Corn Starch Modification

Chapter 19

Corn Starch Modification James N. BeMiller Whistler Center for Carbohydrate Research, Purdue University, West Lafayette, IN, United States

INTRODUCTION Approximately 80% of the world’s commercial production of starch is corn/maize starch, which is isolated from corn kernels (content 64%–80%) by the wet-milling process (Chapter 17). Corn starches are used in a wide range of products and applications. In order for corn starches to be used in such a wide range of products and applications, it must provide a wide range of attributes and withstand a wide range of processing conditions. Yet native normal corn starch provides the desired characteristics for few, if any, of the products and applications. The necessary attributes (physical properties and functionalities) are imparted by modifying normal corn starch genetically (to produce waxy corn [waxy maize] starch or a high-amylose corn [amylomaize] starch) (Chapter 3), chemically, and/or physically. Covered in this chapter are chemical and physical treatments that modify the characteristics of normal corn, waxy maize, and amylomaize starches. Only covered are chemical modifications of corn starches that are currently practiced commercially (rather than all possible modifications). Currently practiced and some potential physical modifications are also described. Hereafter, when the simple term corn starch is used, it refers generically to all three of the commercial corn/maize starches.

BASIC NATURES OF CORN STARCH GRANULES AND MOLECULES Normal corn starch contains small amounts of protein (about 0.35%), lipid (about 0.8%), and ash, and >98% of two polysaccharides: amylose and amylopectin (Chapter 11). All starches occur in the plant in the form of granules that are insoluble in room-temperature water. Normal corn and waxy maize starch granules vary in size from 2 to 30 mm, with most falling in the 12–15 mm range. They also vary in shape, with the cross sections of most being various polygons. Various levels of organization exist within granules ( Jane, 2009; P
erez et al., 2009). Most of the properties of granules are believed to arise from the fact that they are composed of a spherocrystalline assembly of amylopectin molecules oriented radially. The granular, partially crystalline nature of their granules is key to many of the properties and uses of corn and other starches and to chemical and physical modifications of them. When corn starch granules are added to aqueous systems, they take up water and rapidly hydrate. When the temperature of an aqueous dispersion of hydrated granules is raised sufficiently, dramatic changes occur. The water of hydration (water being a plasticizer for starch granules) first disrupts hydrogen bonds in the amorphous regions of the granules. As a result, the granules swell and change shape, becoming more spherical. As the temperature continues to increase and more hydration and more swelling occur in amorphous regions, and because the amorphous and crystalline phases are connected, the crystallites become distorted and loosened, so that the starch chains in them can become, at least partially, hydrated/plasticized to the point that the crystallites melt. This irreversible disruption of both amorphous and crystalline structures is called gelatinization. During gelatinization, some dissolved starch polysaccharide molecules (primarily amylose) leach from swollen granules. Continued heating of starch granules in excess water with some shear results in a process called pasting. During pasting, further granule swelling, additional leaching of soluble starch polymer (primarily amylose) molecules, and granule disruption (because the highly swollen granules are quite fragile) occur. The result is a hot starch paste. Cooling of the hot paste results in the formation of a gel. (The cooled paste is called a gel, although rheologically speaking, both the hot paste and the cool paste are gels.) The process of starch pasting and gel formation is followed with an instrument such as the Rapid Visco Analyzer (RVA). Attributes measured by such instruments are shown graphically in Fig. 19.1. The pasting temperature (Tp) is the temperature at which the rapid viscosity increase begins. The peak viscosity is reached when there is the maximum amount of swollen granules in the system. Disintegration of the swollen granules occurs under the slight shear (stirring) Corn. https://doi.org/10.1016/B978-0-12-811971-6.00019-X Copyright © 2019 AACCI. Published by Elsevier Inc. in cooperation with AACC International. All rights reserved.

537

Final

Peak Tp

Trough

Temperature (– – –)

Viscosity (—)

538 Corn

Time FIG. 19.1 An idealized RVA curve that describes the pasting and paste properties of a starch (Tp ¼ the pasting temperature).

Viscosity (—)

Waxy maize Normal corn

Temperature (– – –)

of the instrument, and the viscosity reaches a minimum (the trough or hot-paste viscosity) just before or just after the system begins to cool. (The starting and ending temperatures in the instrument are usually 50°C, and the maximum (holding) temperature is usually 95°C.) The difference between the peak viscosity and the trough viscosity is called breakdown. Breakdown is an indicator of how susceptible swollen granules are to disintegrating. The final viscosity is usually that recorded when the temperature of the paste/gel reaches 50°C. There is an increase in viscosity upon cooling that occurs, both because the dispersion of polymer molecules has cooled and because some of the starch polymer molecules have associated with each other (forming larger particles) in a system that has less energy. The difference between the final viscosity and the trough viscosity is called setback. Retrogradation is a process in which the starch polymer molecules associate with one another. The end result of such associations may be insolubility (precipitation) or a firmer gel, which often is opaque because of insoluble crystallites. Retrogradation in a hot paste or a gel of a cooked starch system is often undesirable. Characteristic viscosity profiles of normal, waxy, and high-amylose (amylomaize) corn starches during heating of aqueous dispersions (of the same concentrations) to 95°C, holding at 95°C, and cooling to 50°C reveal significant differences (Fig. 19.2). Waxy maize starch produces a high peak viscosity at a relatively low temperature because the granules lack amylose, which would strengthen them and restrict their swelling. For the same reason, the swollen granules disintegrate rather easily and a rapid and significant breakdown occurs. During RVA analysis, little setback occurs in waxy maize starch pastes and gels because amylopectin molecules undergo retrogradation only slowly. Normal corn starch, which contains amylose, has a higher pasting temperature, produces a lower peak viscosity (less volume occupied by swollen granules), less breakdown, and develops a more solid-like gel upon cooling. High-amylose corn starch granules gelatinize and paste very little at 95°C, so almost no viscosity is produced. In large part, corn starches are modified to alter their pasting, paste and gel properties.

Amylomaize Time FIG. 19.2 A comparison of typical RVA pasting and paste profiles of normal corn, waxy maize, and high-amylose (amylomaize) starches.

Corn Starch Modification Chapter

19

539

CHEMICAL MODIFICATIONS Objectives The properties of corn starches are inherently unsuitable for most applications. Corn starches are primarily modified chemically to enhance the positive attributes of either their granular or cooked states, to minimize the defects (negative attributes) of either their granular or cooked states, and/or to provide functionalities that the native starches cannot provide. For food applications (Mason, 2009), the principle reasons for chemical modification (primarily of normal corn and waxy maize starch) can be summarized as follows. 1. To modify properties related to retrogradation (that is, to prolong product stability) a. To prolong cold storage stability b. To prolong freeze-thaw stability c. To decrease syneresis d. To improve gel clarity and sheen 2. To provide tolerance to processing conditions and prevent overcooking a. To provide acid, shear, and heat stability (that is, to decrease breakdown during cooking) b. To decrease or increase peak viscosity c. To increase or decrease the pasting temperature 3. To provide desirable texture a. To decrease or increase viscosity b. To decrease or increase gel formation c. To decrease or increase gel strength One of several chemical modifications called stabilization (or substitution) is used to accomplish objective 1. One of several chemical modifications called cross-linking is used to accomplish objective 2. Combinations of stabilization and crosslinking modifications are used to accomplish objective 3.

Reactions Chemical reactions used to derivatize starch (Chiu and Solarek, 2009; Huber and BeMiller, 2010; Rutenberg and Solarek, 1984; Wurzburg, 1986a) to accomplish the objectives outlined above and others are the following: 1. Reactions of hydroxyl groups a. Esterification b. Etherification c. Oxidation 2. Reactions involving glycosidic linkages (converted starches/products of conversion) a. Depolymerization i. Acid-catalyzed ii. Oxidation plus base/alkali b. Transglycosylation plus depolymerization (dextrinization) 3. Graft copolymerization Although other processes have been investigated, the great majority of corn starch is derivatized in aqueous slurry—usually at 35%–40% solids, at an alkaline pH (8.5–11.3), at ca. 48°C (120°F), in the presence of a swelling-inhibiting salt (usually sodium sulfate at a concentration of about 7.5% [anhydrous basis] to prevent gelatinization and pasting of the granules). The granular product is recovered by centrifugation or filtration, washed, and dried. The chemistry of chemical modifications of starch is simple. Understanding what takes place when starch granules undergo the chemical modifications and effecting the reactions uniformly and consistently on very large scales is not simple.

Stabilization (Substitution) Reaction with monofunctional reagents to produce ethers and esters is used to make products whose hot pastes and gels exhibit less retrogradation and associated processes, such as syneresis. With regard to products to be used in food, which are frequently based on waxy maize starch, it means that their gels have increased clarity. Reduction in setback (retrogradation) occurs because the substituent ether or ester groups along the starch polysaccharide chains sterically prevent chain

540 Corn

association and crystallization. In addition, gelatinization and pasting temperatures are often lowered, granule swelling is increased, gel strength is reduced, and water-holding capacity is increased. Stabilized waxy maize starch is used when stability during refrigerated storage and/or to freezing and thawing is required. A common stabilized corn starch used in preparation of processed food is made by reacting the starch with propylene oxide at pH ca. 11.2–11.3 in a slurry containing a swelling-inhibiting salt to add hydroxypropyl ether groups to the starch polysaccharide molecules (Fig. 19.3). Being attached in an ether linkage, the hydroxypropyl group is stable to both acids and bases. Hydroxypropylated corn starch may also be cross-linked (“Cross-Linking” section) and/or acid-thinned (“Converted Starches” section). Papermaking consumes the majority of isolated normal corn starch (other than that used for sweetener production) in the United States. Two types of derivatized starch products related to hydroxypropylated starch are employed in papermaking. Much of that is hydroxyethylated starch, which is used in both surface sizing and coating operations (Maurer, 2009). Hydroxyethylated starch is made by a reaction similar to that used to make hydroxypropylated starch using ethylene oxide as the reagent (Fig. 19.4). A principal reason for use of a stabilized starch is that prevention of retrogradation in the period between cooking of the starch product and application to the paper sheet (often several hours) is essential. Hydroxyethylated corn starch (usually also acid-thinned [“Converted Starches” section]) is also an excellent film former. Another class of starch ethers used in papermaking is that of cationic starches. The added cationic group may be a tertiary amine or quaternary ammonium group. Fig. 19.5 illustrates preparation of the most common type of cationic starch (that is, that made via addition of a quaternary ammonium group). The primary use of cationic normal corn starch is as a retention or drainage aid in the formation of the paper sheet. A secondary use is as a surface size. Stabilized (substituted) normal corn and waxy maize starches are also made by esterification. Because esters are easily saponified by alkali, the pH used is lower (ca. 8.5) and the reaction time is shorter. There are two primary commercially available organic esters. The most common one is the acetate ester, which is a little less expensive to produce than the hydroxypropyl ether, but is less stable. It can be made in two ways. The only production process in the United States utilizes acetic anhydride (Fig. 19.6). In other countries, these products may be made by transesterification using vinyl acetate (Fig. 19.7); the acetaldehyde coproduct being the reason that this reaction is not used in the United States. The second primary organic ester made is the 2-octenylsuccinate ester, which is made by reacting a starch with 2-octenylsuccinic anhydride (OSA). Starch octenylsuccinates (commonly known as OS [or OSA] starch) represent a special kind of stabilized starch containing hydrophobic groups (Fig. 19.8). It has both emulsifying and emulsionStarchJO– + H2CJCHJCH3DStarchJOJCH2JCHJCH3 O OH FIG. 19.3 The hydroxypropylation reaction affected under alkaline conditions (followed by neutralization). StarchJO– + H2CJCH2

StarchJOJCH2JCH2OH

O FIG. 19.4 The hydroxyethylation reaction affected under alkaline conditions (followed by neutralization). O– JStarch

StarchJO–

ClJCH2JCHJCH2JN+R3

OH–

OH

CH2JCHJCH2N+R3 O

StarchJOJCH2JCH2N+R3 FIG. 19.5 Cationization of starch affected under alkaline conditions (followed by neutralization).

StarchJO + CH3JCJOJCJCH3

FIG. 19.7 Acetylation of starch under alkaline conditions using vinyl acetate.

O

StarchJOJCJCH3 + AcO–

O

K

K

O

StarchJO– + CH3JCJOJCHKCH2

–

K

O

K

O

K

FIG. 19.6 Acetylation of starch under alkaline conditions using acetic anhydride. (AcO ¼ acetate ion).

StarchJOJCJCH3 + CH3CHO

Corn Starch Modification Chapter

19

541

C

J

J

K

O

StarchJO + H3CJ(CH2)4JCHKCHJCH2JCH O J H2C C –

K

J

J

O

K

O

J J

H3CJ(CH2)4JCHKCHJCH2JCHJCJOJStarch CH2 CO2 –

+

K

O

J

H3CJ(CH2)4JCHKCHJCH2JCHJCH2JCJOJStarch CO2

–

FIG. 19.8 Preparation of octenylsuccinylated starch (OS starch) under alkaline conditions.

stabilizing properties and produces water-resistant films. Starch succinate (made using succinic anhydride) is also approved for food use, but if made at all, only in small amounts. One inorganic ester is produced, namely, the phosphate ester. Monostarch phosphate (starch  O  PO3 2 Na2 + ) is produced by reacting starch with either a mixture of orthophosphates (NaH2PO4 + Na2HPO4) or with sodium tripolyphosphate (STPP) (Scheme 19.1). For food starch products, the amount of “add on” or degree of substitution (DS) allowed is determined by country or country groups. The degree of substitution is defined as average number of substituent groups attached to each glucosyl unit. Because each glucosyl unit contains on average (whether or not the starch polysaccharide molecule is branched) three hydroxyl groups, the maximum possible DS is 3.0. The maximum DS permitted for a modified food starch is usually <0.2, which means an average of <2 substituent groups per 10 glucosyl units. When the starch is derivatized with propylene oxide, the hydroxyl group on the glucosyl unit is lost, but a new hydroxyl group, which can now react with the reagent, is created on the hydroxypropyl group (Fig. 19.3). The result is that potentially more than three reagent molecules can react with each glucosyl unit, so the term molar substitution (MS) is used. MS is defined as the total moles of substituent groups per mole of glucosyl units. In the case of hydroxypropyl starch for food use, the allowed level of substitution is so low that there is very little (probably no) reaction of the reagent with the hydroxyl groups of the hydroxypropyl groups, so the DS and MS values are essentially the same. Stabilized starches, in general, decrease the rate and degree of retrogradation, decrease gelatinization and pasting temperatures, and increase granule swelling by introducing protrusions along the starch polysaccharide chains that prevent them from associating with one another. When the added substituent groups are charged (like the phosphate ester group), the repelling nature of the like charges increases solubility and helps to prevent chain association (retrogradation).

Cross-Linking Cross-linking of adjacent starch polysaccharide molecules within granules occurs when the starch is reacted with a bifunctional reagent (Fig. 19.9). Again, reactions are conducted under alkaline conditions. Cross-linking is only practiced with food starches (that is, starch products to be used as ingredients in processed food). Cross-linking strengthens granules. The primary reason for cross-linking is to make granules more resistant to the temperatures, amounts of shear, and acidic environments starch ingredients often encounter during preparation of processed food. Some effects of cross-linking are summarized below.

SCHEME 19.1 Sodium tripolyphosphate.

J

O

K

K

O

PJ

O O– Na+

J

Na+ – O

PJ

O

J

K

O

J

Na+ –OJ

– + JO Na PJ O– Na+

542 Corn

Starch chainJO– + XJRJX Starch chainJOJRJOJStarch chain + 2X– FIG. 19.9 General cross-linking of starch polysaccharide molecules within granules with bifunctional reagents (under alkaline conditions) (X ¼ a leaving group). l l l l l l l l l l l

Modifies cooking characteristics Produces higher or lower peak viscosities Produces higher or lower final viscosities Improves heat stability Improves acid stability Improves shear tolerance Inhibits swelling (delays pasting) Reduces setback (improves storage stability) Reduces gel formation Reduces cohesiveness Improves clarity

Three types of cross-linking are approved for food starches. The most commonly prepared cross-linked product is the distarch phosphate ester, and the most commonly used reagent for this purpose is phosphorus oxychloride (phosphoryl chloride, POCl3). The overall reaction is shown in Fig. 19.10. The reaction is conducted under alkaline conditions to neutralize the HCl formed. Distarch phosphate can also be prepared using sodium trimetaphosphate (STMP) (Scheme 19.2) or a mixture of STMP and sodium tripolyphosphate (STPP). When the inorganic reagents are used, granules are impregnated with an alkaline solution of the salt (or salts), then dried and heated to introduce the diester cross-links. A second type of approved cross-link is the adipic acid diester, which is made by reaction of the starch with acetic-adipic mixed anhydride (Fig. 19.11). The reagent is prepared by dissolving adipic acid in an excess of acetic anhydride. Because of FIG. 19.10 Cross-linking of starch by reaction with phosphorus oxychloride.

2 Starch chainJOH + POCl3

K

O

J

Starch chainJOJ P JOJStarch chain + 3 HCl O–

K

J

O

O– Na+

J

O

J P KO

O

J

K

Na+ –OJP

J

J

PJ OJ O

O– Na+

SCHEME 19.2 Sodium trimetaphosphate.

O

K

O

K

O

K

K

O

2 Starch chainJO + H3CJCJOJCJ(CH2)4JCJOJCJCH3

K

O

O

O

K

–

K

FIG. 19.11 Cross-linking of starch by reaction with acetic-adipic mixed anhydride under alkaline conditions. (Only the reaction of two adjacent starch molecules with the two ends of the adipic acid moiety to produce a cross-link is given.)

Starch chainJOJCJ(CH2)4JCJOJStarch chain + 2 CH3JCJO–

the excess of acetic anhydride, acetylation also occurs, and the product is both cross-linked (distarch adipate) and stabilized (acetylated). A third type of cross-link (the glyceryl diether made by reaction with epichlorohydrin under alkaline conditions [Fig. 19.12]) is approved by the US Food and Drug Administration, but not used in the United States. It is used in some other countries. Because the cross-link is a diether, it is more stable than the distarch adipate cross-link.

Corn Starch Modification Chapter

StarchJOJCH2JCHJCH2JOJStarch

J

J

J

2 StarchJO– + H2CJCHJCH2Cl O

OH

19

543

FIG. 19.12 Cross-linking of starch by reaction with epichlorohydrin under alkaline conditions (followed by neutralization).

Converted Starches Hydrolysis Treatment of an aqueous slurry of a corn starch at an acidic pH value and usually at a temperature of a little below 50°C (120°F) for a relatively short time (one to several hours) results in hydrolysis of some of the glycosidic linkages of amylose and amylopectin (while leaving the starch granules intact) and produces an acid-modified (thinned) starch (Wurzburg, 1986b). The effects of acid modifying (thinning) of granular starches are summarized below. l l l

l l l l

Reduces the energy required to cook (because the granules are weakened) Increases solubility Reduces both hot-paste and gel viscosities (because of depolymerization). (This means that more of the starch can be put into solution.) Increases the tendency to gel May produce stronger gels (especially true of the high-amylose [amylomaize] starches) Increases the ability to form films May increase adhesiveness

More extensive acid- or enzyme-catalyzed hydrolysis will produce maltodextrins, cyclodextrins, and glucose syrups, but these are not considered to be modified starches.

Oxidation Most commercial corn starch has been treated with a little sodium hypochlorite for the purpose of bleaching it. Treatment with greater amounts of the oxidant produces various oxidized glucosyl units (Fig. 19.13) (Wurzburg, 1986b). Because the reagent solution is alkaline, those units containing carbonyl (aldehydic or keto) groups initiate a reaction known as betaelimination that results in chain scission—either in front of or behind the oxidized unit. In addition, the formation of chain units that are different than the units on chains with which the chains might come into contact reduces the probability that the two chains will associate and form a junction zone, so oxidized starches are also stabilized starches. This is particularly

CH2OH O

CH2OH

CH2OH O

O OH

O

O O

OH

O

OH

O

OH O

CH2OH

CH2OH

O

O

CHO CHO COO–

HCKO

O

O O OH

OH O

O OH

OH

O

O

CO2 O2C

FIG. 19.13 Products of oxidation of glucosyl units of starch polysaccharides with hypochlorite. ([O] is the symbol for oxidation.)

544 Corn

true of units with negative charges on them. Because of the bleaching, depolymerization, and stabilization, hypochloriteoxidized starches have the characteristics listed below. l l l l l l

Improved color Reduced energy required to cook Reduced hot-paste viscosity Increased paste stability Reduced gel formation, Improved adhesion

Hypochlorite-oxidized starches find use as textile-sizing agents and as coatings for battered, deep-fried food. Corn starch is also oxidized with hydrogen peroxide in the presence of copper(II) or iron(II) ions at neutral pH (Kesselmans and Bleeker, 2004; Parovuori et al., 1995). This oxidation depolymerizes the starch polysaccharide molecules (decreases paste viscosity) with generation of only low (in some cases insignificant) amounts of carbonyl and carboxylate groups. (Using FeSO4 as the catalyst results in the formation of somewhat more carboxylate groups than does using CuSO4.) Acid-modification, hypochlorite-oxidation, and hydrogen peroxide oxidation all weaken granules, reduce the average molecular weights of the starch polysaccharide and molecules, and allow production of higher-solids pastes (which can increase the body, that is, improve the mouthfeel, of the food product), but the products are not identical. Other oxidants approved for at least bleaching and sterilization of food starches are peracetic acid and potassium permanganate, but changes in molecular structures imparted by them have not been described. Corn starch is also oxidized with ozone, but use of the products as food ingredients would not be allowed in the United States.

Dextrinization The term dextrin is used to describe depolymerized (degradation) products of starch (Wurzburg, 1986b). (Products of extensive enzyme-catalyzed hydrolysis of starch maltodextrins and cyclodextrins, but are not modified starches and not covered in this chapter.) A particular group of products called starch dextrins are produced by heating dry starch, and thus, they are also often called pyrodextrins. The first step in the production of a pyrodextrin is to spray the starch with a solution of an acid, alkali, or buffer to adjust the pH as desired. (The starch may be predried at this point.) Then the starch is heated at the desired temperature for the desired time. Finally, the product needs to be cooled. Pyrodextrins are grouped into three categories: white dextrins, British gums, and yellow or canary dextrins. White dextrins are made by heating the starch at a relatively low temperature in the presence of an acid. British gums are made by heating the starch at a relatively high temperature in the presence of an alkali. British gums have a darker color than do white dextrins. Yellow dextrins are what are known as highly converted products. They are made by heating the starch to relatively high temperatures in the presence of an acid. Three types of reactions may take place during dextrinization. One is hydrolysis of glycosidic linkages, which occurs primarily when an acid catalyst is used. Moisture is required for acid-catalyzed hydrolysis, so most hydrolysis occurs during the predrying and the initial stages of dextrinization. During these periods, there is a continual decrease in the average molecular weights of the starch polysaccharides and a continual loss of water (so that depolymerization slows down as the starch loses moisture and may eventually stop). Hydrolysis is the predominant reaction in the production of white dextrins. The second type of reaction is transglycosylation (transglycosidation), which is the transfer of a portion of a starch molecule to another starch molecule. This reaction is also acid-catalyzed and also results from cleavage of glycosidic bonds. However, without water molecules present (with which the unit that is now at the reducing end of one of the two new molecules can react), the unit reacts with a hydroxyl group of another molecule, forming a new glycosidic linkage. As a result, the starch molecules become more branched. Transglycosylation reactions primarily occur late in the process when the moisture content is low. Transglycosylation is the predominant reaction in the production of British gums. Both hydrolysis and transglycosylation occur in the production of yellow dextrins, the former early in the process and the latter later. (Some re-polymerization of small fragments [a process called reversion] may occur in the preparation of yellow dextrins.) All the while, the starch remains granular. Yellow dextrins and British gums are used as remoistenable adhesives, adhesives for making paper tubes, and mining, foundry, and printing applications. White dextrins are used as crispness enhancers for batters (in food products), as coatings for pharmaceutical tablets, and in textile finishing.

Corn Starch Modification Chapter

StarchJOH or StarchJOJCH2JCH2OH

Radical initiator

StarchJO• or StarchJOJCH2JCH2–O•

19

545

FIG. 19.14 A generalized scheme for producing starch graft copolymers.

Vinyl or acrylic monomer Starch graft copolymer

Graft Copolymerization Graft copolymers are made by generating free radicals from hydroxyl groups of native or hydroxyalkylated starch molecules, followed by reaction with an unsaturated monomer (Fig. 19.14). A wide range of starch graft copolymers have been made using different methods of producing starch free radicals, different vinyl and acrylic monomers, mixtures of monomers, different modified starch products, and different reaction conditions. Corn starch-g-polyacrylic acid has been produced for use in paper coating, and starch-g-styrene-butadiene latex and similar products have been investigated for compounding with styrene-butadiene rubber latex.

Multiple Modifications It is common for starches to have undergone more than one modification. For use as food ingredients, they are often both cross-linked and stabilized, and perhaps also acid-modified (thinned) or oxidized, or they may only be either cross-linked or stabilized and acid-modified or oxidized. Approved for food use in the United States are hydroxypropylated distarch phosphates, phosphorylated distarch phosphates, acetylated distarch phosphates, and acetylated distarch adipates (all distarch phosphates being made with POCl3). Multiple modifications are performed to obtain at least some of the attributes of the two or three modifications, such as lower pasting temperatures but increased paste and gel viscosities. Using multiple modifications, a variety of textures and rheological properties can be achieved.

PHYSICAL MODIFICATIONS Objectives Recently, there has been considerable interest in the physical modification of starches. Physical modifications of granular corn starch are investigated and practiced for starches intended for food applications (BeMiller and Huber, 2015; BeMiller, 2018). Physical modifications are modifications imparted by physical treatments that do not modify the chemical structures of the D-glucopyranosyl units of the starch polysaccharides, although some depolymerization (chain scission) may occur. There are several reasons for wanting a physically modified starch. As already pointed out, native corn starches are inherently undesirable for food applications for one or more reasons. Chemical modifications are practiced to minimize the negative attributes or to enhance the positive attributes of the starch or to introduce new attributes to the starch, but a chemically modified starch used in a food formulation requires a label listing (in the United States) of “modified food starch” or “food starch modified”. The foremost reason for wanting a starch that has been physically modified so that it has more desirable properties and imparts more desirable functionalities than provided by the native starch is that such modified starches do not need to be labeled as modified starches, making them known as “clean-label,” “label friendly,” or “functional native” starches. (For this reason, major corn starch-producing companies offer such starches made by proprietary processes.) There are other reasons for wanting physically modified starches. If physical modification can result in acceptable functionalities, it would be less expensive and easier to accomplish compared to chemical modifications and would produce no effluents (containing salts, unreacted reagents, or reagent byproducts), making the process simpler, “greener,” and more desirable. Another interest is that some physical treatments produce increased amounts of slowly digesting and resistant starch. Finally, some physical treatments are used or investigated for the nonthermal processing of food. As a result, their effects on the starch used as an ingredient have also been investigated. Physical modification processes are usually categorized by being either thermal or nonthermal. 1. Thermal treatments a. Pregelatinization b. Preparation of granular cold-water-swelling starch

546 Corn

c. Heat-moisture treatments d. Annealing e. Heating of dry starch 2. Some nonthermal treatments a. Ultrasound b. Milling c. High-pressure d. Pulsed electric field In general, physical treatments produce changes in or destroy the packing arrangements of the amylose and amylopectin molecules within granules. Such structural changes can alter the properties and functionalities of the starch, but the changes usually are neither as dramatic nor as thermostable (that is, the properties may not remain during and after cooking of the physically modified starch—with the exception of some pregelatinized starch products) as are those produced by chemical modification.

Thermal Treatments Pregelatinization Pregelatinized starches (also called “instant starches”) are precooked starches and are established articles of commerce. They are starches that have been cooked and dried under conditions that allow no or little retrogradation. Because a wide variety of types of equipment and processing conditions are used, commercial products have a variety of attributes; but granules are extensively, but often not completely, destroyed in all products. Pregelatinized starches hydrate rapidly and are described as being cold-water soluble, but most produce additional viscosity upon heating aqueous dispersions of them. Three basic types of equipment and processes are used to make pregelatinized starch. In one, a slurry of starch in water is applied to a superheated steam-heated roll or into the nip between two counter-rotating superheated steam-heated rolls. The starch is rapidly gelatinized, pasted, and dried on the roll. The dry film is scraped from the roll and ground to powders of different mesh sizes. In another general process, moistened starch is cooked in an extruder. Again, the dried extrudate is ground to powders of different mesh sizes. Because of the variables involved, there is more variability in starches pregelatinized by extrusion than in products made by the hot-roll process. The variables include the type of starch, starch-water ratio, temperature, screw configuration, screw speed, and residence time. Pregelatinized starch made by extrusion is depolymerized to some extent during the process. In a third process, starch is pregelatinized by treatment with superheated steam—first in a two-fluid nozzle, then in a spray-drying chamber. Both native and chemically modified starches can be, and are, pregelatinized. A pregelatinized starch is used in a food application when no heat is available, when no processing step requires sufficient heat to cook the starch, when heat cannot be applied because of the thermal lability of another ingredient(s), or when convenience of use (such as in dry mixes designed for home use) makes it desirable. Pregelatinized starch products are often used in cake and pudding mixes, cream fillings, frostings and toppings, as pharmaceutical tablet binders, and as laundry starch. Those made from chemically modified starches retain some of the attributes imparted by the modification.

Granular Cold-Water-Swelling Starch Granular cold-water-swelling starch (GCWS, CWS) products are also referred to as “instant” starches. They are also called cold-water-soluble starches, although pregelatinized starch products are usually more soluble in room-temperature water under low shear conditions than they are. GCWS products contain gelatinized, but intact granules. Such amorphous granules swell extensively when placed in room-temperature aqueous systems. Three main processes, two of which are used commercially, have been used to produce GCWS. In all three processes, granules gelatinize (that is, swell to the point of losing their crystalline order) without pasting (that is, while maintaining their integrity). In one process, a slurry of normal corn or a mixture of normal corn and waxy maize starches is heated in an aqueous alcohol solution (the alcohol restricting granule swelling and pasting). In another process, a starch slurry is very quickly heated in a special spray-drying nozzle, and the droplets containing gelatinized granules are dried in a spray drier. A popular laboratory preparation method treats the starch with an alkaline, aqueous alcohol solution at, or at a little above, room temperature. The unique characteristic of GCWS starch products is that their granules hydrate rapidly and swell without granule disintegration when placed in a room-temperature aqueous system (that is, without cooking)—even when little or no shear

Corn Starch Modification Chapter

19

547

is applied. GCWS corn starch can be used to prepare gum candies, in instant pudding mixes, as a fat mimetic in spoonable salad dressings, and in muffin batters containing fruit or nut pieces.

Heat-Moisture Treatment Heat-moisture treatments (HMT) are hydrothermal processes consisting of heating moistened starch granules. A wide range of conditions can be, and have been, used because the variables are the type of starch, its moisture content (usually 20%–30%), the temperature (usually 80–140°C [175–285°F]), the length of heating (usually 1–24 h), and the source of heat. Heat-moisture treatments may be employed with acid- and chemically modified starches, but have most often been applied to native starches. Due to the diverse possible combinations of conditions employed, a variety of products can be made and it is not possible to describe the properties of HMT starches in a way that applies to all products. There are exceptions, but during HMT, granules usually remain intact. Common property changes that occur upon HMT are increases in the pasting temperature and hot-paste viscosity and decreases in granule swelling, peak viscosity, breakdown (increased stability to cooking conditions), and solubility (leaching of amylose from swollen granules), giving the starch cooking characteristics similar to those of a lightly cross-linked starch. Other changes frequently seen are a broadening of the gelatinization temperature range, increases in gel hardness, and the formation or perfection of amylose-native lipid complexes. The granule characteristics and the pasting and paste properties of HMT and annealed (next section) starches are similar, but not identical. HMT starches usually contain slightly to moderately more thermostable slowly digestible starch (SDS) and resistant starch (RS). Treatment of starch with acid before or conducting HMT under slightly acidic conditions affects depolymerization and increases the amount of thermostable RS (total dietary fiber). Also, addition of a fatty acid (especially lauric acid) to normal corn starch before HMT increases the amounts of thermostable SDS and RS. The pasting temperature of the product also increases and the peak viscosity, breakdown, and setback decrease when the moisture content of the lauric acid-treated starch is raised from 10% to 30% before HMT, but peak viscosity, breakdown, setback, and final viscosity then increases when the moisture content of the starch to be subjected to HMT is raised from 30% to 50%—an indication of the wide variety of physically modified products with different properties that can be made from just normal corn starch.

Annealing Annealing is also a hydrothermal treatment that changes the physical properties of starches. Annealing occurs when starch granules in an excess of water (>40%) are held at a temperature below that at which gelatinization begins (for corn starch generally 60–70°C [140–160°F] and for waxy maize starch generally 50–60°C [120–140°F]). The duration of the process has varied from minutes to days. Again, because of the diverse combinations of potential variables involved (type of starch, temperature, time), a variety of products can be made, and it is not possible to describe the properties of annealed starches in a way that applies to all products. During annealing, granules remain intact. There are fewer more common changes to the properties of starch brought about by annealing as compared to HMT. While there are exceptions, annealed normal corn starch often exhibits an increased gelatinization temperature and gel firmness, reduced swelling power, solubility (leaching of amylose from the swollen granules), breakdown (increased shear stability during cooking), and a narrowing of the gelatinization temperature range. As with HMT, the characteristics are similar to those of a lightly cross-linked starch. There is no cleavage of starch chains. Only slight or no increases in thermostable RS and SDS values have been reported. Annealing has been done both as single- and multistep processes, prior to and following HMT, prior to and following chemical modification, and following acid-thinning.

Heating Dry Starch Starches with acid-, shear-, and heat-tolerance profiles similar to those of chemically cross-linked starches with a low degree of cross-linking are prepared by heating a starch with a low (<15%) moisture content at a temperature above 100°C (212°F), but below that which effects thermal degradation. Making the starch alkaline and drying to <1% moisture before heating facilitates the property changes.

Nonthermal Treatments Milling Milling (sometimes called micronization because a decrease in particle size results from it) is most often used with flours. Milling is classified as a nonthermal process, but there is a thermal component (in addition to the mechanical component)

548 Corn

involved because of high temperatures at the points of impact and an overall increase in temperature with time during milling. Milling generally results in a mixture of undamaged granules, damaged granules, and fragments of damaged granules, with the amounts of each determined by the type and duration of milling and the type of starch. Waxy maize starch is more susceptible to milling damage than is normal corn starch. Milling can result in a loss of crystallinity, a decrease in the gelatinization temperature, and an increase in water vapor adsorption, solubility, and susceptibility to the action of amylases. Milling is likely to reduce the size of some amylopectin molecules via chain cleavage. An increase in fragmented amylopectin molecules increases the stickiness of food products.

Ultrasonication Differences in property changes result when starch granules are treated with ultrasound. Because of differences in the natures of the starches used and differences in treatment conditions (the variables of which are the nature of the medium, the nature of dissolved gases [the atmosphere above the medium], ultrasound input [frequency, power, amplitude, treatment time], temperature, and starch concentration), it is again impossible to detail changes in starch properties in a way that applies to all products. The shock waves produced during sonication generally erode and pit the granule surface. Ultrasound energy is generally insufficient to disrupt, or even distort, the crystallites of starch granules, but it can impact amorphous regions. Sonication generally increases water sorption, swelling power, and solubility and decreases paste viscosity (due to depolymerization of some starch polysaccharide [especially amylopectin] molecules).

High-Pressure Treatment High-pressure treatments have been used as nonthermal food processes to kill bacteria and inactivate enzymes. Several kinds of high-pressure treatments have been applied to corn starches—two of which are ultrahigh-pressure (UHP) (also known as high hydrostatic pressure [HHP]) treatment and treatment resulting from passing a starch slurry through a high-pressure homogenizer. UHP treatment is a static treatment wherein a slurry of starch granules is subjected to a pressure above 400 MPa (58,000 psi). Variables are the type and concentration of the starch, pressure, time, and temperature. UHP treatment is believed to force water into granules without inducing granule swelling. Forcing water into granules effects gelatinization (which occurs over a pressure range); so by employing various combinations of the variables, it is possible to achieve different degrees of gelatinization (from 0% to 100%) while maintaining the granular form of the starch (when normal corn starch is used), which is quite different from thermal gelatinization, which is a rather all-or-none process in which the granular nature of the starch is at least partially destroyed. UHP treatment of normal corn starch results in decreased swelling power and viscosity. Since granules are gelatinized while maintaining their form, they have characteristics similar to those of a cold-water-swelling product. Waxy maize starch is more susceptible to UHP treatment than is normal corn starch. UHP treatment of waxy maize starch results in increased granule swelling and viscosity and extensive destruction of granules. High-pressure homogenizers produce high shear, turbulence, and cavitation. When a starch slurry is passed through a high-pressure homogenizer, granules may swell and fragment; they may also aggregate due to surface gelatinization. The gelatinization temperature decreases as granules lose some of their crystallinity. Solubility may increase.

Pulsed Electric Field When starch granules in an aqueous slurry are subjected to a pulsed electric field, they become distorted, crystallinity is lost, and gelatinization temperatures decrease.

REFERENCES BeMiller, J.N., 2018. Physical modification of starch. In: Sj€oo€, M., Nilsson, L. (Eds.), Starch in Food, second ed. Elsevier, Duxford (Chapter 5 ). BeMiller, J.N., Huber, K.C., 2015. Physical modifications of food starch functionalities. Annu. Rev. Food Sci. Technol. 6, 19–69. Chiu, C.-W., Solarek, D., 2009. Modification of starches. In: BeMiller, J., Whistler, R. (Eds.), Starch: Chemistry and Technology, third ed. Academic Press, New York (Chapter 17). Huber, K.C., BeMiller, J.N., 2010. Modified starch: chemistry and properties. In: Bertolini, A.C. (Ed.), Starches: Characterization, Properties, and Applications. CRC Press, Boca Raton, FL (Chapter 8). Jane, J.L., 2009. Structural features of starch granules II. In: BeMiller, J., Whistler, R. (Eds.), Starch: Chemistry and Technology, third ed. Academic Press, New York (Chapter 6). Kesselmans, R.P.W., Bleeker, I.P., 2004. Oxidation of Starch. U.S. Patent 6822091. Mason, W.R., 2009. Starch use in foods. In: BeMiller, J., Whistler, R. (Eds.), Starch: Chemistry and Technology, third ed. Academic Press, New York (Chapter 20).

Corn Starch Modification Chapter

19

549

Maurer, H.W., 2009. Starch in the paper industry. In: BeMiller, J., Whistler, R. (Eds.), Starch: Chemistry and Technology. third ed. Academic Press, New York (Chapter 18). Parovuori, P., Hamunen, A., Forssell, P., Autio, K., Poutnen, K., 1995. Oxidation of potato starch by hydrogen peroxide. Starch/St€arke 47, 19–23. P
erez, S., Baldwin, P.M., Gallant, D.J., 2009. Structural features of starch granules I. In: BeMiller, J., Whistler, R. (Eds.), Starch: Chemistry and Technology, third ed. Academic Press, New York (Chapter 5). Rutenberg, M.W., Solarek, D., 1984. Starch derivatives: production and uses. In: Whistler, R.L., BeMiller, J.N., Paschall, E.F. (Eds.), Starch: Chemistry and Technology, second ed. Academic Press, Orlando, FL (Chapter 10). Wurzburg, O.B. (Ed.), 1986a. Modified Starches: Properties and Uses. CRC Press, Boca Raton, FL. Wurzburg, O.B., 1986b. Converted starches. In: Wurzburg, O.B. (Ed.), Modified Starches: Properties and Uses. CRC Press, Boca Raton, FL (Chapter 2).