgalactopyranosyl-(1-6)- D-glucopyranosyl-β-Dfructofuranoside
Composition
β-galactoβ-D-galactopyranosyl-(1-6) oligosaccharides or -[β-D-galactopyranosyl]ntransgalactoolig(1-4) -D-glucose osaccharides (TOS)
-galactooligosaccharides (raffinose, stachyose/other soy oligosaccharides)
Name
FIGURE 3.6 — Commercial Nondigestible Polysaccharides - Continued
94 FUNCTIONAL FOODS
Composition
Levan-type
Fructans
Neogalactobiose (NGB) Isogalactobiose (IGB) Galsucrose (GAS) Isolactose I (IL1) Isolactose II (IL2) Isolactose III (IL3) lactose trimer
β-D-(2,6)-fructofura-nosyl)n -D-glycopyranoside
β-D-galactopyranosyl-βD-glucopyranoside β-D-galactopyranosyl-D-glucopyranoside -D-galactopyranosylβ-D-glucopyranoside -D-galactopyranosyl-βD-fructofuranosyl-(2-6)-βD-fructofuranoside
Synthetic galactooligosaccharides
Name
Produced by Bacillus polymyxa on sucrose
Chemically by the Koenigs-Knorr reaction
Occurrence & Production
No primary commercial application
Proposed for functional foods
Food Applications
Data indicate that addition of inulin-type fructans affect the bacterial composition and the metabolic pattern of the intestinal microbiota. Studies have shown production of short chain fatty acids and a relatively high production of propionate and butyrate, necessary for colonic health and systemic influences on blood glucose and lipids
No animal or human data exists. Unknown if these products are digested in the upper GI tract
Human Physiological Effects
FIGURE 3.6 — Commercial Nondigestible Polysaccharides - Continued
DIETARY FIBER 95
Composition
β-1-4-D-mannose (backbone), Guar gum (Cyamopsis Sauces, salad dressings, -1-6-D-galactose tetragonolobus), locust ice cream, frozen desserts, bean gum (Ceratonia siliqua) partially hydrolyzed used as highly soluble fiber
Galactomannan residues
Galactomannan (guar gum, locust bean gum)
Guar gum oligosaccharides
Produced by partial hydrolysis of guar gum
Functional foods
Fermented by colon microbiota. Lipid lowering, plasma glucose lowering
Readily fermented in human gut with bifidogenic effects, improves bowel function, shows hypolipidemic effects reduces postprandial glycemia
Fat/sugar replacement, Data indicate that addition of texture modification. inulin-type fructans affect the Dressings and sauces, bacterial composition and the beverages, baked products, metabolic pattern of the fillings, icings, frozen intestinal microbiota. Studies desserts, dairy products, have shown production of processed meats, low fat short chain fatty acids and a spreads, toppings, relatively high production of extruded products, propionate and butyrate, dietary supplements necessary for colonic health (prebiotic functional foods) and systemic influences on blood glucose and lipids
Mixtures of 1- kestose, nystose & 1f-β-fructofuranosylnystose β-D(2,1)-fructofuranosyl)n β-D-fructofuranoside
Fructooligosaccharides
Produced by transfructosylation of a β-fructosidase of Aspergillus niger on sucrose. By partial enzymatic degradation of native inulin
Human Physiological Effects
Fat/sugar replacement, Data indicate that addition of texture modification. inulin-type fructans affect the Dressings and sauces, bacterial composition and the beverages, baked products, metabolic pattern of the fillings, icings, frozen intestinal microbiota. Studies desserts, dairy products, have shown production of processed meats, low fat short chain fatty acids and a spreads, toppings, relatively high production of extruded products, propionate and butyrate, dietary supplements necessary for colonic health (prebiotic functional foods) and systemic influences on blood glucose and lipids
Food Applications
β-D-(2,1)-fructofuranosyl)n -Dglucopyranoside Naturally occurring in Jerusalem artichokes, chicory, onion, and so on. Produced by extraction from chicory root.
Occurrence & Production
Inulin-type
Fructans - Continued
Name
FIGURE 3.6 — Commercial Nondigestible Polysaccharides - Continued
96 FUNCTIONAL FOODS
Naturally occuring as the husk of the psyllium seed
Extracted from seeds of tamarind tree (Tamarindus indica)
Partial enzymatic hydrolysis Functional foods of polyxylan by xylanase from Trichoderma sp.
Dehydration and purification of konjac tubers (Amorphophallus konjac)
Polymer of arabinoxylans with 1,4 and 1,3 linkages
β-(1-4) linked D-glucose, partially substituted with -D-xylopyranose.
β-D-((1,4)-xylose)n
β-1,4-linked D-glucose and D-mannose (glucomannan)
Psyllium seed husk
Xyloglucan
Xylooligosaccharides
Konjac (flour) mannan
Binder in meat, often used with k-carrageenan and xanthan gum for gelling
Food additives in Japan, sauces, dressing, ice cream, mayonnaise
Functional food development as a fiber source
Functional food development, dietary supplements
Highly branched (β-1-3 & (β-1-6) D-arabinose and D-galactose sub-units) 6:1 ratio
Arabinogalactan
Extracted from the pulp of Western Larch trees
Complex mixture of polymers Dried exudates of Asiatic sp. Salad dressings, pickle of D-galacturonic acid, of Astragalus, Leguminosae relish, pulpy beverages, galactose, arabinose, xylose, (Astragalus gummifer) milkshakes, ice cream traces starch and cellulose
Human Physiological Effects
Ability to reduce serum cholesterol and serum triglyceride levels and influence glucose and insulin responses
Not hydrolyzed by human enzymes, Changes in metabolic pattern of intestinal flora observed in rats. Blood lipid effects
Fermented in the human colon. Adds viscosity in the small intestine
Reduced risk of coronary heart disease (health claim, at least 1.7 g per RA. Reduced cholesterol
Data indicate gut fermentation, increasing Lactobacilli populations. Studies show immunological properties
Adds viscosity, fermented in the human gut
Powdered doughnuts, Adds viscosity, fermented French dressings, ice in the human gut pops, cheese spreads, ground meats, meringues
Food Applications
Gum tragacanth
Dried exudate of the Indian tree Sterculia urens
Occurrence & Production
Acetylated galacturonic acid+rhamnose+galactose
Composition
Gum karaya
Name
FIGURE 3.6 — Commercial Nondigestible Polysaccharides - Continued
DIETARY FIBER 97
High in mannuronic and glucuronic residues
Mannanoligosaccharide mixture containing less than 50% oligosaccharides
Alginate oligosaccharides
Mannanoligosaccharides
Adapted from Tungland and Meyer, 2002
β-1-4-D-mannuronic acids and -1-4-L-guluronic acid
Composition
Alginate
Name
By Saccharomyces cerevisiae on sucrose
Produced by enzymatic degradation of alginate
Extracted from brown algae (Phaeophyceae)
Occurrence & Production
Human Physiological Effects
Limited data, algal oligosaccharides have an effect on either the metabolism or composition of the intestinal microbiota
Used as growth promoter No data available on in animal feed industry fermentation
Functional foods
Dairy products, bakery Adds viscosity, fermented in products, salad dressings, human gut to short chain dessert puddings, foam fatty acids. stabilization, fabricated foods, dietetic products
Food Applications
FIGURE 3.6 — Commercial Nondigestible Polysaccharides - Continued
98 FUNCTIONAL FOODS
DIETARY FIBER
99
Guar Gum Guar gum is a galactomannan isolated from the seed of Cyamopsis tetragonolobus (guar). In its unmodified form, this food additive is used as a thickener in a large variety of food products. Partial enzymatic hydrolysis results in a product that can be used as a soluble dietary fiber. The physiological effects of this fiber source comply with what might be expected from a soluble fiber. Guar gum is readily fermented by the human fecal microbiota, and it has bifidogenic effects, at least with enteral feeding (Okubo et al., 1994). It improves bowel functioning, reducing diarrhea in enterally fed patients and relieves constipation in patients. It shows a hypolipidemic effect in humans, lowering both serum cholesterol and triglycerides, and it reduces postprandial glycemia. Gum Arabic This is exudate from the acacia tree and is a complex arabinogalactan polysaccharide in admixture with a glycoprotein. It has a high molecular weight and it is used as an additive in many food applications as a stabilizer and emulsifier. The physiological effects from human studies include its complete fermentation in the human colon with indications for a bifidogenic effect and its ability to lower serum triglyceride and cholesterol levels (Michel et al., 1998). Fructans Fructans can be divided in 2 classes. Levans are β-2,6 -linked fructans with variable degrees of β-2,1 -linked side chains that are produced by a large variety of bacteria. Inulins, on the other hand, are composed of β-2,1 -linked fructosyl units, and are produced by many dicotyledonous plants as a reserve carbohydrate. Fructans are discussed in detail in the chapter “Prebiotics, probiotics and symbiotics”. Galactooligosaccharides These oligosaccharides are produced from lactose by the transglycosylating activity of β-galactosidase. They consist of a number of β-1,6-linked galactosyl residues linked to a terminal glucose unit via an -1,4-bond. Galactooligosaccharides are not digested in the human alimentary tract, acting as soluble dietary fiber. Reports show a change in colon flora composition and activity following consumption of these compounds (Alles et al., 1999). Various human studies show that these oligosaccharides may relieve constipation, improve calcium absorption, and retard the development of colon cancer in rat model systems.
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FUNCTIONAL FOODS
Another type of galactooligosaccharides is natural isolates from soybeans. These -galactooligosaccharides (galactosylsucroseoligosaccharides) include raffinose, stachyose, and verbascose and consist of galactose residues linked -1,6 to the glucose moiety of sucrose. Their physiological effects appear similar to the β-linked galactose oligomers, such as gut pathogen reduction with corresponding effects, as expected from this change in colon microbiota. However, no data are available at present. Lactulose Lactulose is a disaccharide of D-galactose linked β-1,4 to fructose. It is manufactured from lactose, where alkali isomerization is used to convert the glucose moiety in the lactose into a fructose residue. The disaccharide is not digested by humans, and promotes growth of bifidobacteria in the colon (Strohmaier, 1996). Lactulose is mostly used as a pharmaceutical in prevention of constipation and in portosystemic encephalopathy. It is not allowed for food use in Europe, due to its drug status, but seems to find its way into the food supplement market (mainly in The Netherlands). Although its use is clearly for physiological reasons and it has well-established health effects, its legal status limits use in food. The physiological effects of lactulose are exploited in Japan. Other Oligosaccharides As indicated in Figure 3.6, there is a wide variety of oligosaccharides, all of which have properties characteristic of dietary fiber, that have effects on the activity and/or composition of the human colon flora. PROPERTIES OF ISOLATED FIBER IN FOOD APPLICATIONS Dietary fiber components, isolated from their native plants, provide many functional properties that affect the technological function of foods. These functional properties also influence the food product’s properties during its processing and its final product quality and characteristics. The primary properties provided by isolated fiber ingredients for food development are related to their solubility, viscosity and gelation-forming ability, water-binding capacity, oil-binding capacity, and mineral and organic molecule-binding capacity. The solubility of fiber as a technological property refers to its solubility in water. Four primary structural features of the polymeric backbone primarily increase solubility. More branching generally results in greater solubility (for example, gum acacia); the presence of ionizing
DIETARY FIBER
101
groups also tend to increase solubility (for example, pectin methoxylation); the potential for inter unit positional bonding (for example, β-glucans with mixed β-1-3 and β-1-4 linkages) also increase solubility; and alterations of the monosaccharide units or their molecular form (- or β-form) further increase solubility (for example, gum acacia, arabinogalactan, and xanthan gum). Viscosity is another technological property of fiber that provides rheological change in food systems. Generally, as the molecular weight or chain length of the fiber increases, the viscosity of the fiber in solution increases. However, the concentration of the fiber in solution, the temperature, pH, shear conditions of processing, and ionic strength all substantially depend on the fiber used. Primarily, long chain polymers, such as the gums (for example, guar gum, locust bean gum, tragacanth gum, etc.), bind significant water and exhibit high solution viscosity. These are used as thickening agents in foods at low concentrations. While these fiber sources are typically necessary to make food-based delivery systems functional, they are limited, due to their high water binding capacity, in their ability to be used at high levels that may provide significant benefit as a fermentable food source for colon microorganisms. However, in general, highly soluble fibers, those that are highly branched or are relatively short chain polymers, such as gum arabic, isolated arabinogalactans, inulins, and oligosaccharides have low viscosities. These low viscosity fibers are generally used to modify texture or rheology, manage water migration, influence the colligative properties of the food system, and improve the marketability of the food product as a health-promoting or functional food product. These fiber sources can be used in food products at relatively high levels, as they typically enhance the food product’s taste, mouthfeel, and shelf life without significantly altering the specific application characteristics. For example, sugar-free and fat-free products also have potential for high fiber claims and marketed as supplements. Gelation is an important attribute of some fiber ingredients as a means to add form or structure to various food products. Gelation represents the association of polymer units to form a network of junction zones. The gel formed by this process encapsulates water and other components in solution to form a firm 3-dimensional structure. Gel formation depends on the type of gum, its concentration, temperature, presence of ions (for example, calcium), pH, and the presence of other rheology modifiers in the food system. Gums, including guar gum, gum arabic, karaya, carrageenan, and tragacanth gum, as molecular gel forming polymers, are typically used as rheology modifiers or stabilizers
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FUNCTIONAL FOODS
in food systems. Particle gel forming polymers, such as starch and inulin, are typically used at much higher concentrations than the molecular gel forming gums, and are used with the gums in systems to influence system rheology and overall texture. Fiber ingredients interact with water differently, dictating how the fiber is used and how it functions in a food system. This interaction is generally described as water uptake, hydration, adsorption, absorption, binding, or holding, with the 2 most common being water-binding capacity (WBC) and water-holding capacity (WHC). While being used interchangeably, the terms are differentiated based on the ability of a fiber to retain water under stress. WHC refers to the amount of water the gel system retains within its structure without pressure or stress, while WBC refers to the amount of water the gel system retains after it is stressed, as following centrifugation. The WBC likely has greater practicality, because food manufacturing/processing typically uses some form of physical stress (for example, extrusion, mixing or kneading, homogenization, and so on). The fiber source does not directly influence the WBC, but rather the source determines the physicochemical properties of the fiber ingredient, such as fiber length, particle size, and porosity. These properties in turn influence the WBC and its use and the conditions of its use in food development. However, other factors in the food system can also influence WBC, such as pH, ionic strength, concentration of the fiber component, and interaction with other waterbinding ingredients (that is, sugar, starches, and so on). By contrast, water interactions with soluble fibers are more greatly influenced by pH and ionic strength. Many dietary fibers are fat and/or oil dispersible, and some also bind oil. Oil binding is in part related to its chemical composition, but is more largely a function of the porosity of the fiber structure rather than the affinity of the fiber molecule for oil. By hydrating a fiber with water, the water occupies the fiber pores, significantly reducing oilbinding. This technique is used successfully with batters and film coatings to reduce the oil uptake during frying operations, and reduces the total fat content of the final food product, enhancing crispiness. Some dietary fibers, fruit and vegetable fiber that have cation exchange capacity (CEC) from unmethylated galacturonic acid residues and phytic acid from cereal fiber, are also able to bind cations such as calcium, cadmium, zinc, and copper. The CEC is influenced by the type of fiber (its makeup), the system pH and ionic strength, and the chemical nature of the cation. Cho et al (1997) reported that some dietary fibers have also been shown to absorb organic molecules (for example, lignin
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103
binds bile acids, and wheat bran binds certain carcinogens like benzopyrazine). However the effect is pH dependent. These primary functional properties of several isolated fiber sources provide the means to make high-fiber foods with high eating quality, The main technological functions in food of isolated fiber components, such as pectin and guar gum, are as gelling and thickening agents. However, other food (fiber) ingredients are also available to modify or stabilize the texture of food product (for example alginates, carrageenans, cellulose and its derivatives, modified starch). Due to their relatively low level of use in prepared food systems, these polysaccharides are used mainly for their technological properties rather than their physiological significance. Yet they fit the general definition of dietary fiber and are considered as dietary fibers according to their physiology. In contrary to the fiber polysaccharides, NDO, and, more specifically, inulins, are used in food products because of their physiological functionality as prebiotic fiber as well as their technological characteristics. These dietary components function particularly well when used in sugar and fat replacer systems, as having synergy with high-water binding thickening agents to add texturizer, provide bulk, and enhance rheology in food product development. These combined properties provide a means to enhance health promoting, high-fiber food product development, without compromising taste. Worldwide Fiber Recommendations And Intake Most data in literature regarding intake of total dietary fiber (TDF) are dependent on the method(s) used to define their dietary content and are estimates using standard food tables. These data are derived from analysis of foods using official methods for fiber labeling, such as AOAC method 985.29. Typically, there has been little indication of the individual components making up this TDF value. More recent studies have used various analytical methods to determine intakes of various individual segments of the dietary fiber, such as the insoluble and soluble nonstarch polysaccharides. While specific official analytical methods are now available for meeting the requirements for measuring these dietary fiber components, only limited data are available. The National Cancer Institute (NCI) recommended the adult fiber consumption should be increased to 20 to 30 g daily, not to exceed 35 g due to research data suggesting fiber-containing foods provide some protection against colon and rectal cancer (Butrum et al., 1988). It was suggested that if these guidelines were followed, they may help reduce risk of these
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FUNCTIONAL FOODS
cancers. Reviewing recommendations for healthy populations from other agencies and countries suggest that fiber intakes should be increased, but the recommendations are somewhat unclear as to the amounts and types of fiber being recommended. Beneficial Claims For DF (1) A grain product, fruit, or vegetable that contains dietary fiber; low fat and good source of dietary fiber (without fortification) may be beneficial in preventing some types of cancer. (2) A grain product, fruit, or vegetable that contains dietary fiber: low saturated fat, low cholesterol, and low fat, particularly soluble fiber (0.6 g per Reference Amount [RA] without fortification), may reduce the risk of coronary heart disease. (3) A fruit or vegetable, low in fat with good source of vitamin A, vitamin C or dietary fiber (without fortification) may reduce the risks of some types of cancer. Soluble fiber must be labeled. (4) Soluble fiber from: (1) β-glucan soluble fiber from oat bran, rolled oats (oatmeal) and whole oat flour; and (2) psyllium husks may reduce the risk of heart disease if they are low in fat, saturated fat, cholesterol and include 0.75 g of whole oat soluble fiber or 1.7 g of psyllium husk soluble fiber per RA. Soluble fiber must be labeled. (5) Diets rich in whole grain foods and other plant foods and low in total fat, saturated fat, and cholesterol may reduce the risk of heart disease and some cancers. The food must contain 51% or more whole grain ingredients by weight per serving, and a dietary fiber content of at least 3.0 per RA of 55 g, 2.8 g per RA of 50 g, 2.5 g per RA of 45 g, 1.7 g per RA of 35 g, and be low in fat. OATS Oats (Avena sativa) is a whole kernel cereal consumed as groats milled to yield various products – rolled oats, oatmeal, oat flour. FDA has allowed a health claim for an association between consumption of diets high in oat meal, oat bran, or oat flour and reduced risk of coronary heart disease. This is the first health claim for a specific food under Nutrition Labeling and Education Act (NLEA). Oats based functional foods, oats yogurt, and a symbiotic oats based beverage have been developed in the author’s laboratory. Oat Bran Oat bran is the food which is produced by grinding clean oat groats or rolled oats and separating the resulting oat flour by sieving,
DIETARY FIBER
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bolting and/or other suitable means into fractions so that the oat bran fraction is not more than 50% of the starting material, and has total βglucan content of at least 5.5% (dry weight basis) and a total dietary fiber content of at least 16% (dry weight basis), and so that one third of the total dietary fiber is soluble fiber (AACC). Oat bran concentrate Natureal GI from GTC Nutrition, Golden, Colo., is high in β-glucan and said to slow the uptake and release of energy from a meal and exert positive control over healthy blood glucose levels and subsequent inulin response. Tappy et al (1996) found that 5 g of βglucan reduced glycemic response by 50% in a 35 g carbohydrate meal. The soluble fiber in Natureal oat bran concentrate has been associated with benefits for blood sugar control and satiety, and with weight management. The effectiveness of ingredient for control of glycemic response is attributed to the viscous nature of β-glucan. Viscosity is critical to slowing stomach emptying and regulating the uptake of energy from a meal. This effect is important to controlling glycemic response and blunting the after-meal insulin surge, which is thought to have positive effects for healthy weight maintenance. β -glucan β-glucan is distributed throughout the endosperm and located in the endosperm cell walls and constitutes 75%. It is a linear, unbranched polysaccharide composed of 4-O-linked β-D-glucopyranosyl units (70%) and 3-O-linked β-D-glucopyranosyl units (30%), MW=1.5-3.0 X 106. There is 3-11% in barley, 3-7% in oats and <1% in wheat. β -glucan Isolate There is no pure form of oat β-glucan. A new product high in β-glucan called Oatrim by Quaker Oats is used as soluble fiber or a fat replacer. Oatrim is prepared by extraction of oats or oat bran with hot water containing heat soluble -amylase. Oats Based Functional Foods Dietary oats have been shown to confer a number of significant physiological effects in the prevention or alleviation of disease and thus may be considered as a multifunctional food. Soluble fiber favors the growth of probiotic bacteria. Oats contain a high percentage of desirable complex carbohydrates that may reduce the risk of certain cancers and constipation and also promotes a good balance of fatty acids. The key cholesterol lowering ingredient in oats is soluble fiber. It works by binding cholesterol-containing bile acids produced in the liver and speeding their exit from the body. It also helps control diabetes by
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FUNCTIONAL FOODS
preventing erratic swings in blood sugar levels. Soluble fiber slows down the absorption of sugar from the intestines into the blood. It also increases the cell’s insulin sensitivity and assists the cells in drawing sugar from the blood. Due to these benefits, there is a great interest in increasing the consumption of products based on oats that contain both soluble and insoluble fibers. However, consumption of oat-based products is low mainly due to the lack of acceptable food products based on oats. So oats based symbiotic beverage and yogurt were developed in the author’s laboratory with the idea of preparing products having two healthy components, dietary fiber of oats and probiotic lactic acid bacteria wherein the oats is fermented by probiotic bacteria itself. Symbiotic Oats Beverage Oats flour (3-7%, w/v), sugar (4-8%, w/v), inulin (0.2%, w/v) and whey protein concentrate (0.5%, w/v) were blended in water to make a homogenous slurry. The slurry was slowly heated at the rate of 1°C per min. to boiling and boiled for 3 min. to break down the starch content of oats. The cooked slurry was sterilized at 121°C for 15 min. The slurry was then cooled to 37°C stirring at intervals to avoid formation of a layer on the surface. This was inoculated with probiotic culture and mixed well so that the culture was distributed evenly in the slurry. Fermentation was carried out at 37°C in aerobic environment. Oats flour was the main ingredient in the product. We chose to use 5% because less than that would not provide the required amount of dietary fiber, and it was required to add more of inulin to meet the dietary fiber requirement. But if more was used the product would be more viscous, and it would not be a beverage. 0.2% inulin was used to increase the soluble dietary fiber content to meet the requirement (0.75 g/serving). Inulin acts as a water binder, stabilizer and texturizer in addition to being a prebiotic. Whey protein concentrate was used at the level of 0.5%. This was just enough to make a homogenous product, and more than this would coagulate the slurry when it was autoclaved. Whey protein concentrate acts as a stabilizer that makes the product homogenous. Sugar (4%) was added as energy source for the cultures and to give a balanced taste of sweet and sour to the product. All the ingredients and water were blended in a blender to make a homogenous slurry. The slurry was cooked and autoclaved. The beverage was homogenous, free flowing and had smooth texture compared to the unfermented slurry. The pH, titratable acidity and viscosity of the control sample (without fermentation) were 6.3, 0.032% and 420 mPas, respectively. After fermentation for 12 h, there was a
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significant change in pH which reduced to 3.63, titratable acidity which increased to 0.21% and viscosity that decreased to 222 mPas. The product had a good balance of sweet and sour tastes. The beverage provides 0.80 g of soluble dietary fiber per serving (150 mL) which meets the FDA requirements (0.75 g per serving). Since this product contains only plant origin raw materials and whey protein concentrate, it can be considered as low in saturated fat and cholesterol free. It is also suitable for enrichment with traditional flavors. This oats based beverage is typically a non-dairy vegetarian product containing no milk. It serves as an alternative to both dairy and soy beverages and suits a healthy life-style whether vegetarian-oriented or not. Symbiotic Oats Yogurt Symbiotic oats yogurt was formulated as a multifunctional product that delivered dietary fiber and viable probiotic lactic acid bacteria, in which the bacteria fermented the oats, and prepolymerized whey proteins were utilized to form a gel. Preparation of symbiotic oats yogurt is presented in Figure 3.7. The oats yogurt provides 1.68 g of soluble dietary fiber per serving (240ml) which meets the FDA requirement (0.75 g per serving) for labeling. Since this product contains only raw materials from plant origin and whey protein isolate, it can be considered low in saturated fat and cholesterol free. Other oats-based functional foods are being developed in the author’s laboratory. SUMMARY Dietary fiber, an essential part of a healthy diet, provides many health benefits which have been researched to a large extent. This chapter deals in detail about its role in human health and also its functionality in foods as a functional ingredient and/or food. Dietary fiber also known as roughage or bulk includes all parts of plant foods that our body can’t digest or absorb. The term ‘Dietary fiber’ was coined by Hipsley in 1953 and is defined as the edible parts of the plant and analogous carbohydrate that are resistant to digestion and absorption in the human small intestine with complete or partial fermentation in the large intestine. It includes polysaccharides, lignin and associated plant substances. Dietary fiber exhibits one or more of laxation (fecal bulking and softening; increased frequency; and/or regularity), blood cholesterol attenuation, and/or blood sugar regulation.
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FIGURE 3.7 — Preparation Of Symbiotic Oats Yogurt
Dietary fiber is classified into different types based on solubility, fermentability and the arrangement of monomeric units. There are also different methods of analysis of dietary fiber which can be modified based on which fiber type we want to analyse. The physiological benefit of various dietary fiber types depends on how they are metabolized in the body which in turn depends on their physicochemical characteristics. Dietary fiber has been found to have
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an important role in preventing chronic diseases such as cancer, cardiovascular disorders, diabetes, gastrointestinal disorders and also in improving mineral bioavailability. The primary properties provided by isolated fibers for food product development are solubility, viscosity, and gelation-forming ability, waterbinding capacity, oil-binding capacity, and mineral and organic moleculebinding capacity. Oats have been popularly known as a functional food. The first health claim that FDA has allowed for a specific food under The Nutrition Labeling and Education Act (NLEA) is a health claim for an association between consumption of diets high in oat meal, oat bran, or oat flour and reduced risk of coronary heart disease. The main component that makes oats a functional food is β-glucan, a soluble dietary fiber. Focusing on the physiological health aspect of dietary fiber and the types of fibers meeting a physiological definition will help to make the consumer understand the importance of fiber in their diets. References Alles, M. S., Hartemink, R., Meyboom, S., Harry, J. L., van Laere, K. M. J., van Nagengast, F. M. and Hautvast, J. C. A. J. 1999. Effect of transgalactooligosaccharides on the composition of the human intestinal microflora and on putative risk markers for colon cancer. Am. J. Clin. Nutr. 69:980-991. Anderson, J. W., Story, L., Sieling, B., Chen, W. J. L., Petro, M. S. and Story, I. 1984. Hypocholesterolemic effects of oat-bran or bean intake for hypercholesterolemic men. Am. J. Clin. Nutr. 40:1146-1155. Anderson, J. W., Allgood, L. D., Lawrence, A., Altringer, L. A., Lerdack, G. R., Hengehold, D. A. and Morel, J. G. 2000. Cholesterol-lowering effects of psyllium intake adjunctive to diet therapy in men and women with hypercholesterolemia: meta-analysis of 8 controlled trials. Am. J. Clin. Nutr. 71:472-479. Baghurst, P. A., Baghurst, K. I. and Record, S. J. 1996. Dietary fiber, nonstarch polysaccharides and resistant starch: a review. Food Australia 48(3):1-36S. Birkett, A., Muir, J., Phillips, J., Jones, G. and O’Dea, K. 1996. Resistant starch lowers fecal concentrations of ammonia and phenols in humans. Am. J. Clin. Nutr. 63:766772. Boeckner, L. S., Schnepf, M. I. and Tungland, B. C. 2000. Inulin: A review of nutritional and health implications. Adv. Food Nutr. Res. 43:1-63. Bourquin, L. D., Titgemeyer, E. C., Garleb, K. A. and Fahey, G. C. 1996. Fermentation of various dietary fiber sources by human fecal bacteria. Nutr. Res. 16:1119-1131. Brown, L., Rosner, B., Willett, W. W. and Sacks, F. M. 1999. Cholesterol-lowering effects of dietary fiber: a meta-analysis. Am. J. Clin. Nutr. 69:30-42. Butrum, R. R., Clifford, C. K. and Lanza, E. 1988. Dietary guidelines: Rationale. Am. J. Clin. Nutr. 48:888-895. Byers, T. 2000. Diet, colorectal adenomas, and colorectal cancer. N. Engl. J. Med. 342(16):1206-1207. Causey, J. L., Feirtag, J. M., Gallaher, D. D., Tungland, B. C. and Slavin, J. L. 2000. Effects of dietary inulin on serum lipids, blood glucose and the gastrointestinal environment in hypercholesterolemic men. Nutr. Res. 20(21):191-201.
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Ohta, A., Motohashi, Y. and Sakuma K. 1998. Dietary fructooligosaccharides change the concentration of calbindin-D9k differently in the mucosa of the small and large intestine of rats. J. Nutr. 128:934-939. Okubo, T., Ishihara, N., Takahashi, H., Fujisawa, T., Kim, M., Yamamoto, T. and Misuoka T. 1994. Effects of partially hydrolyzed guar gum intake in human intestinal microflora and its metabolism. Biosci. Biotechnol. Biochem. 58:1364-1369. Sakamoto, J., Nakaji, S., Sugawara, K., Iwane, S. and Munakata, A. 1996. Comparison of resistant starch with cellulose on 1,2-dimethylhydrazine-induced colon carcinogenesis in rats. Gastroenterol. 110:116-120. Satchithanandam, S., Vargofcak-Apker, M., Calvert, R. J., Leeds, A. R. and Cassidy, M. M. 1990. Alteration of gastrointestinal mucin by fiber feeding in rats. J. Nutr. 120:1179-1184. Scheppach, W. 1998. Butyrate and the epithelium of the large intestine. In: Guillon, F., Amado, R., Amaral-Collaco, M. T., Andersson, H., Asp, N. G., Bach Knudsen, K. E., Champ, M., Mathers, J., Robertson, J. A., Rowland, I., Van Loo, J. editors. Proceedings of the Profiber Conference: Functional Properties of Nondigestible Carbohydrates. Lisbon, Portugal; Feb. P 215. Silk, D. B. A., Walters, E. R., Duncan, H. D., Green, C. J. 2001. The effect of a polymeric enteral formula supplemented with a mixture of six fibres on normal human bowel function and colonic motility. Clin. Nutr. 20:49-58. Smith, T., Brown, J. C. and Livesey, G. 1998. Energy balance and thermogenesis in rats consuming nonstarch polysaccharides of various fermentabilities. Am. J. Clin. Nutr. 68:802-819. Strohmaier, W. 1996. Lactulose, an innovative food ingredient - physiological aspects. In: Proceedings of FIE Conference; Paris; 12-14 Nov. 1996. Maarssen, Netherlands: Miller Freeman. P 69-72. Tappy, L., Gugolz, E. and Wursch, P. 1994. Effects of breakfast cereals containing various amounts of beta-glucan fibers on plasma glucose and insulin responses in NIDDM subjects. Diabetes Care. 19:831-834. Tungland, B. C. and Meyer, D. 2002. Nondigestible Oligo- and Polysaccharides (Dietary Fiber): Their Physiology and Role in Human Health and Food. Comprehensive Reviews in Food Science and Food Safety. 3:73-92. Vanhoof, K. and De Schrijver, R. 1996. Nitrogen metabolism in rats and pigs fed inulin. Nutr. Res. 16:1035-1039. Wasan, H. S. and Goodlad, R. A. 1996. Fiber-supplemented foods may damage your health. Lancet 348:319-320. Younes, H., Garleb, K., Behr, S., Remsey, C. and Demigne, C. 1995. Fermentable fibers or oligosaccharides reduce urinary nitrogen excretion by increasing urea disposal in the rat caecum. J. Nutr. 125:1010-1016. Younes, H., Demigne, C. and Remesy, C. 1996. Acidic fermentation in the caecum increases absorption of calcium and magnesium in the large intestine of the rat. Br. J. Nutr. 75:301-314. (Guo, M.R., Gokavi, S.)