STARCH | Modified Starches

5576 STARCH/Modified Starches

Modified Starches J N BeMiller, Purdue University, West Lafayette, IN, USA Copyright 2003, Elsevier Science Ltd. All Rights Reserved.

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Food processors generally require starches with better behavioral characteristics than provided by native starches. Cereal starches produce particularly weak-bodied, cohesive, rubbery pastes and undesirable gels when cooked. However, via modification, the functional properties of starches can be improved. Modification is done to introduce specific functionalities and to make resultant cooked products better able to withstand the conditions of heat, shear, and pH (acid) associated with processing conditions. The final products, modified food starches, are abundant, functional, and useful food ingredients, generally macroingredients. (See Starch: Functional Properties.) Modifications can be chemical or physical. Chemical modifications are oxidation, cross-linking, stabilization, and depolymerization. Physical modifications make pregelatinized and cold-water-swelling products. 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. Chemical derivatives found in modified food starches (in the USA) are the following:

1. Stabilized starches a. Hydroxypropyl starches (starch ether) b. Starch acetates (starch ester) 0005 c. Starch octenylsuccinates (monostarch ester) 0006 d. Monostarch phosphate (ester) 0007 2. Cross-linked starches 0008 a. Distarch phosphate 0009 b. Distarch adipate 0010 3. Cross-linked and stabilized starches 0011 a. Hydroxypropylated distarch phosphate 0012 b. Phosphorylated distarch phosphate 0013 c. Acetylated distarch phosphate 0014 d. Acetylated distarch adipate 0015

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. Easier redispersibility when pregelatinized e. Greater clarity of pastes and gels 3. Cross-linked (phosphorylated) 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 4. Cross-linked and stabilized starches a. Lower gelatinization and pasting temperatures, but increased paste viscosity b. Other attributes of stabilized and cross-linked products 5. Thinned (depolymerized) starches a. Decreased viscosity of pastes b. Lower gelatinization and pasting temperatures c. Increased solubility Any starch (corn, potato, tapioca/cassava, wheat, rice, etc.) can be modified, but modification is practiced significantly only on corn (both common corn and waxy maize) and potato starches and, to a much lesser extent, on tapioca and wheat starches. This article is written primarily from the point of view of corn and waxy maize starches. (See Starch: Sources and Processing.)

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Property improvements that can be obtained by chemical modifications include the following: 1. Hypochlorite-oxidized starches a. Whiter b. Lower gelatinization and pasting temperature 0018 c. Decreased maximum paste viscosity 0019 d. Softer, clearer gels 0020 2. Stabilized (hydroxypropylated or acetylated) 0021 starches 0016

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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 mill is introduced into a stirred reaction tank. Sodium chloride or sodium sulfate is added to inhibit granule swelling. The pH is adjusted with sodium hydroxide (up to values of  11.5, depending on the reaction). Chemical reagents are added. Temperature is controlled. Reactions may be done at temperatures up to 50  C, but gelatinization 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 that can be made in this manner. (The degree of substitution is the average number of hydroxyl groups per a-dglucopyranosyl unit (the monomeric unit of starch)

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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 starch is recovered by centrifugation or filtration, washed, and dried. Chemical reactions currently both allowed and used to prepare modified food starches in the USA are as follows: . Esterification with acetic anhydride, succinic anhydride, the mixed anhydride of acetic and adipic acids, 1-octenylsuccinic 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 . Oxidation with sodium hypochlorite . Various combinations of these reactions Other reagents may be used in other countries. Waxy maize starch modifications are especially popular in the US food industry because the inherent properties of waxy maize starch make them preferred over modifications to common corn starch. Cross-Linking

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Cross-linking is the most important modification of a food starch. Cross-linking occurs when starch granules are reacted with difunctional 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, i.e., reduce sensitivity to processing conditions (high temperature; extended cooking times; low pH; high shear during mixing, milling, homogenization, and/or pumping). Cooked pastes of cross-linked starches are more viscous, heavier-bodied, shorter-textured, 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; for example, one cross-link for every approximately 1200 a-d glucopyranosyl units greatly reduces both the rate and the degree of granule swelling, greatly increases paste stability, and changes dramatically both the viscosity profile as the starch is cooked and the textural characteristics of its paste. Three times that much crosslinking, for example, produces a product in which granule swelling is restricted to the point that a peak viscosity is never reached in a slurry heated to 95  C and held at

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that temperature with moderate stirring. As the number of cross-links increases, the granules become more and more tolerant to physical conditions and acidity, and swell and disintegrate (solubilize) upon cooking less and less. Energy requirements to reach maximum swelling and viscosity are also increased. By far the most common cross-links are distarch phosphate esters. These distarch phosphates are prepared with either phosphoryl chloride or sodium trimetaphosphate. Phosphoryl chloride is very reactive and undoubtedly reacts near granule surfaces. To prepare cross-linked starches with phosphoryl chloride, the reagent is added to an aqueous starch suspension of pH 8–12. 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. In this case, the cross-links are undoubtedly more evenly distributed throughout the granule. A relatively small amount of cross-linked starch is made by reaction of corn starch with the mixed anhydride of adipic and acetic acids in aqueous alkaline suspension.

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Stabilization

Derivatization of a starch with monofunctional reagents reduces the intermolecular associations which result in gelation of its paste and/or precipitation of the starch polymers (combined processes termed retrogradation or setback). Pastes of unmodified 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 very limited extent at room temperature, but will become cloudy and chunky and exhibit syneresis when stored under refrigerator or freezing conditions.) The most common derivatives employed for starch stabilization are the hydroxypropyl ether and acetate and monostarch phosphate esters. Acetylation is accomplished by treating a starch slurry with acetic anhydride at pH 7–11, the optimum pH depending on the reaction temperature. Acetylation of starch lowers the 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, i.e., less retrogradation. Acetylated starches with an acetyl content of up to 2.5% (degree of substitution, DS, 0.09) can be used in food products (USA). (A DS of 0.09 indicates an average of nine acetyl groups per 100 a-d-glucopyranosyl units.) Sodium phosphate monoesters are prepared by impregnating the starch with a solution of sodium

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5578 STARCH/Modified Starches

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tripolyphosphate. After adjustment of the pH to 5.0–8.5, the slurry is mixed, then filtered, and the filter cake is dried and heated. Sodium tripolyphosphate is used to make products of up to 0.002 DS (one phosphate group per 500 a-d-glucopyranosyl units), the maximum allowed in the USA. Monosodium orthophosphate in the pH range 5.0–6.5 is also used to produce monostarch phosphates in the same way. Monostarch phosphates produce stable pastes that are clear and have a long, cohesive texture. Paste viscosity can be controlled by varying the concentrations of phosphate salt, time of reaction, temperature, and pH. Increasing substitution lowers the gelatinization temperature; products become cold-waterswelling at DS 0.07. Corn starch phosphates of DS 0.01–0.03 produce pastes with hot viscosity, clarity, stability, and texture more like those of potato starch. Starch phosphates are good emulsion stabilizers and produce pastes with improved freeze–thaw stability. Hydroxypropyl ether derivatives of starches are prepared by reacting an alkaline slurry with propylene oxide. To a starch slurry is added sodium sulfate and sodium hydroxide. The reactor is charged with propylene oxide and sealed. Reaction is continued for about 24 h at about 49  C. The maximum allowable moles of substitution in the USA is 0.2 (7.0% of hydroxypropyl groups). (Moles of substitution, MS, is essentially the same as DS but is used in place of DS because each hydroxypropyl group contains a hydroxyl group that can itself be etherified, so that the maximum number of substituent groups per glucosyl unit can be more than 3.) Low-MS hydroxylpropylstarches behave much like low-DS starch acetates and are used because of similar improvements in texture and appearance. The hydroxypropyl ether linkage is, however, much more stable than an ester linkage. Starch succinate half-esters are prepared by reacting starch with succinic anhydride. Starches with Hydrophobic Groups

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Reaction of starch with 1-octenylsuccinic anhydride introduces hydrophobic substituent groups. Such derivatives can be used as emulsifiers and emulsion stabilizers in products based on oil-in-water emulsions, such as pourable dressings and flavored beverages. Flavor oil emulsions containing a thin-boiling starch or dextrin (see below) derivatized with 1-octenylsuccinate 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. Gum arabic is, however, usually the material of choice for this application. 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 USA is 0.02. Acid Modification

Thin-boiling starches are prepared by treating a suspension of a native or derivatized starch with dilute mineral acid at a temperature below the gelatinization temperature. When a product that gives the desired 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. Thin-boiling starches are used as film formers and adhesives in products such as pan-coated nuts and candies, whenever a strong gel is desired, e.g., in gum candies such as jelly beans, jujubes, orange slices, and spearmint leaves, and in processed cheese loaves. To prepare especially strong and fast-setting gels, a high-amylose corn starch is used. More extensive modification with acid produces dextrins. (See Dextrins.)

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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 association of amylose molecules, i.e., results in some stabilization via introduction of small amounts of carboxylate and carbonyl groups. Oxidized starches produce intermediate-viscosity and soft gels and are used when these properties are needed. They are also used to improve adhesion of starch batters to fish and meat and in breadings. Mild treatment with sodium hypochlorite, hydrogen peroxide, or potassium permanganate simply bleaches the starch and reduces the count of viable microbes.

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Pregelatinization

Pregelatinized starches are precooked starches that can be dispersed (dissolved) in water at temperatures below the gelatinization temperatures of the parent starches; thus, these ‘instant’ starches need no cooking. To prepare a pregelatinized starch, a slurry is simultaneously cooked and dried on hot drums. Because pregelatinized starch products are powders prepared from dried pastes, generally no granules are present, although granule fragments may be. Both chemically modified and unmodified starches can be used. The resulting products contain no intact starch

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STARCH/Resistant Starch

granules. 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 0064

Starch products that are gelatinized starches, i.e., starches that have lost their crystallinity, but which retain their granular form, in contrast to standard pregelatinized starches, are called cold-water-swelling starches. There are several ways that such products can be prepared; one way is to heat a starch in an aqueous alcohol solution with sufficient water to allow gelatinization and sufficient alcohol that granule integrity is maintained. Cold-water-swelling products swell rapidly and thicken unheated aqueous systems. (A granular, cook-up starch requires heating a slurry to the pasting temperature before thickening occurs.) Multiple Modifications

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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 common corn starch. Characteristics that can be controlled/improved by multiple modifications include, but are not limited to, one or more of the following: . . . . . . . . . . . .

Adhesion Clarity of solutions/pastes Color Emulsion stabilization Film formation Flavor release Hydration rate Moisture retention and control in product Mouth feel of product Oil migration control in product Paste texture/consistency Product form (liquid, semisolid, solid)

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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)

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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 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. (See Carbohydrates: Digestion, Absorption, and Metabolism.)

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See also: Carbohydrates: Digestion, Absorption, and Metabolism; Dextrins; Starch: Sources and Processing; Functional Properties

Further Reading Eliasson A-C (ed.) (1996) Carbohydrates in Food. New York: Marcel Dekker. Light JM (1990) Modified food starches. Cereal Foods World 35: 1081–1092. Whistler RL, BeMiller JN and Paschall EF (eds) (1984) Starch: Chemistry and Technology, chapters 10, 17, 19. Orlando, Florida: Academic Press. Wurzburg OB (ed.) (1986) Modified Starches: Properties and Uses, chapters 2–4, 6, 7, 9, 12. Boca Raton, Florida: CRC Press.

Resistant Starch R M Faulks, Institute of Food Research, Norwich, UK Copyright 2003, Elsevier Science Ltd. All Rights Reserved.

Introduction The term ‘resistant starch’ was first coined by Englyst, Wiggins, and Cummings in 1982 to describe a small

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