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Starch: Modification
Starch: Modification JN BeMiller, Purdue University, West Lafayette, IN, USA ã 2016 Elsevier Ltd. All rights reserved.
Topic Highlights
• • • • • •
Most starch used in processed food and other industrial applications has been modified. Modification is done to enhance the starch’s desirable attributes, to minimize its undesirable characteristics, or to add a new functionality. Modifications can be chemical and/or physical. Cross-linking is the most important modification of a food starch. Stabilization is important for both processed food and nonfood applications. Hydroxyethylated starches and cationic starches are widely used in the paper industry.
Learning Objectives
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To identify the major means of modifying starch chemically and physically. To name some properties that can be modified by each type of modification. To point out some major applications of modified starches.
Introduction Commercial sources of starches include cereal grains such as corn/maize (including waxy maize and high-amylose corn), wheat, and rice (various types). (The sago palm and roots and tubers, such as potato, cassava/tapioca, and arrowroot, are other sources.) Corn is by far the principal source of starch, with cornstarch making up 80% of the world’s commercial supply of starch. An additional 8% is obtained from wheat. The largest industrial use of starch is the production of fuel ethanol and glucose and high-fructose syrups. The remaining isolated starch is used in processed food and other industrial applications, most often after being chemically and/or physically modified in at least one way and often in multiple ways. Modification is done to enhance the starch’s desirable attributes and to minimize its undesirable characteristics. Of the starch that is used industrially (in the United States), the greatest amount is used in papermaking, which (in the United States) consumes approximately two-thirds of the remaining starch. This article emphasizes modified food starch. Starch to be used in papermaking undergoes modifications different than those to be used in food applications. Nevertheless, modified food starches exemplify why and how starches are modified.
Background Food processors generally require starches with better behavioral characteristics than are provided by native starches. Cereal
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starches produce weak-bodied, cohesive, rubbery pastes and undesirable gels when cooked. However, the functional properties of starches can be greatly improved via small amounts of modification. Modification is done to introduce specific functionalities; to make resultant cooked products better able to withstand the conditions of heat, shear, and pH (acid) associated with processing conditions; or to improve the stability of the product in which the starch is used as an ingredient. The final products are abundant, functional, and useful food ingredients, generally macroingredients. Modifications can be chemical, physical, or genetic (not covered in this article). Chemical modifications are oxidation, cross-linking, stabilization, and depolymerization. Commercial physical modifications are various types of thermal treatments. Chemical modifications have the greatest effects on functionalities. Modifications can be single modifications, but modified starches are often prepared by combinations of two, three, and sometimes four processes. Ether and ester derivatives found in modified food starches (in the United States) are the following: 1. Stabilized starches a. Hydroxypropyl starches (starch ether) b. Starch acetates (starch ester) c. Starch octenylsuccinates (monostarch ester) d. Monostarch phosphate (ester) e. Starch succinate (ester) 2. Cross-linked starches a. Distarch phosphate b. Distarch adipate 3. Stabilized and cross-linked starches a. Hydroxypropylated distarch phosphate b. Phosphorylated distarch phosphate c. Acetylated distarch phosphate d. Acetylated distarch adipate Some property improvements that can be obtained by chemical modifications include the following: 1. Cross-linked starches a. Increased gelatinization and pasting temperatures b. Increased shear resistance c. Increased acid stability d. Decreased setback of pastes and gels (improved paste stability) e. Increased viscosity of pastes 2. Stabilized starches a. Lower gelatinization and pasting temperatures b. Improved freeze–thaw stability of pastes and gels c. Decreased setback of pastes and gels (improved paste stability) d. Greater clarity of pastes and gels e. Easier redispersibility when pregelatinized 3. Hydrolytic cleavage a. Acid-modified starches
Encyclopedia of Food Grains, Second Edition
http://dx.doi.org/10.1016/B978-0-12-394437-5.00147-9
PROCESSING OF GRAINS | Starch: Modification
i. Decreased viscosity of pastes ii. Lower gelatinization and pasting temperatures iii. Increased solubility iv. Increased gel strength v. Increased clarity of gels b. Dextrins i. Highly water-soluble ii. Better film formation 4. Hypochlorite-oxidized starches a. Lower gelatinization and pasting temperature b. Decreased maximum paste viscosity c. Softer, clearer gels d. Whiter 5. Cross-linked and stabilized starches a. Lower gelatinization and pasting temperatures b. Increased paste viscosity c. Other attributes of stabilized and cross-linked products Any starch (corn, wheat, rice, potato, tapioca/cassava, etc.) can be modified, but modification is practiced significantly only on corn (both normal corn and waxy maize) and potato starches and, to a lesser extent, on wheat and tapioca/cassava starches. This article is written primarily from the point of view of normal corn and waxy maize starches.
Methods of Production and Applications Cross-linked and/or stabilized starch products are prepared by chemical derivatization of a starch, most often in an aqueous slurry in a batch process. In such a process, a slurry of 30–45% solids (starch) as obtained from the wet-milling operation is introduced into a stirred reaction tank. Sodium sulfate or sodium chloride is added to inhibit granule swelling. The pH is adjusted with sodium hydroxide (to values of 8.5 to 11.3, depending on the reaction). A chemical reagent is added. Reactions may be done at controlled temperatures up to 50 C, but pasting must be avoided to allow recovery of the modified starch in granule form by filtration or centrifugation. Because the gelatinization temperature may be lowered by the modification, there may be, and often is, a limit to the degree of substitution (DS) that can be made in this manner. (The DS is the average number of hydroxyl groups per a-D-glucopyranosyl unit (the monomeric unit of starch) that have been derivatized, the maximum being 3.) In some reactions, the pH needs to be controlled by the metered addition of dilute sodium hydroxide solutions. Following modification to the desired level, the reaction mixture is neutralized and the starch is recovered by centrifugation or filtration, washed, and dried. Chemical reactions currently both allowed and used to prepare modified food starches in the United States are as follows:
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Esterification with acetic anhydride, succinic anhydride, the mixed anhydride of acetic and adipic acids, 2octenylsuccinic anhydride, phosphoryl chloride, sodium trimetaphosphate, sodium tripolyphosphate, or monosodium orthophosphate Etherification with propylene oxide Acid modification with hydrochloric or sulfuric acids Bleaching with hydrogen peroxide, peracetic acid, potassium permanganate, or sodium hypochlorite
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Oxidation with sodium hypochlorite Various combinations of these reactions
Other reagents may be used in other countries. The most important are cross-linking with epichlorohydrin, acetylation by transesterification with vinyl acetate, and oxidation with hydrogen peroxide plus copper(II). The first two are allowed modifications in the United States, but are not practiced. Waxy maize starch modifications are especially popular in the US food industry because the inherent properties of waxy maize starch are preferred over modifications to normal corn starch.
Cross-linking Cross-linking is the most important modification of a food starch. Cross-linking occurs when starch granules are reacted with bifunctional reagents to connect hydroxyl groups on two different molecules within the granule. Cross-links reinforce the granule and reduce both the rate and the degree of granule swelling and subsequent disintegration, that is, reduce sensitivity to processing conditions (high temperature, extended cooking times, low pH, and high shear during mixing, milling, homogenization, and/or pumping). Cooked pastes of most cross-linked starches are more viscous, heavier-bodied, shortertextured, and less likely to break down with extended cooking times, greater acidity, or severe agitation than are pastes of the native starches from which they are prepared. Only a small amount of cross-linking is required to produce a noticeable effect. Most cross-linked starches contain less than one crosslink for every 1000 a-D-glucopyranosyl units. At this very low level of cross-linking, both the rate and the degree of granule swelling are reduced, paste stability is increased, and both the viscosity profile as the starch is cooked and the textural characteristics of its paste are improved, giving products in which the starch is used as an ingredient desirable bulk and body. By far, the most common cross-links are distarch phosphate esters. These distarch phosphates are prepared primarily by reaction with phosphorus oxychloride (phosphoryl chloride). To prepare cross-linked starches with phosphorus oxychloride, the reagent is added to an aqueous starch suspension of pH 8–12. Sodium trimetaphosphate or mixtures of sodium trimetaphosphate and sodium tripolyphosphate may also be used to make distarch phosphates. To cross-link a starch with sodium trimetaphosphate, it is slurried in a solution of the reagent at pH 5.0–8.5; the suspension is filtered, and the starch is dried and heated. Some cross-linked starch is made by reaction of corn starch with the mixed anhydride of adipic and acetic acids in an aqueous alkaline suspension.
Stabilization The process of introducing stabilization to a starch is sometimes called substitution. Derivatization of a starch with monofunctional reagents reduces the intermolecular associations that result in gelation of its paste and/or precipitation of the starch polymers. (The combined processes are termed retrogradation or setback.) The use of a stabilized starch product
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PROCESSING OF GRAINS | Starch: Modification
improves both the texture and the appearance of the food product in which it is used as an ingredient. Pastes of unmodified normal starches generally will gel, and the gels will usually be cohesive, rubbery, long-textured, and prone to syneresis. (Waxy maize starch pastes gel to a limited extent at room temperature, but will become cloudy and chunky and exhibit syneresis when stored under freezing or refrigerated conditions.) Stabilization/substitution of starch lowers its gelatinization temperature, an indication of a weakening of granules. Upon cooking, a higher peak viscosity is obtained due to greater granule swelling. Upon cooling of the resulting paste, the viscosity becomes lower than that obtained from the unmodified starch, an indication of improved stability, that is, less retrogradation, and the product changes less with time, even under refrigerated or freeze–thaw conditions. Derivatives most often employed for starch stabilization are the hydroxypropyl ether, the acetate ester, and the monostarch phosphate ester. Hydroxypropyl ether derivatives of starches are prepared by reacting a starch with an alkaline (pH 11.3) slurry containing a swelling-inhibiting salt. The reactor is charged with propylene oxide and sealed. Reaction is continued for 24 h at 49 C. In the United States, 0.2 (7.0% of hydroxypropyl groups) is the maximum allowable moles of substitution (MS). (MS is the moles of hydroxypropyl ether groups per a-D-glucopyranosyl unit. A MS value of 0.2 indicates an average of two hydroxypropyl groups per ten glucosyl units. In the context of these stabilized starch products, DS and MS are essentially the same.) Acetylation is accomplished by treating a starch slurry with acetic anhydride at pH 7–11, the optimum pH depending on the reaction temperature. Acetylated starches with an acetyl content of up to 2.5% (DS, 0.09) can be used in food products (the United States). (A DS of 0.09 indicates an average of nine acetyl groups per 100 a-D-glucopyranosyl units.) The acetate ester linkage is less stable than the hydroxypropyl ether linkage. Sodium phosphate monoesters are prepared by impregnating the starch with a solution of sodium orthophosphate. After adjustment of the pH to 5.0–6.5, the slurry is mixed and then filtered, and the filter cake is dried and heated. Monostarch phosphates produce stable pastes that are clear and have a long, cohesive texture. Paste viscosity can be controlled by varying the concentration of phosphate salt, time of reaction, temperature, and pH. Increasing substitution lowers the gelatinization temperature; products become cold-water-swelling at DS 0.07. Cornstarch phosphates of DS 0.01–0.03 produce pastes with hot viscosity, clarity, stability, and texture more like those of a potato starch. Starch phosphates are good emulsion stabilizers and produce pastes with improved freeze–thaw stability. Sodium tripolyphosphate may be used to make products of up to 0.002 DS (one phosphate group per 500 a-Dglucopyranosyl units), the maximum allowed in the United States. The process for this reaction is essentially the same as that using orthophosphate except that the pH is 5.0–8.5. Starch succinate half-esters are prepared by reacting starch with succinic anhydride.
Starches with Hydrophobic Groups Reaction of starch with 2-octenylsuccinic anhydride introduces hydrophobic substituent groups. Such derivatives can be used
as both emulsifiers and emulsion stabilizers in products based on oil-in-water emulsions, such as pourable dressings and flavored beverages. Flavor oil emulsions containing a thinboiling starch or dextrin (see later text) derivatized with 2octenylsuccinate ester groups may be spray-dried. The flavor oil in the resulting powder is protected against oxidation, and the emulsion will reform when the powder is stirred into an aqueous medium. Higher-DS products are nonwetting and are used as release agents for dusting on dough sheets and as processing aids. The maximum DS level allowed in the United States is 0.02.
Hydrolytic Cleavage Acid-modified (also known as thinned, thin-boiling, fluid, and fluidity) starches are prepared by treating a suspension of a native or derivatized starch with dilute mineral acid at a temperature below its gelatinization temperature. When a product that gives the desired cooked paste viscosity is produced, the acid is neutralized, and the product is recovered by centrifugation or filtration, washed, and dried. Even though only a few glycosidic bonds are hydrolyzed, granules disintegrate more easily and after only a small degree of swelling. Acid-modified starches form gels with improved clarity and increased strength, even though their pastes are less viscous. They are used as film formers and adhesives in products such as pancoated nuts and candies and in processed cheese loaves and whenever a strong gel is desired, for example, in gum candies such as jelly beans, jujubes, orange slices, and spearmint leaves. To prepare especially strong and fast-setting gels, a high-amylose corn starch is used. More extensive modification with acid produces dextrins. Dextrins are both more extensively depolymerized and more highly branched and, hence, are more water-soluble than are thin-boiling starches. They find use as encapsulators and carriers of flavors, especially spray-dried flavors, and in coatings, the latter because of their film-forming and adhesive properties. Yet more extensive hydrolysis, done with various amylases, produces maltodextrins, cyclodextrins, various types of glucose syrups, and high-fructose syrups.
Oxidation Depolymerization, viscosity reduction, and decreased pasting temperature can also be achieved by oxidation with sodium hypochlorite (chlorine in an alkaline solution). Oxidation also reduces the association of amylose molecules, that is, results in some stabilization, via the introduction of small amounts of carboxylate and carbonyl groups. Because the starch polymer molecules of hypochlorite-oxidized starches are reduced in molecular weight, that is, depolymerized, to some extent, such oxidized starches have some of the characteristics of acid-modified products. Oxidized starches produce intermediate-viscosity pastes and soft gels and are used when these properties are needed. They are also used to improve adhesion of starch batters to fish, meat, and vegetables and in breadings. Mild treatment with sodium hypochlorite, hydrogen peroxide, or potassium permanganate simply bleaches the starch and reduces the count of viable microbes.
PROCESSING OF GRAINS | Starch: Modification
Pregelatinization Pregelatinized starches are precooked starches that can be dissolved in water at temperatures below the gelatinization temperatures of the parent starches; thus, these ‘instant’ starches need no cooking. Two processes are used to prepare a pregelatinized starch. In one, a starch slurry is simultaneously cooked and dried on hot rolls. In the other, a starch slurry is heated and subjected to high shear in the presence of less moisture in an extruder. Both types of products are then milled to the desired mesh size. Because pregelatinized starch products are powders prepared from dried pastes, generally few intact granules are present, although granule fragments may be. Both chemically modified and unmodified starches can be pregelatinized. If chemically modified starches are used, the properties introduced by the modification(s) are found in the pregelatinized products; thus, paste properties, such as freeze–thaw stability, can be characteristics of pregelatinized starches. Several physical forms of pregelatinized starches are produced. For example, some will produce smooth solutions; others will produce pulpy or grainy dispersions and find use in fruit drinks and tomato products. Pregelatinized starches are often used in dry mixes, as are maltodextrins, because they disperse readily, even when mixed with other ingredients. Starches that are not pregelatinized are known as cook-up starches.
Cold-Water-Swelling Starches Products that are gelatinized starches, that is, starches that have lost their crystallinity, but that retain their granular form, in contrast to standard pregelatinized starches, are called coldwater-swelling starches or cold-water-soluble starches, even though standard pregelatinized starches are generally even more soluble in unheated water. There are two ways that such products are prepared commercially. One way is to heat a normal starch in an aqueous alcohol solution with sufficient water to allow gelatinization and sufficient alcohol that granule integrity is maintained. The other way is to quickly heat a starch slurry in a special spray-drying nozzle and dry the droplets in a spray dryer. Cold-water-swelling products swell rapidly and thicken unheated aqueous systems. (A granular, cook-up starch requires heating a slurry to its pasting temperature before thickening occurs.) They are particularly useful in making gum candies. Both types of pregelatinized starches are useful when no heat is available, thickening is desired before heating the formulation, no step utilizes sufficient heat to cook a starch, or heat cannot be applied because of the thermal lability of another ingredient.
Other Thermal Treatments Heat-moisture treatment (HMT) involves heating starch granules with a moisture content of <35% at 80–140 C (above the starch’s glass transition temperature, but below its gelatinization temperature, at the moisture content employed). When HMT starches are cooked, they exhibit a higher pasting temperature, reduced granule swelling, reduced paste viscosity, and reduced leaching of amylose and an increase in hardness of the resulting gel. Slowly digestible starch (SDS) and resistant starch (RS) contents are somewhat increased (RS > SDS). HMT may also be practiced with flours.
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Annealing occurs when granular starch is heated in water close to, but below, the temperature at which the onset of gelatinization occurs. Annealing results in an increase in granule stability, the pasting temperature, and the hardness of any resulting gel and a decrease in granule swelling, amylose leaching, and retrogradation. The content of RS often increases. Because there are so many variables involved (type of starch, moisture content, time and temperature of heating, and heating method) in HMT and annealing, there is a wide range of attributes possible in both products. Starches with acid-, shear-, and temperature-tolerance profiles similar to those of chemically cross-linked starches can be prepared by heating a starch with a low (<15%) moisture content at a temperature above 100 C, but below that which effects thermal degradation. Drying to <1% moisture before heating and a pH of 8.0–9.5 helps change the properties of the starch. This process is practiced to increase the contents of RS and SDS and thereby to increase the content of dietary fiber in food products.
Multiple Modifications Modified food starches are tailor-made for specific applications. Most modified food starches are made by cross-linking, introduction of monosubstituent groups (stabilization), or a combination of these two approaches. Many products, in fact, have received two or more modifications. For example, a modified food starch may be a cross-linked and stabilized waxy maize starch; another may be a stabilized, acid-thinned, and pregelatinized normal corn starch. Characteristics that can be controlled/improved by multiple modifications include, but are not limited to, one or more of the following:
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Adhesion Clarity of solutions/pastes Color Emulsion stabilization Film formation Flavor release Hydration rate Moisture retention and control in product Mouthfeel of product Oil migration control in product Paste texture/consistency Product form (liquid, semisolid, and solid) Sheen of product Shelf stability of product Stability to acids Stability to heat Stability to shear Tackiness Temperature required to cook Viscosity (hot paste and cold paste)
Digestion and Metabolism Various regulations concerning reagents that may be used and the maximum allowable modification of a starch for food use, alone or in combination with another modification, are in
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PROCESSING OF GRAINS | Starch: Modification
effect around the world. Generally, the level of substitution in a derivatized food starch is below DS 0.1 and in the range DS 0.002–0.2. Because of this low level of modification, the digestion, metabolism, and caloric values of modified food starches are reduced only to a minor, usually unmeasurable, extent as compared to native starches. Because only monosaccharides (D-glucose in this case) are absorbed, fragments containing esterified, etherified, or oxidized a-D-glucopyranosyl units should not be absorbed from the small intestine.
Modifications for Nonfood Applications The papermaking industry uses modified starches not approved for food use. In addition to hypochlorite-oxidized starch (approved for food use), starches oxidized with ammonium persulfate are used as a surface size. Hydroxyethyl ethers of starches, made by reacting a starch with ethylene oxide, are the products of choice for surface sizing and coating binding. Cationic starches are substituted with ether groups that contain a quaternary ammonium group. They are used in the formation of the paper sheet as retention or drainage aids. Carboxymethyl starch is an anionic derivative used as a thickener for coating colors and a coating binder. (When used in the pharmaceutical industry as a tablet disintegrant, carboxymethyl starch is called sodium starch glycolate.) Amphoteric starches may also be employed in papermaking. Graft copolymers, such as starch-g-styrene–butadiene latex, have been made for coating paper. Starch-g-polyacrylate/acrylamide is a superabsorbent and forms hydrogels. Dialdehyde starch is used as a wet strength agent in the production of tissue. Some of the same and similar modified starches are used in oil and gas production and in other applications.
Exercises for Revision
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What property improvements can be obtained by the following modifications: cross-linking, stabilization, acidmodification, dextrinization, and oxidation? What property changes can be imparted by pregelatinization, HMT, and annealing? What is the principle of making cold-water-swelling starch? What is the principle behind cross-linking?
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What is the principle behind stabilization? Why are starches often modified in more than one way?
Exercises for Readers to Explore the Topic Further
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What are the names and general structures of the starch polymers? What are the differences between a normal, waxy, and highamylose starch (e.g., in maize)? What are the meanings of the following starch terms: gelatinization, gelatinization temperature, pasting, pasting temperature, breakdown, setback, and retrogradation? Each native starch is unique. What are general characteristics of the following starches: normal corn/maize, waxy maize, high-amylose maize (amylomaize), wheat, rice, potato, and cassava/tapioca? What is the wet-milling process by which corn starches are isolated from kernels?
See also: Barley, Rice and Maize Processing: Maize: Wet Milling; Carbohydrates: Resistant Starch and Health; Starch: Chemistry; Starch: Starch Architecture and Structure; Genetics of Grains: Maize: Genetics; Maize: Other Maize Mutants; Rice: Genetics; Wheat Genetics and Genomics; Non-food Products from Grains: Fuel Alcohol Production; Processing of Grains: Starch: Uses of Native Starch; The Cereal Grains: Maize: Overview; Rice: Overview; Wheat: An Overview of the Grain That Provides ‘Our Daily Bread’.
Further Reading BeMiller JN (2007) Carbohydrate Chemistry for Food Scientists. St. Paul, MN: American Association of Cereal Chemists. BeMiller JN and Whistler RL (eds.) (2009) Starch: Chemistry and Technology, third ed. New York: Academic Press. Bertolini AC (ed.) (2010) Starch. Boca Raton, FL: Taylor & Francis. Eliasson A-C (ed.) (1996) Carbohydrates in Food. New York: Marcel Dekker. Light JM (1990) Modified food starches. Cereal Foods World 35: 1081–1092. Thomas DJ and Atwell WA (1997) Starches. St. Paul, MN: Eagen Press. Whistler RL, BeMiller JN, and Paschall EF (eds.) (1984) Starch: Chemistry and Technology, second ed. Orlando, FL: Academic Press. Wurzburg OB (ed.) (1986) Modified Starches: Properties and Uses. Boca Raton, FL: CRC Press.
Topic Highlights
• • • • • •
Most starch used in processed food and other industrial applications has been modified. Modification is done to enhance the starch’s desirable attributes, to minimize its undesirable characteristics, or to add a new functionality. Modifications can be chemical and/or physical. Cross-linking is the most important modification of a food starch. Stabilization is important for both processed food and nonfood applications. Hydroxyethylated starches and cationic starches are widely used in the paper industry.
Learning Objectives
• • •
To identify the major means of modifying starch chemically and physically. To name some properties that can be modified by each type of modification. To point out some major applications of modified starches.
Introduction Commercial sources of starches include cereal grains such as corn/maize (including waxy maize and high-amylose corn), wheat, and rice (various types). (The sago palm and roots and tubers, such as potato, cassava/tapioca, and arrowroot, are other sources.) Corn is by far the principal source of starch, with cornstarch making up 80% of the world’s commercial supply of starch. An additional 8% is obtained from wheat. The largest industrial use of starch is the production of fuel ethanol and glucose and high-fructose syrups. The remaining isolated starch is used in processed food and other industrial applications, most often after being chemically and/or physically modified in at least one way and often in multiple ways. Modification is done to enhance the starch’s desirable attributes and to minimize its undesirable characteristics. Of the starch that is used industrially (in the United States), the greatest amount is used in papermaking, which (in the United States) consumes approximately two-thirds of the remaining starch. This article emphasizes modified food starch. Starch to be used in papermaking undergoes modifications different than those to be used in food applications. Nevertheless, modified food starches exemplify why and how starches are modified.
Background Food processors generally require starches with better behavioral characteristics than are provided by native starches. Cereal
282
starches produce weak-bodied, cohesive, rubbery pastes and undesirable gels when cooked. However, the functional properties of starches can be greatly improved via small amounts of modification. Modification is done to introduce specific functionalities; to make resultant cooked products better able to withstand the conditions of heat, shear, and pH (acid) associated with processing conditions; or to improve the stability of the product in which the starch is used as an ingredient. The final products are abundant, functional, and useful food ingredients, generally macroingredients. Modifications can be chemical, physical, or genetic (not covered in this article). Chemical modifications are oxidation, cross-linking, stabilization, and depolymerization. Commercial physical modifications are various types of thermal treatments. Chemical modifications have the greatest effects on functionalities. Modifications can be single modifications, but modified starches are often prepared by combinations of two, three, and sometimes four processes. Ether and ester derivatives found in modified food starches (in the United States) are the following: 1. Stabilized starches a. Hydroxypropyl starches (starch ether) b. Starch acetates (starch ester) c. Starch octenylsuccinates (monostarch ester) d. Monostarch phosphate (ester) e. Starch succinate (ester) 2. Cross-linked starches a. Distarch phosphate b. Distarch adipate 3. Stabilized and cross-linked starches a. Hydroxypropylated distarch phosphate b. Phosphorylated distarch phosphate c. Acetylated distarch phosphate d. Acetylated distarch adipate Some property improvements that can be obtained by chemical modifications include the following: 1. Cross-linked starches a. Increased gelatinization and pasting temperatures b. Increased shear resistance c. Increased acid stability d. Decreased setback of pastes and gels (improved paste stability) e. Increased viscosity of pastes 2. Stabilized starches a. Lower gelatinization and pasting temperatures b. Improved freeze–thaw stability of pastes and gels c. Decreased setback of pastes and gels (improved paste stability) d. Greater clarity of pastes and gels e. Easier redispersibility when pregelatinized 3. Hydrolytic cleavage a. Acid-modified starches
Encyclopedia of Food Grains, Second Edition
http://dx.doi.org/10.1016/B978-0-12-394437-5.00147-9
PROCESSING OF GRAINS | Starch: Modification
i. Decreased viscosity of pastes ii. Lower gelatinization and pasting temperatures iii. Increased solubility iv. Increased gel strength v. Increased clarity of gels b. Dextrins i. Highly water-soluble ii. Better film formation 4. Hypochlorite-oxidized starches a. Lower gelatinization and pasting temperature b. Decreased maximum paste viscosity c. Softer, clearer gels d. Whiter 5. Cross-linked and stabilized starches a. Lower gelatinization and pasting temperatures b. Increased paste viscosity c. Other attributes of stabilized and cross-linked products Any starch (corn, wheat, rice, potato, tapioca/cassava, etc.) can be modified, but modification is practiced significantly only on corn (both normal corn and waxy maize) and potato starches and, to a lesser extent, on wheat and tapioca/cassava starches. This article is written primarily from the point of view of normal corn and waxy maize starches.
Methods of Production and Applications Cross-linked and/or stabilized starch products are prepared by chemical derivatization of a starch, most often in an aqueous slurry in a batch process. In such a process, a slurry of 30–45% solids (starch) as obtained from the wet-milling operation is introduced into a stirred reaction tank. Sodium sulfate or sodium chloride is added to inhibit granule swelling. The pH is adjusted with sodium hydroxide (to values of 8.5 to 11.3, depending on the reaction). A chemical reagent is added. Reactions may be done at controlled temperatures up to 50 C, but pasting must be avoided to allow recovery of the modified starch in granule form by filtration or centrifugation. Because the gelatinization temperature may be lowered by the modification, there may be, and often is, a limit to the degree of substitution (DS) that can be made in this manner. (The DS is the average number of hydroxyl groups per a-D-glucopyranosyl unit (the monomeric unit of starch) that have been derivatized, the maximum being 3.) In some reactions, the pH needs to be controlled by the metered addition of dilute sodium hydroxide solutions. Following modification to the desired level, the reaction mixture is neutralized and the starch is recovered by centrifugation or filtration, washed, and dried. Chemical reactions currently both allowed and used to prepare modified food starches in the United States are as follows:
•
• • •
Esterification with acetic anhydride, succinic anhydride, the mixed anhydride of acetic and adipic acids, 2octenylsuccinic anhydride, phosphoryl chloride, sodium trimetaphosphate, sodium tripolyphosphate, or monosodium orthophosphate Etherification with propylene oxide Acid modification with hydrochloric or sulfuric acids Bleaching with hydrogen peroxide, peracetic acid, potassium permanganate, or sodium hypochlorite
• •
283
Oxidation with sodium hypochlorite Various combinations of these reactions
Other reagents may be used in other countries. The most important are cross-linking with epichlorohydrin, acetylation by transesterification with vinyl acetate, and oxidation with hydrogen peroxide plus copper(II). The first two are allowed modifications in the United States, but are not practiced. Waxy maize starch modifications are especially popular in the US food industry because the inherent properties of waxy maize starch are preferred over modifications to normal corn starch.
Cross-linking Cross-linking is the most important modification of a food starch. Cross-linking occurs when starch granules are reacted with bifunctional reagents to connect hydroxyl groups on two different molecules within the granule. Cross-links reinforce the granule and reduce both the rate and the degree of granule swelling and subsequent disintegration, that is, reduce sensitivity to processing conditions (high temperature, extended cooking times, low pH, and high shear during mixing, milling, homogenization, and/or pumping). Cooked pastes of most cross-linked starches are more viscous, heavier-bodied, shortertextured, and less likely to break down with extended cooking times, greater acidity, or severe agitation than are pastes of the native starches from which they are prepared. Only a small amount of cross-linking is required to produce a noticeable effect. Most cross-linked starches contain less than one crosslink for every 1000 a-D-glucopyranosyl units. At this very low level of cross-linking, both the rate and the degree of granule swelling are reduced, paste stability is increased, and both the viscosity profile as the starch is cooked and the textural characteristics of its paste are improved, giving products in which the starch is used as an ingredient desirable bulk and body. By far, the most common cross-links are distarch phosphate esters. These distarch phosphates are prepared primarily by reaction with phosphorus oxychloride (phosphoryl chloride). To prepare cross-linked starches with phosphorus oxychloride, the reagent is added to an aqueous starch suspension of pH 8–12. Sodium trimetaphosphate or mixtures of sodium trimetaphosphate and sodium tripolyphosphate may also be used to make distarch phosphates. To cross-link a starch with sodium trimetaphosphate, it is slurried in a solution of the reagent at pH 5.0–8.5; the suspension is filtered, and the starch is dried and heated. Some cross-linked starch is made by reaction of corn starch with the mixed anhydride of adipic and acetic acids in an aqueous alkaline suspension.
Stabilization The process of introducing stabilization to a starch is sometimes called substitution. Derivatization of a starch with monofunctional reagents reduces the intermolecular associations that result in gelation of its paste and/or precipitation of the starch polymers. (The combined processes are termed retrogradation or setback.) The use of a stabilized starch product
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improves both the texture and the appearance of the food product in which it is used as an ingredient. Pastes of unmodified normal starches generally will gel, and the gels will usually be cohesive, rubbery, long-textured, and prone to syneresis. (Waxy maize starch pastes gel to a limited extent at room temperature, but will become cloudy and chunky and exhibit syneresis when stored under freezing or refrigerated conditions.) Stabilization/substitution of starch lowers its gelatinization temperature, an indication of a weakening of granules. Upon cooking, a higher peak viscosity is obtained due to greater granule swelling. Upon cooling of the resulting paste, the viscosity becomes lower than that obtained from the unmodified starch, an indication of improved stability, that is, less retrogradation, and the product changes less with time, even under refrigerated or freeze–thaw conditions. Derivatives most often employed for starch stabilization are the hydroxypropyl ether, the acetate ester, and the monostarch phosphate ester. Hydroxypropyl ether derivatives of starches are prepared by reacting a starch with an alkaline (pH 11.3) slurry containing a swelling-inhibiting salt. The reactor is charged with propylene oxide and sealed. Reaction is continued for 24 h at 49 C. In the United States, 0.2 (7.0% of hydroxypropyl groups) is the maximum allowable moles of substitution (MS). (MS is the moles of hydroxypropyl ether groups per a-D-glucopyranosyl unit. A MS value of 0.2 indicates an average of two hydroxypropyl groups per ten glucosyl units. In the context of these stabilized starch products, DS and MS are essentially the same.) Acetylation is accomplished by treating a starch slurry with acetic anhydride at pH 7–11, the optimum pH depending on the reaction temperature. Acetylated starches with an acetyl content of up to 2.5% (DS, 0.09) can be used in food products (the United States). (A DS of 0.09 indicates an average of nine acetyl groups per 100 a-D-glucopyranosyl units.) The acetate ester linkage is less stable than the hydroxypropyl ether linkage. Sodium phosphate monoesters are prepared by impregnating the starch with a solution of sodium orthophosphate. After adjustment of the pH to 5.0–6.5, the slurry is mixed and then filtered, and the filter cake is dried and heated. Monostarch phosphates produce stable pastes that are clear and have a long, cohesive texture. Paste viscosity can be controlled by varying the concentration of phosphate salt, time of reaction, temperature, and pH. Increasing substitution lowers the gelatinization temperature; products become cold-water-swelling at DS 0.07. Cornstarch phosphates of DS 0.01–0.03 produce pastes with hot viscosity, clarity, stability, and texture more like those of a potato starch. Starch phosphates are good emulsion stabilizers and produce pastes with improved freeze–thaw stability. Sodium tripolyphosphate may be used to make products of up to 0.002 DS (one phosphate group per 500 a-Dglucopyranosyl units), the maximum allowed in the United States. The process for this reaction is essentially the same as that using orthophosphate except that the pH is 5.0–8.5. Starch succinate half-esters are prepared by reacting starch with succinic anhydride.
Starches with Hydrophobic Groups Reaction of starch with 2-octenylsuccinic anhydride introduces hydrophobic substituent groups. Such derivatives can be used
as both emulsifiers and emulsion stabilizers in products based on oil-in-water emulsions, such as pourable dressings and flavored beverages. Flavor oil emulsions containing a thinboiling starch or dextrin (see later text) derivatized with 2octenylsuccinate ester groups may be spray-dried. The flavor oil in the resulting powder is protected against oxidation, and the emulsion will reform when the powder is stirred into an aqueous medium. Higher-DS products are nonwetting and are used as release agents for dusting on dough sheets and as processing aids. The maximum DS level allowed in the United States is 0.02.
Hydrolytic Cleavage Acid-modified (also known as thinned, thin-boiling, fluid, and fluidity) starches are prepared by treating a suspension of a native or derivatized starch with dilute mineral acid at a temperature below its gelatinization temperature. When a product that gives the desired cooked paste viscosity is produced, the acid is neutralized, and the product is recovered by centrifugation or filtration, washed, and dried. Even though only a few glycosidic bonds are hydrolyzed, granules disintegrate more easily and after only a small degree of swelling. Acid-modified starches form gels with improved clarity and increased strength, even though their pastes are less viscous. They are used as film formers and adhesives in products such as pancoated nuts and candies and in processed cheese loaves and whenever a strong gel is desired, for example, in gum candies such as jelly beans, jujubes, orange slices, and spearmint leaves. To prepare especially strong and fast-setting gels, a high-amylose corn starch is used. More extensive modification with acid produces dextrins. Dextrins are both more extensively depolymerized and more highly branched and, hence, are more water-soluble than are thin-boiling starches. They find use as encapsulators and carriers of flavors, especially spray-dried flavors, and in coatings, the latter because of their film-forming and adhesive properties. Yet more extensive hydrolysis, done with various amylases, produces maltodextrins, cyclodextrins, various types of glucose syrups, and high-fructose syrups.
Oxidation Depolymerization, viscosity reduction, and decreased pasting temperature can also be achieved by oxidation with sodium hypochlorite (chlorine in an alkaline solution). Oxidation also reduces the association of amylose molecules, that is, results in some stabilization, via the introduction of small amounts of carboxylate and carbonyl groups. Because the starch polymer molecules of hypochlorite-oxidized starches are reduced in molecular weight, that is, depolymerized, to some extent, such oxidized starches have some of the characteristics of acid-modified products. Oxidized starches produce intermediate-viscosity pastes and soft gels and are used when these properties are needed. They are also used to improve adhesion of starch batters to fish, meat, and vegetables and in breadings. Mild treatment with sodium hypochlorite, hydrogen peroxide, or potassium permanganate simply bleaches the starch and reduces the count of viable microbes.
PROCESSING OF GRAINS | Starch: Modification
Pregelatinization Pregelatinized starches are precooked starches that can be dissolved in water at temperatures below the gelatinization temperatures of the parent starches; thus, these ‘instant’ starches need no cooking. Two processes are used to prepare a pregelatinized starch. In one, a starch slurry is simultaneously cooked and dried on hot rolls. In the other, a starch slurry is heated and subjected to high shear in the presence of less moisture in an extruder. Both types of products are then milled to the desired mesh size. Because pregelatinized starch products are powders prepared from dried pastes, generally few intact granules are present, although granule fragments may be. Both chemically modified and unmodified starches can be pregelatinized. If chemically modified starches are used, the properties introduced by the modification(s) are found in the pregelatinized products; thus, paste properties, such as freeze–thaw stability, can be characteristics of pregelatinized starches. Several physical forms of pregelatinized starches are produced. For example, some will produce smooth solutions; others will produce pulpy or grainy dispersions and find use in fruit drinks and tomato products. Pregelatinized starches are often used in dry mixes, as are maltodextrins, because they disperse readily, even when mixed with other ingredients. Starches that are not pregelatinized are known as cook-up starches.
Cold-Water-Swelling Starches Products that are gelatinized starches, that is, starches that have lost their crystallinity, but that retain their granular form, in contrast to standard pregelatinized starches, are called coldwater-swelling starches or cold-water-soluble starches, even though standard pregelatinized starches are generally even more soluble in unheated water. There are two ways that such products are prepared commercially. One way is to heat a normal starch in an aqueous alcohol solution with sufficient water to allow gelatinization and sufficient alcohol that granule integrity is maintained. The other way is to quickly heat a starch slurry in a special spray-drying nozzle and dry the droplets in a spray dryer. Cold-water-swelling products swell rapidly and thicken unheated aqueous systems. (A granular, cook-up starch requires heating a slurry to its pasting temperature before thickening occurs.) They are particularly useful in making gum candies. Both types of pregelatinized starches are useful when no heat is available, thickening is desired before heating the formulation, no step utilizes sufficient heat to cook a starch, or heat cannot be applied because of the thermal lability of another ingredient.
Other Thermal Treatments Heat-moisture treatment (HMT) involves heating starch granules with a moisture content of <35% at 80–140 C (above the starch’s glass transition temperature, but below its gelatinization temperature, at the moisture content employed). When HMT starches are cooked, they exhibit a higher pasting temperature, reduced granule swelling, reduced paste viscosity, and reduced leaching of amylose and an increase in hardness of the resulting gel. Slowly digestible starch (SDS) and resistant starch (RS) contents are somewhat increased (RS > SDS). HMT may also be practiced with flours.
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Annealing occurs when granular starch is heated in water close to, but below, the temperature at which the onset of gelatinization occurs. Annealing results in an increase in granule stability, the pasting temperature, and the hardness of any resulting gel and a decrease in granule swelling, amylose leaching, and retrogradation. The content of RS often increases. Because there are so many variables involved (type of starch, moisture content, time and temperature of heating, and heating method) in HMT and annealing, there is a wide range of attributes possible in both products. Starches with acid-, shear-, and temperature-tolerance profiles similar to those of chemically cross-linked starches can be prepared by heating a starch with a low (<15%) moisture content at a temperature above 100 C, but below that which effects thermal degradation. Drying to <1% moisture before heating and a pH of 8.0–9.5 helps change the properties of the starch. This process is practiced to increase the contents of RS and SDS and thereby to increase the content of dietary fiber in food products.
Multiple Modifications Modified food starches are tailor-made for specific applications. Most modified food starches are made by cross-linking, introduction of monosubstituent groups (stabilization), or a combination of these two approaches. Many products, in fact, have received two or more modifications. For example, a modified food starch may be a cross-linked and stabilized waxy maize starch; another may be a stabilized, acid-thinned, and pregelatinized normal corn starch. Characteristics that can be controlled/improved by multiple modifications include, but are not limited to, one or more of the following:
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Adhesion Clarity of solutions/pastes Color Emulsion stabilization Film formation Flavor release Hydration rate Moisture retention and control in product Mouthfeel of product Oil migration control in product Paste texture/consistency Product form (liquid, semisolid, and solid) Sheen of product Shelf stability of product Stability to acids Stability to heat Stability to shear Tackiness Temperature required to cook Viscosity (hot paste and cold paste)
Digestion and Metabolism Various regulations concerning reagents that may be used and the maximum allowable modification of a starch for food use, alone or in combination with another modification, are in
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effect around the world. Generally, the level of substitution in a derivatized food starch is below DS 0.1 and in the range DS 0.002–0.2. Because of this low level of modification, the digestion, metabolism, and caloric values of modified food starches are reduced only to a minor, usually unmeasurable, extent as compared to native starches. Because only monosaccharides (D-glucose in this case) are absorbed, fragments containing esterified, etherified, or oxidized a-D-glucopyranosyl units should not be absorbed from the small intestine.
Modifications for Nonfood Applications The papermaking industry uses modified starches not approved for food use. In addition to hypochlorite-oxidized starch (approved for food use), starches oxidized with ammonium persulfate are used as a surface size. Hydroxyethyl ethers of starches, made by reacting a starch with ethylene oxide, are the products of choice for surface sizing and coating binding. Cationic starches are substituted with ether groups that contain a quaternary ammonium group. They are used in the formation of the paper sheet as retention or drainage aids. Carboxymethyl starch is an anionic derivative used as a thickener for coating colors and a coating binder. (When used in the pharmaceutical industry as a tablet disintegrant, carboxymethyl starch is called sodium starch glycolate.) Amphoteric starches may also be employed in papermaking. Graft copolymers, such as starch-g-styrene–butadiene latex, have been made for coating paper. Starch-g-polyacrylate/acrylamide is a superabsorbent and forms hydrogels. Dialdehyde starch is used as a wet strength agent in the production of tissue. Some of the same and similar modified starches are used in oil and gas production and in other applications.
Exercises for Revision
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What property improvements can be obtained by the following modifications: cross-linking, stabilization, acidmodification, dextrinization, and oxidation? What property changes can be imparted by pregelatinization, HMT, and annealing? What is the principle of making cold-water-swelling starch? What is the principle behind cross-linking?
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What is the principle behind stabilization? Why are starches often modified in more than one way?
Exercises for Readers to Explore the Topic Further
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What are the names and general structures of the starch polymers? What are the differences between a normal, waxy, and highamylose starch (e.g., in maize)? What are the meanings of the following starch terms: gelatinization, gelatinization temperature, pasting, pasting temperature, breakdown, setback, and retrogradation? Each native starch is unique. What are general characteristics of the following starches: normal corn/maize, waxy maize, high-amylose maize (amylomaize), wheat, rice, potato, and cassava/tapioca? What is the wet-milling process by which corn starches are isolated from kernels?
See also: Barley, Rice and Maize Processing: Maize: Wet Milling; Carbohydrates: Resistant Starch and Health; Starch: Chemistry; Starch: Starch Architecture and Structure; Genetics of Grains: Maize: Genetics; Maize: Other Maize Mutants; Rice: Genetics; Wheat Genetics and Genomics; Non-food Products from Grains: Fuel Alcohol Production; Processing of Grains: Starch: Uses of Native Starch; The Cereal Grains: Maize: Overview; Rice: Overview; Wheat: An Overview of the Grain That Provides ‘Our Daily Bread’.
Further Reading BeMiller JN (2007) Carbohydrate Chemistry for Food Scientists. St. Paul, MN: American Association of Cereal Chemists. BeMiller JN and Whistler RL (eds.) (2009) Starch: Chemistry and Technology, third ed. New York: Academic Press. Bertolini AC (ed.) (2010) Starch. Boca Raton, FL: Taylor & Francis. Eliasson A-C (ed.) (1996) Carbohydrates in Food. New York: Marcel Dekker. Light JM (1990) Modified food starches. Cereal Foods World 35: 1081–1092. Thomas DJ and Atwell WA (1997) Starches. St. Paul, MN: Eagen Press. Whistler RL, BeMiller JN, and Paschall EF (eds.) (1984) Starch: Chemistry and Technology, second ed. Orlando, FL: Academic Press. Wurzburg OB (ed.) (1986) Modified Starches: Properties and Uses. Boca Raton, FL: CRC Press.