starch,' in: ullmann's encyclopedia of industrial chemistry · 2012 wiley-vch verlag gmbh...

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Article No : a25_001

Starch

JAMES N. BEMILLER,Whistler Center for Carbohydrate Research, Purdue University,

West Lafayette, Indiana, USA

KERRY C. HUBER, School of Food Science, University of Idaho, Moscow, Idaho, USA

1. Introduction. . . . . . . . . . . . . . . . . . . . . . 113

2. Sources of Commercial Starches . . . . . . 114

3. Worldwide Starch Production . . . . . . . . 114

3.1. Starch Production and Consumption in

the EU . . . . . . . . . . . . . . . . . . . . . . . . . . 114

3.2. The USA Corn Wet-Milling Industry . . 115

3.3. Tapioca Starch Production in Thailand . 115

4. Molecular Structures and Properties of

Amylose and Amylopectin . . . . . . . . . . . 115

5. Structures and Properties of Starch

Granules . . . . . . . . . . . . . . . . . . . . . . . . 117

5.1. Granule Structure . . . . . . . . . . . . . . . . . 117

5.2. Physicochemical Properties . . . . . . . . . . 119

5.2.1. Melting/Gelatinization . . . . . . . . . . . . . . . 121

5.2.2. Retrogradation. . . . . . . . . . . . . . . . . . . . . 121

5.2.3. Pasting and Viscoelastic Properties. . . . . . 122

6. Minor Components of Starch Granules . 124

7. Starch Biosynthesis and Genetics. . . . . . 124

8. Industrial Starch Production Processes . 124

8.1. Corn/Maize Wet Milling . . . . . . . . . . . . 124

8.1.1. Grain Cleaning . . . . . . . . . . . . . . . . . . . . 124

8.1.2. Kernel Steeping. . . . . . . . . . . . . . . . . . . . 124

8.1.3. Kernel Milling and Fraction Separation . . 124

8.1.4. Starch Drying . . . . . . . . . . . . . . . . . . . . . 125

8.2. Wheat Starch . . . . . . . . . . . . . . . . . . . . . 125

8.3. Potato Starch . . . . . . . . . . . . . . . . . . . . . 125

8.4. Tapioca/Cassava Starch . . . . . . . . . . . . . 126

8.5. Rice Starch . . . . . . . . . . . . . . . . . . . . . . 126

9. Modified Starches . . . . . . . . . . . . . . . . . 126

9.1. Chemically Modified Starches . . . . . . . . 128

9.1.1. Converted Starches and Hydrolyzates . . . . 1289.1.1.1. Thinned Products. . . . . . . . . . . . . . . . . . . 1289.1.1.2. Starch Dextrins . . . . . . . . . . . . . . . . . . . . 1299.1.1.3. Dextrose Equivalency . . . . . . . . . . . . . . . 1299.1.1.4. Maltodextrins . . . . . . . . . . . . . . . . . . . . . 1309.1.1.5. Syrup Solids . . . . . . . . . . . . . . . . . . . . . . 1309.1.1.6. Syrups and Crystalline D-Glucose . . . . . . . 1309.1.1.7. High-Fructose Syrups and Crystalline

D-Fructose . . . . . . . . . . . . . . . . . . . . . . . . 1319.1.1.8. Other Starch Conversion Products . . . . . . 131

9.1.2. Cross-linked Starches. . . . . . . . . . . . . . . . 131

9.1.3. Stabilized Starches . . . . . . . . . . . . . . . . . 132

9.1.4. Cationic Starch . . . . . . . . . . . . . . . . . . . . 132

9.1.5. Starch Graft Copolymers . . . . . . . . . . . . . 133

9.2. Thermally Modified Starches. . . . . . . . . 133

9.2.1. Instant Starches . . . . . . . . . . . . . . . . . . . . 133

9.2.2. Annealed and

Heat-moisture-treated Starches . . . . . . . . . 134

9.2.3. Dry Heating of Starches. . . . . . . . . . . . . . 134

9.2.4. Destructurized and Thermoplastic Starch . 134

9.3. Genetically Modified Starches . . . . . . . . 135

9.4. Multiple Modifications of Starches . . . . 135

10. Examples of Uses of Starches and

Products Derived from Starches . . . . . . 136

10.1. Uses of Starches . . . . . . . . . . . . . . . . . . . 136

10.2. Uses of Products derived from Starches

via Extensive Depolymerization . . . . . . . 138

11. Starch Digestibility . . . . . . . . . . . . . . . . 139

References . . . . . . . . . . . . . . . . . . . . . . . 139

1. Introduction

Starch [9005-25-8] is widely used in nonfoodindustrial applications, especially in the produc-tion of paper and paperboard products, as a textilesizing agent, and in fermentation processes toproduce ethanol and other products, and in theprocessed food industry as a thickener/stabilizer,gelling agent, and a starting material for theproduction of sweeteners and polyols. It is alsothe principal source of dietary calories for the

world’s human population. The chemistry andtechnology of starch has been reviewed in severalbooks [1–7].

Native starch occurs as discrete particlescalled granules. A starch from a specific biologi-cal (plant) source is unique among starches, i.e.,starch granules from the various plant sourcesdiffer in appearance, particle size distribution,fine structure of the constituent polymer mole-cules, and physical properties. Most applicationsof starch are realized only after the starch is

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heated (cooked) in the presence of water, aprocess that destroys its granular structure andreleases its constituent polymer molecules.

Granular starch is generally composed of twotypes of molecules, amylose and amylopectin(Chap. 4). Amylose [9005-82-7] is a predomi-nantly linear (1,4)-a-D-glucan (Fig. 1), althoughsome amylose molecules are slightly branched.Amylopectin [9037-22-3] (Fig. 2) has a branch-on-branch structure consisting of mostly short

chains of (1,4)-linked a-D-glucopyranosyl units(sometimes referred to as anhydroglucose units)linked to other short, linear chains via a-(1,6)branch points. The amylopectin fraction is com-posed of much larger molecules than those ofamylose. Most normal starches contain 70–75%amylopectin molecules by weight. Different mo-lecular structures and sizes, amylose/amylopec-tin ratios, and granular architectures give eachtype of starch its unique properties.

2. Sources of Commercial Starches

Major crops used for starch production includecorn (maize), cassava, potato, and wheat. Cornused to produce starch includes hybridized ge-netic variants of corn (Chap. 4). Lesser amountsof starch are isolated industrially from arrowroot,mung bean, rice, sago palm, sweet potato, yam,yellow pea, and other plants.

3. Worldwide Starch Production

In 2006, it was reported that 99% of the globalstarch production of ca. 60�106 t originatedfrom crops of corn/maize, cassava/tapioca,wheat and potatoes, with 73% being maizestarch, 14% tapioca starch, 8.1% wheat starch,and 3.7% potato starch [8]. Starch is predomi-nantly produced in the USA, the EU, Japan, andThailand. The largest commercial starch produ-cers worldwide are Cargill (9.2�106 t, 14.6%),CPI (5.2�106 t, 8.2%), ADM (5.2�106 t, 8.2%),National Starch and Chemical Co. (1.2�106 t,1.9%), and Avebe (0.6�106 t, 1.0%). Approxi-mately 33.6�106 t (53.1%) of starch is producedby small to medium-sized companies.

3.1. Starch Production andConsumption in the EU

The starch industry in Western Europe is com-prised of 24 different companies with 68 plants,which in 2005, produced 9.6�106 t of starchdistributed as follows: 46% corn/maize starch(4.4�106 t), 36% wheat starch (3.4�106 t), and18% potato starch (1.7�106 t) [8]. Production ofstarch from yellow pea has also been instituted.

Figure 1. A three-glucosyl unit segment of an unbranched(i.e., linear) portion of an amylose or amylopectin molecule

Figure 2. A representation of amylopectin molecules show-ing crystalline packing of double-helical pairs of branchchains(Reprinted with permission from A. Imberty, A. Bul�eon, V.Tran, S. Perez, Starch/St€arke 43 (1991) 375.)

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In 2005, 9.0�106 t of starch products wereconsumed in the EU for production of starchhydrolysates, including high-fructose syrups(5.1�106 t, 57%), native starches (2.1�106 t,23%), and modified starches (1.8�106 t, 20%).Approx. 57% of the 9.0�106 t was used in theprocessed food industry and 43% in the nonfoodsector. The breakdown by application areas/mar-ket sectors was as follows: sweets and drinks(confectionary, beverages, fruit processing)30%, processed food (convenience food, bakery,food ingredients and food preparations, dairyproducts and ice cream) 27%, paper and corru-gated board manufacture 28%, chemical, fer-mentation, and other industrial products 14%(Chap. 10).

3.2. The USA Corn Wet-MillingIndustry

By far themajority of starch produced in theUSAis isolated from corn/maize, with relatively smallamounts of potato and wheat starch also beingproduced. Corn starch is produced by a wet-milling process (Section 8.1). Production figuresfor member companies of the Corn Refiner’sAssociation (USA) for 2008 are given in Table1 [9]. The production ratio of high-fructose syr-ups to glucose syrups was approx. 67:33. Inaddition, 4.5�109 L of fuel ethanol [64-17-5]was produced by corn wet millers, although mostof the fuel ethanol made in the USA by fermen-tation was obtained from corn by a dry-grindprocess. The sale of coproducts (see Table 1)from the wet-milling process is important to theeconomic viability of the industry.

3.3. Tapioca Starch Production inThailand

The greatest amount of tapioca starch is producedin Thailand. Lesser amounts are produced inIndonesia, Brazil, and China. In 2009, Thailandexported 1.8�106 t of native tapioca starch,0.7�106 t of modified tapioca starch, and0.02�106 t of tapioca pearls (total ¼ 2.5�106 t)[10].

4. Molecular Structures andProperties of Amylose andAmylopectin

Amylopectin. The major polysaccharide ofstarch, amylopectin, is a very large, highly-branched molecule. The branch points constitute4–5% of the total glycosidic linkages and occurin clusters. There are three general classes ofchains. A chains are those that are connected toanother chain via a (1, 6) linkage, but are them-selves unbranched. B chains are those chains thatare connected to another chain via a (1, 6) linkageand have one or more A chains or other B chainsattached to them, i.e., they are further branched.B chains can be subdivided into chains of variouslengths/sizes. The C chain is the one possessingthe lone reducing end-unit of the amylopectinmolecule. In starch granules, the A and short Bchains occur as pairs of chains entwined aroundeach other in double helices (Fig. 2), which packtogether to give rise to the crystallinity of starchgranules. Average molecular masses of amylo-pectin molecules are at least 107 and may be aslarge as 109.

The original cluster model for amylopectinhas been refined several times, but has kept itsbasic form [11–14] until recently when a varia-tion was proposed [15] that accounts for thesuperhelical nature of the amylopectinmoleculesproposed to be present in at least some starches.

Amylopectin is present in all known starches,constituting about three-fourths of most normalstarches (Tables 2 and 3); indeed, some starchesconsist entirely of amylopectin (i.e., they lackamylose). Starches containing only amylopectin(all-amylopectin starches) are often referred to aswaxy starches. Average fine structures, averagemolecular masses, molecular mass ranges, and

Table 1. Products from the corn wet-milling industry in the United

States for 2008

Producta Production, 106 t

Sweetenersb 5.2

Starchesc 1.5

Coproductsd 5.9

aDry weight.bAlso called conversion products. Includes high-fructose syrups

(42% and 55% fructose), glucose syrups, crystalline glucose

(dextrose), crystalline fructose, maltodextrins, and syrup solids.c Includes native corn starches, modified corn starches, and

dextrins.d Includes corn oil, corn oil meal, corn gluten feed, corn gluten

meal, and steepwater.

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perhaps, the shapes of amylopectin moleculesvary from starch to starch and are importantdeterminants of the physical properties of astarch, as well as foods and/or other productsthat contain starch.

Amylose, the other naturally occurringstarch biopolymer, is an essentially linear chainof (1,4)-linkeda-D-glucopyranosyl units (Fig. 1).Some amylose molecules are slightly branched,with branch points constituting 0.3–0.5% of thetotal glycosidic linkages. Because there are onlya few branches, with the branches usually beingeither very long or very short chains, and because

the branch points are usually far apart, amylosemolecules behave as linear polymer molecules.The average molecular masses of amyloses fromdifferent commercial starches are in the 105–106

range, which means that they have average de-grees of polymerization (DP, number of glucosylunits per molecule) of ca. 600–6000. The natureof the glycosidic linkages in amylose chainsproduces a natural right-handed helix (Fig. 1),which has consequences for its properties.

Most starches contain 25–30% amylose (asshown in Tables 2 and 3, where their generalproperties are outlined) and are known as normalstarches. Starches containing more than this

Table 2. General properties of granules and pastes of native corn/maize starches

Normal corn starch Waxy maize starch High-amylose corn starcha

Granule size, mm 2–30 2–30 2–24

Amylose (approx.)b, % 28 0 50–70

Gelatinization/pasting temperaturec, �C 74–81 66–71 66–71d

Relative viscosity medium medium-high very low

Paste rheology (body) short long short

Paste clarity opaque slightly cloudy slightly opaque

Tendency to gel/retrograde high very low very high

Gel consistency firm nongelling very firm

Lipid, % dse 0.8 0.2 –

Protein, % dse 0.35 0.25 0.4

Starch-bound phosphorus, % dse 0 0 0

X-ray diffraction pattern type A A B

aAlso known as amylomaize starch.bThe percentage of amylopectin is the difference between 100% and the percentage of amylose.cFrom the initial temperature of gelatinization to complete cookout.dUnder ordinary cooking conditions, where the slurry is heated to 95–100 �C, high-amylose corn starch produces little viscosity.

Gelatinization of high-amylose corn/maize starch with ca. 50% apparent amylose content may begin as low as 66 �C and end as high as

166 �C, depending on the moisture content. Gelatinization of high-amylose corn/maize starch with ca. 70% apparent amylose content may

begin as low as 66 �C and end as high as 171 �C, depending on the moisture content.eds ¼ dry solids.

Table 3. General properties of granules and pastes of potato, tapioca, and wheat starches

Potato starch Tapioca starch Wheat starch

Granule size, mm 5–100 4–35 0.5–45a

Amylose (approx.)b, % 21 17 28

Gelatinization/pasting temperaturec, �C 58–65 52–65 52–85

Relative viscosity very high high medium-low

Paste rheology (body) very long long short

Paste clarity clear clear cloudy

Tendency to gel/retrograde medium-low low high

Gel consistency salve-like soft soft

Lipid, (% ds)d < 0.1 < 0.1 0.9

Protein, % dsd 0.1 0.1 0.4

Starch-bound phosphorus, % dsd ca. 0.08 0 0

X-ray diffraction pattern type B A A

aBimodal population.bThe percentage of amylopectin is the difference between 100% and the percentage of amylose.cFrom the initial temperature of gelatinization to complete cookout.dds ¼ dry solids.

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amount of amylose are known as high-amylosestarches. Two commercial corn starches, knownas high-amylose corn starches or amylomaizestarches, have apparent amylose contents of ca.50% and >70%.

Several particular properties of amylose areimportant with regard to applications of starch.Heating of starches in water with shear formswhat are called pastes. Pastes from amylose-containing starches are generally opaque andusually form firm gels upon cooling. Precipita-tion, rather than gelation, may occur in dilutepastes as they cool. The opacity and the forma-tion of a gel or a precipitate results from amylosemolecules associating with each other in crystal-line order, a process that is called retrogradation(see Section 5.2.2), or when it is associated withdetermining paste properties, setback (see Sec-tion 5.2.3).

Helical amylose chains have hydrophobicinteriors capable of forming inclusion complexeswith linear hydrophobic portions of moleculesthat can fitwithin the lumen (i.e., the inner cavity)of the helix. When a hot, aqueous dispersion ofstarch is stirred with a slightly polar, slightlywater-soluble organic compound, such as butan-1-ol, and then cooled, amylose complexes crys-tallize out and can be isolated by centrifugation.Complexation stabilizes the helix and convertsthe long, linear molecules into more uniform andmore rod-like structures. Because only the amy-lose molecules crystallize, such complexationcan be employed to isolate pure amylose.

Iodine (as I�3 ) complexes with amylose andamylopectin molecules, with complex formationalso occurring within the hydrophobic interiorsof helical segments of starchmolecules. The longhelical segments of amylose allow long chains ofpoly(I�3 ) to form and produce the blue color thatis a diagnostic test for starch (more specifically,amylose). The amylose–iodine complex contains19% iodine, and determination of the amount ofiodine complexed is used to measure the amountof apparent amylose in a starch. Amylopectinforms reddish brown complexes with iodinebecause its branches are too short for the forma-tion of long chains of poly(I�3 ).

Polar lipids (surfactants/emulsifiers and fattyacids) affect starch pastes and starch-based foodsin one or more of three ways as a result ofcomplex formation: (1) by affecting starch gela-tinization (see Section 5.2.1) and pasting, (2) by

modifying the rheological behavior of the result-ing paste, and (3) by inhibiting (or in some casesaccelerating) the crystallization of starch mole-cules associated with the retrogradation process.Specific changes to a paste that are observed uponthe addition of a lipid depend on its structure andthe starch employed, i.e., different lipids/surfac-tants affect the gelatinization and pasting beha-viors of a given starch differently, and each lipid/surfactant affects starches from different botani-cal sources differently. Because complex forma-tion with emulsifiers occurs much more readilywith amylose and, as a result, has a much greatereffect on amylosemolecules than on amylopectinmolecules, polar lipids have a much greatereffect on normal starches than on waxy (all-amylopectin) starches. Addition of most lipids/surfactants/emulsifiers to starches containingamylose inhibits the processes associated withgelatinization and pasting, but some speed upthese processes and/or cause them to occur atlower temperatures. Polar lipids inherent to na-tive cereal starches generally inhibit retro-gradation.

The ability of polar lipids to form complexeswith amylose and amylopectin is associated withtheir chain length, their degree of unsaturation,and the nature of their hydrophilic group. Ingeneral, esters of myristic (C14, saturated) andpalmitic (C16, saturated) acids aremost effective.

5. Structures andProperties of StarchGranules

5.1. Granule Structure

Within plants, synthesized starch molecules areassembled in the form of semicrystalline aggre-gates, termed granules, which vary according tosize (< 1–100 mm) and shape (spherical, ellip-soidal, polygonal, lenticular, etc.) depending ontheir botanical origin [16]. Starch granule bio-synthesis occurs within an organelle called theamyloplast and originates at a point called thehilum (the approximate center of the granule),from which site starch molecules are depositedin a radial, spherocrystalline arrangement [17].The precise architecture of starch granules israther complex and consists of multiple levelsof structural organization. At the most basic

Vol. 34 Starch 117


organizational level, the granule is comprised ofalternating semicrystalline and less crystalline(frequently called amorphous) shells, commonlyreferred to as growth rings [17–19] (Fig. 3). On ahigher organizational level, a semicrystallineshell or growth ring itself is comprised of alter-nating crystalline and amorphous lamellae,which correspond to the molecular features ofamylopectin molecules. The branching regionsof amylopectin molecules comprise amorphousareas within the crystalline regions of both shells,while the short, linear segments of amylopectinbranch chains organized in double helices (Chap.4) constitute the crystalline lamellae. Anotherlevel of organization, intermediate to those al-ready described, involves aggregation of adja-cent double-helical segments of amylopectin toform crystalline blockets [17, 18]. In short, am-ylopectin is primarily responsible for the organi-zation of starch granules and accounts for theirsemicrystalline nature. In contrast, amylose isbelieved to be located in amorphous regions,where it is proposed to be randomly interspersed

among amylopectin clusters and/or to be in-volved in complexeswith native polar lipids [17].

The spherocrystalline radial arrangement ofstarch molecules in the granule is inferred fromthe birefringence (as evidenced by the Maltesecross) that is seen onmicroscopic examination ofstarch granules under plane-polarized light [17](Fig. 4). As determined by X-ray diffraction,native starch granules generally exhibit crystal-linities ranging from 15–45% [20, 21]. X-raydiffraction of native starches reveals three dis-tinct patterns, A, B, and C (not to be confusedwith the A, B, and C chains of amylopectinmolecules already described), which define thespecific allomorphic packing arrangement ofstarch double helices (i.e., crystallites) presentwithin granules (Fig. 5) [17, 20]. The specifictype of crystallite packing observed in a starch isprimarily a function of both the temperatureconditionwithin the plant during starch synthesisand the chain length. An average chain length inexcess of DP 12 tend to favor the formation of theB allomorph over the A allomorph (average DP

Figure 3. Schematic diagram of the starch granuleA) Structural relationship between the semicrystalline and amorphous shells (growth rings); B) Structural relationship betweencrystalline and amorphous lamellae; C) Amylopectin molecular structure(Reprinted with permission fromP.J. Jenkins, R.E. Cameron, A.M.Donald,W. Bras, G.E. Derbyshire, G.R.Mant, A.J. Ryan, J.Polym. Sci., B-Polym. Phys. 32 (1994) 1579.)

118 Starch Vol. 34


10–12) [17]. In general, tuber starches produce Bcrystallites (starch synthesized underground un-der cool conditions), while cereal starches pos-sess an A crystalline arrangement (starch synthe-sized above ground under hot, dry conditions);legume starches exhibit the C allomorph, whichis a mixture of A and B crystalline allomorphs[17, 20].

At themacrostructural level, granules of corn/maize and wheat starches possess pores on ex-ternal granule surfaces [22–24], which featuresrepresent openings to channels leading into thegranule interior [23–26]. Channels within gran-ules of these starches have been demonstrated toimpact granular patterns of amylolytic degrada-tion and chemical modification by facilitatingaccess of enzymes and chemical reagents to thegranule interior [22, 27–29].

5.2. Physicochemical Properties

Native starch granules, which possess complexlong-range and short-range molecular order, areinsoluble in cold water, but are neverthelesscapable of absorbing water (0.48–0.56 g/g drystarch) due to their semi-amorphous nature [30].In hydrated granules, water has a plasticizingeffect on amorphous regions, effectively lower-ing the overall glass transition temperature (Tg)and enhancing mobility of the biopolymers. Hy-dration (at 25 �C) produces increases in granulediameter (i.e., swelling) of approximately 10%and 25% for normal and waxy corn starch,respectively, which swelling is reversible innature.

The most significant physicochemical prop-erties of starch are associated with irreversiblethermal transitions that result in increased starchgranule swelling and dissolution of the starchbiopolymers. Heating of starch granules in ex-cess water (i.e., water-starch ratios > 1.5:1)brings about gelatinization (the irreversible loss

color

fig

Figure 4. Depiction of the Maltese cross observed withinstarch granules (potato) viewed under plane-polarized light

Figure 5. Depiction of A and B polymorphic structures for starch double helices(Reprinted with permission from H.F. Zobel, Starch/St€arke 40 (1988) 1.)

Vol. 34 Starch 119


of granular/molecular order), which is accompa-nied by increased granule hydration, swelling,and leaching of soluble components (primarilyamylose) (Fig. 6) [31]. Upon heating, hydratedmolecules within granule amorphous regionsbegin to vibrate, inducing progressive disruptionof hydrogen bonds to further increase hydrationand plasticization of starch molecules withingranules. Granule swelling becomes irreversibleas the strain imposed within granule amorphousregions becomes sufficient to bring aboutassisted disruption of starch crystallites. Gelati-nization is marked by the disappearance ofbirefringence and a complete loss of granulecrystallinity [30]. When starch is present in suf-ficient concentration (generally 2–7%), swollengranules begin to press against one another asthey occupy nearly the entire volume of theaqueous continuous phase, leading to greatlyincreased viscosity.

In the presence of shear, highly swollengranules are fragile and disintegrate to form apaste, which is comprised of hollow, swollengranules (ghosts) and broken granule remnants(ghost fragments) dispersed within a continuousphase of partially dissolved starch molecules.As a paste is cooled, amylose molecules reas-sociate or recrystallize in a process called ret-rogradation, which involves formation of dou-ble-helical structures that aggregate to form athree-dimensional gel network (Fig. 6) or pre-

cipitate [32]. Starch gels are a two-phase, mixedgel comprised of both amylose-rich and amylo-pectin-rich regions due to incompatibility of thetwo polymers [31]. Due to their branch-on-branch structure and relatively short branchchains, amylopectin molecules undergo verylimited intermolecular associations and retro-grade very slowly (although they may graduallycrystallize somewhat with time) [32]. Thus,waxy starches, which consist primarily of amy-lopectin, lack the ability to form strong gelnetworks. Starch paste characteristics are de-fined by: (1) the rheology of the continuousphase, (2) the rigidity of granule remnants andthe volume occupied by them, and (3) the extentand nature of interactions between the continu-ous and dispersed phases [30].

Short and long flow are terms that remain inuse within the starch industry to describe thenature of a starch paste, although they have beenlargely replaced by more exact rheological char-acteristics (Section 5.2.3). Short and long flowrefer to the draining behavior from a pipette orfunnel. As the forming drop gets larger, it be-comes heavier and the flow rate increases. If theweight of the stream causes it to break so that thefluid exits in small drops, the fluid is said to have ashort flow. Such a fluid is pseudoplastic, i.e.,shear-thinning. Fluids without shear-thinningbehavior exit in long strings (long flow, slimytexture).

Figure 6. Schematic representation of the structural changes associated with the gelatinization and pasting of native starchgranules. Gelatinization (loss of granular molecular order) is accompanied by granule swelling and leaching of soluble starchcomponents (amylose) during heating in an aqueous system. With the application of shear, swollen granules undergo furtherdisintegration to yield a paste, which is comprised of a continuous phase of solubilized starch and a dispersed phase of granuleremnants. Upon cooling, amylose retrogradation (depicted by the cross-hatching betweenmolecules within the paste) results inthe formation of a gel network. (Reprinted with permission from K.C. Huber, A. McDonald, J.N. BeMiller: ‘‘CarbohydrateChemistry’’, inY.H.Hui, F. Sherkat (eds.):Handbook of Food Science, Technology, andEngineering, vol. 1, Taylor&Francis/CRC Press, Boca Raton, 2006.)

120 Starch Vol. 34


5.2.1. Melting/Gelatinization

Melting and gelatinization transitions of starchesare endothermic in nature and may be bothdetected and quantified via differential scanningcalorimetry (DSC). Due to the heterogeneousnature of crystallites present within granules,starch thermal transitions occur over a tempera-ture range rather than at a defined temperature.

At low moisture levels (water/starch ratios <1:1), the term ‘‘melting’’ is used to define thedisappearance of starch crystallinity in responseto heating [31].Melting of native starch producesup to four endothermic transitions – the first twobeing associatedwith disordering of amylopectindouble helices and the latter two reflecting orderto disorder transitions of complexes of amylosewith the native lipids of the starch granules [30].Thermal transitions occurring under limitedmoisture conditions (as low as 11%) are gener-ally shifted to elevated temperatures (as high as180 �C), depending on the amount of waterpresent for plasticization [30, 31]. Melting phe-nomena are likely to be encountered within high-temperature/low-moisture processes such as ex-trusion or baking of low-moisture products.

Gelatinization, which is characterized by thethermal disordering of starch (amylopectin) crys-tallites in excess water (water/starch ratios >1.5:1) [31], generally occurs over a 10–15 �Ctemperature range [30]. Thus, a gelatinizationendotherm obtained via DSC is defined by anonset (To), peak (Tp), and completion (Tc) tem-perature, as well as a gelatinization enthalpy(DH). For a native normal starch (i.e., amylosecontent in the range of 20–30%), the gelatiniza-tion temperature range generally falls between 55and 80 �C, with the precise temperature rangebeing specific to the botanical source of the starch[16, 30, 33]. High-amylose starches exhibit muchhigher and broader gelatinization temperatureranges (70–130 �C) relative to normal starchesdue to their abundance of long amylopectinbranch chains [16, 33]. Aside from the eventsdescribed for gelatinization, a second endother-mic event (90–120 �C) may be observed forstarches possessing amylose–lipid complexes(V-structures) (Chap. 4) [30, 31].

Starch gelatinization temperatures and rangesare impacted by the presence of other systemconstituents, particularly solutes. Sugars gener-ally tend to increase the starch gelatinization

temperature, but reduce the gelatinization tem-perature range [30]. Hypotheses proposed toexplain this effect include competition of sugarswith starch for water (solvent), physical sugar-starch interactions, and/or anti-plasticization ef-fects of the sugar-water cosolvent [30, 31]. Lowsalt concentrations inherent to most foods mayincrease the starch gelatinization temperature;salt effects on gelatinization become much morevaried and complex at high (salt) concentrations[30]. Monoacyl and other polar lipids, eithernative or incorporated, may increase starch ge-latinization temperatures due to complexationwith amylose chains [31]. At typical concentra-tions encountered in food systems, hydrocolloidsexhibit minimal impact on starch gelatinizationcharacteristics (as measured by DSC) [30], al-though they do significantly influence starchpaste rheology (Section 5.2.3).

5.2.2. Retrogradation

Retrogradation involves reassociation (crystalli-zation) of starch chains within a paste uponcooling (below Tm) to form either a viscoelasticgel network or a precipitate [31]. Although amy-lose and amylopectin chains are both capable ofcrystallization, the two polymers retrograde onvastly different timescales, with amylose crys-tallization being very rapid (minutes to hours)and amylopectin reassociation being very slow(days to weeks). For starch pastes comprised ofboth amylose and amylopectin, gelation involvesboth phase separation (formation of polymer-richand polymer-deficient regions) and crystalliza-tion (primarily involving amylose chains), al-though the precise sequence of these two eventsis not fully resolved [30, 31]. Junction zoneswithin normal starch gels are comprised almostexclusively of aggregates of amylose double-helical chain segments [30].

Starch crystallization is defined by three pri-mary processes: nucleation (crystal initiation),propagation (crystal growth), and maturation(crystal perfection), of which nucleation is therate-limiting step [30]. For starch pastes of lessthan 50% by weight concentration, nucleation ismost favored at temperatures approaching T

0g (ca.�5 �C), whereas propagation (i.e., crystal

growth) is enhanced at temperatures approachingthe melting temperatures of the crystallites (Tm)

Vol. 34 Starch 121


[30]. A hold or storage temperature of less than60 �C, promotes formation of B crystallites (Sec-tion 5.1) [30]. The melting temperatures of amy-lose and amylopectin crystallites differ dramati-cally (140–160 �C and 45–60 �C, respectively),with amylose associations being much morethermoresistant [30]. Retrogradation may bemonitored via a multitude of techniques includ-ing DSC, gel turbidity, light scattering, opticalrotation, FTIR, NMR, and viscometry [30].Within foods, excessive starch retrogradationoften leads to product defects, such as syneresis,loss of viscosity, or staling of bakery products(amylopectin crystallization is thought to have apartial role here). Conversely, retrogradationmay also be considered beneficial to various foodproducts and processes (e.g., in the preparation ofcroutons, frozen potato products, and resistantstarch) and may be promoted via defined coolingand/or temperature cycling schemes. Retrogra-dation is very much a phenomenon to be avoidedin papermaking processes, particularly in sizingand coating operations.

The tendency for a starch to retrograde can bedecreased through chemical modification (i.e.,stabilization; Section 9.1.3). Starch retrograda-tion may also be greatly influenced by othersystem components, though the specific roles ofparticular classes of constituents are often variedand difficult to categorize in unified fashion.However, there is reasonable consensus for theeffects of monacyl and polar lipids, which gen-erally decrease the extent of retrogradation bypromoting formation of amylose-lipid com-plexes (Chap. 4) [30, 31], although the reversehas also been reported. Effects of various low-molecular mass sugars and oligosaccharides onstarch crystallization vary according to the ste-reochemistry of the solute. Generally, solutesthat exhibit favorable hydrogen bonding interac-tions with the native water structure are sug-gested to reduce starch crystallization, whereassolutes that disrupt the water network tend topromote starch retrogradation [30]. Water-solu-ble polysaccharides can enhance the starch ret-rogradation rate (via phase separation and con-centration of starch polymer microdomains), butmay either increase or decrease gel rigidity de-pending on the specific hydrocolloid added [30,31]. In general, nonstarch polysaccharides tendto enhance the water-holding capacities andfreeze-thaw behaviors of starch gels [31, 34].

5.2.3. Pasting and Viscoelastic Properties

Upon cooking in excess water, a starch granuleslurry experiences dramatic physical changes inresponse to swelling, gelatinization, pasting, andretrogradation events. A pasting profile providesa definitive, systematic fingerprint that embodiesthe flow behavior of a starch-containing materialas it undergoes these defined events. While theBrabender ViscoAmylograph was traditionallyused for generating pasting profiles, the RapidVisco Analyser (RVA) has now emerged as theinstrument of choice, due to a smaller samplerequirement for analysis, reduced analysis times,and a defined unit of viscosity measure (cP[mPa�s] or Rapid Visco Analyser Units [RVU]).(A Brabender Micro Visco-Amylo-Graph withsimilar functions as the RVA is also available[30].) A pasting profile is generated by theseinstruments by monitoring the viscosity of anaqueous starch suspension over the course of aspecified heating-cooling cycle, in the presenceof shear, over a defined analysis time period.Heating and cooling rates, length of hold, rate ofshear, and heating and cooling temperatures mayall be varied by the operator, though traditionalprofiles generally heat to 95 �C, hold at 95 �C,and cool to 50 �C.Defined pasting attributes (i.e.,pasting temperature, peak viscosity, etc.) may beobtained at various points of a pasting profile toyield specific information about starch swellingproperties, as well as paste stability and retrogra-dation tendency (Fig. 7).

Pasting time represents the point in time (for adefined heating rate) at which sufficient starchgranule swelling has occurred to register aninitial rise in viscosity (above that of the originalungelatinized granule slurry). This value is oftenreported in regard to temperature (i.e., pastingtemperature). However, temperature values pro-vided by the RVA instrument reflect the instru-ment heating block temperature rather than thatof the actual paste. Thus, there is a lag in the truepaste temperature relative to that of the heatingblock, although the extent of lag can be mathe-matically corrected to estimate the true pastetemperature for a given heating rate value [35].Pasting temperature provides an estimate of theminimum temperature needed to cook a starchunder given slurry conditions [36].

Peak viscosity results from a rapid swelling ofgranules upon gelatinization. Paste composition

122 Starch Vol. 34


at the peak is comprised of a combination of bothhighly swollen and ruptured granules within acontinuous phase of solubilized starch. Peakviscosity is indicative of starch thickening power[30], and can be used to provide a rough estimateof the viscous load likely to be achieved in acooking/mixing operation [36].

During the hold period at peak temperature(usually 95 �C), a starch paste will experience aloss in viscosity due to rupturing of the fragile,swollen granules until a trough viscosity (i.e., hotpaste viscosity) is reached. (Trough viscosityrepresents the viscosity of the solubilized starchcontinuous phase containing dispersed granuleremnants). Breakdown (peak viscosity minustrough viscosity) indicates the relative stabilityof a paste to high temperature and shear condi-tions. Chemical cross-linking of starch is con-ducted to stabilize granules against paste break-down (Section 9.1.2).

Upon cooling a starch paste, amylose mole-cules begin to reassociate to form junction zonesto achieve a final viscosity. Setback (final viscos-ity minus trough viscosity) reflects the extent ofamylose recrystallization occurring within astarch paste, and is generally correlated positive-ly with syneresis tendency [36]. Waxy starchpastes exhibit minimal setback, due to the lackof amylose. Normal starches, which generallyexhibit a high propensity toward syneresis, may

be chemically substituted to reduce the tendencyof the starch to retrograde (Section 9.1.3).

Beyond pasting properties, more sophisticatedanalysis of starch dispersion and/or gel propertiesmay be conducted under conditions of constantstress or constant strain using uniaxial compres-sion testing (both large and small deformationtests), a rotaryviscometer,oradynamicoscillatoryrheometer [30]. These analyses determine starchviscoelastic properties, defined as the degree ofsolid-like (storage modulus,G0) versus liquid-like(loss modulus, G00) behavior present within a gel.

Nonstarch constituents present in a starchdispersion may have a significant effect on pasterheological behavior. High solute concentrationsgenerally inhibit starch swelling, increase starchpasting temperature, and decrease paste peakviscosity [30]. Sugars may either increase ordecrease gel strength, depending on their specificstereochemistry [30]; this effect was previouslydescribed in regard to retrogradation behavior(Section 5.2.2). Because of the essentially neutralcharacter of starch, low concentrations of saltshave little effect on gelation. Acids tend todecrease starch viscosity by promoting thehydrolysis of glycosidic linkages, effectivelydecreasing the molecular size of the starchpolymers (Section 9.1.1). Polar lipids, throughcomplexation with amylose chains (Chap. 4),restrict granule swelling, increase starch pasting

Figure 7. Example of a pasting profile depicting the viscosity of a wheat starch granule suspension over the course of a heatingand cooling cycle. Specific pasting attributes obtained from the pasting profile are defined and labeled accordingly (bold line¼starch viscosity profile; thin line ¼ temperature profile)

Vol. 34 Starch 123


temperature, and reduce overall paste viscosity[31]. Hydrocolloid addition may alter starchpasting properties (pasting temperature, peakviscosity, breakdown, setback, final viscosity),as well as both the storage and loss moduli ofstarch gels, although both the nature and degreeof effects vary according to the specific polysac-charide [34]. To explain these effects, directstarch-hydrocolloid interactions and/or phaseseparation of the mixed polymer gel system havebeen proposed [31, 34].

6. Minor Components of StarchGranules

In addition to amylose and/or amylopectin mo-lecules, starches contain other components, in-cluding lipid, protein, and ash. Only cerealstarches contain significant amounts of lipids.Normal cereal starches contain ca. 0.6–1.2%total lipid by weight, consisting primarily of freefatty acids and lysophospholipids of differingratios and compositions in different starches.Starches without amylose, such as waxy maizestarch, contain only ca. 0.2% lipid. Noncerealstarches, such as potato starch and tapioca starch,contain less than 0.1% lipid. Normal cerealstarches contain ca. 0.35–0.40% protein byweight, while waxy maize starch contains ca.0.25% protein and potato and tapioca starchescontain ca. 0.1% protein. Potato starch containsmonostarch phosphate ester groups (0.06%–0.1% P by weight). Starch ash contents usuallyrange between 0.1 and 0.5% and are related to thelipid, protein, and phosphate ester contents.

7. Starch Biosynthesis and Genetics

The biosynthesis of amylose and amylopectinhas been and continues to be an area of intensestudy [37] due to the complex natures of themolecules, especially the clustered branchingpattern of amylopectin, and because of the desireto understand the impact of the biopolymerstructures on the properties of a starch. Thegenetics of starch biosynthesis has also receivedintense study [38] because of the potential im-portance of altering such characteristics as theamylose/amylopectin ratio and the fine structureof amylopectin.

8. Industrial Starch ProductionProcesses

8.1. Corn/Maize Wet Milling

Production of corn starch occurs primarily in theUSA, China, EU, Japan, SouthKorea, Brazil, andArgentina. Corn wet milling consists of severalsteps: (1) cleaning the grain, (2) steeping ofkernels, (3) milling of kernels and separation offractions (i.e., starch and nonstarch components),and (4) drying of the isolated starch [39].

8.1.1. Grain Cleaning

Grain is first cleaned by screening and by mag-neticmeans to remove foreignmaterial includingmetal.

8.1.2. Kernel Steeping

Cleaned grain is steeped to soften the kernel andto facilitate its separation into the desired com-ponents. This process involves countercurrentsteeping in water containing 0.10% sulfur diox-ide at 48–52 �C for 30–40 h. Sulfur dioxide aidsin the dissolution of the protein matrix to releasethe starch, combats growth of spoilage organ-isms, and maximizes the starch yield. The propersteeping temperature is critical for optimalgrowth of lactic acid bacteria, which lower thepH, restricting the growth of other organisms.The lactic acid bacteria also release proteolyticenzymes, which break down protein. SteepwaterpH is typically buffered at ca. 4.0.

Steepwater may be further processed to re-move phytic acid. In either form, it is thenconcentrated and sold as a nutrient solution forcommercial fermentation to produce, for exam-ple, antibiotics, ormore often dried by absorptiononto corn fiber and sold as cattle feed.

8.1.3. Kernel Milling and FractionSeparation

After steeping, the grain is milled to obtainseparation of components. Steeped grain is firstput through an attrition mill to release the oil-

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containing germ, which is removed using hydro-clones (liquid cyclones, hydrocyclones) anddried prior to extraction with hexane or pressedto remove the oil. Next, the residual grain isground to release starch granules. The slurry isthen screened to separate the corn hull or seedcoat, which is recovered as a fibrous mass.

Starch is isolated from the remaining slurryusing centrifuges or by passage through batteriesof hydroclones that separate out not only thestarch, but also the insoluble protein (called corngluten in the industry). The process also washesthe starch to lower the protein and ash contents.The protein (gluten) is less dense (1.1 g/cm3)than starch (1.5 g/cm3) and is easily separated bycentrifugal forces. Starch thus produced contains1–2% protein, which may be removed by asecond washing with water to give a starch witha final protein content of < 0.38%.

The final starch slurry is recovered by filtra-tion or centrifugation, and the starch cake is driedin continuous driers (usually flash driers) (Sec-tion 8.1.4) and sold as unmodified (native) starch.Alternatively, the starch slurry can be pumped,cooked, and subjected to enzyme-catalyzed hy-drolysis to produce syrups and other products(Section 9.1.1), pumped into tanks for chemicalmodification (Sections 9.1.2, 9.1.3, 9.1.4), orpumped onto hot rolls for production of prege-latinized starch (Section 9.2.1).

8.1.4. Starch Drying

Starch is usually dried by flash drying. In thismethod, the starch slurry is centrifuged or filteredand the moist starch cake is introduced at thebottom of a stream of rapidlymoving hot air (93–127 �C). Drying is rapid and the starch is col-lected using cyclones. Drying parameters can beused to control both bulk density and particlesize.

8.2. Wheat Starch

Wheat starch (along with starches of barley, rye,and triticale) is comprised of at least two distinctgranule populations, commonly designated theA- and B-types. The respective A- and B-typegranule populations possess different sizes (>10m vs.< 10 mm), shapes (lenticular vs. spherical/

polygonal), and starch characteristics and prop-erties [40–42]. Of the two primary granule types,A-type granules are most industrially important,although small granule starches (high B-typegranule contents) are also produced commercial-ly [41].

In contrast to corn wet-milling, commercialprocesses for wheat starch isolation involve dry-milling of grain to flour (72% extraction orgreater), which is the preferred starting materialfor starch isolation. Most commercial wheatstarch isolation methods take advantage of thematrix forming properties of gluten proteins,combined with the insoluble nature of starchgranules, to facilitate simultaneous separation ofstarch and gluten coproducts. Wheat starch isisolated on a commercial scale by one of severalmethods, including the Martin (dough ball), bat-ter, Fesca, Raisio, hydrocyclone (dough-batter),or high pressure disintegration/tricanter process-es [41]. The latter two processes are most com-monly employed industrially, with advantages ofreduced energy and water usage. In short, theprocess utilizing hydrocyclones (hydroclones)involves kneading and vigorous agitation ofwheat flour-water mixtures to agglomerate glu-ten into thread-like pieces, after which hydro-cyclones are used to separate starch and glutenstreams. A series of hydrocyclones is furtherutilized to concentrate and purify the primarystarch stream (yielding awheat starch with a highconcentration ofA-type granules). Gluten, small-granule starch (predominantly B-type granules),and fiber (pentosans) may be recovered via addi-tional processing. The high-pressure disintegra-tion/tricanter process utilizes a combination ofhigh-shear homogenization and decanter centri-fugation processes to efficiently fractionatewheat flour batters into A-type granule starch,B-type granule starch, gluten, and fiber productstreams. The primary producers of wheat starchare France, Germany, USA, and China.

8.3. Potato Starch

Potato starch is produced in largest quantities inEurope, especially in the Netherlands, Germany,and France [43]. Potato starch is also produced inChina in substantial amounts. Industrial potatoesare washed with water to remove dirt and otherforeign matter. They are then disintegrated in the

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presence of an antioxidant using drum rasps.Potato juice is removed using a decanter centri-fuge. Sieving is used to remove fiber. Aftersieving, a disk-type, continuous centrifuge isused to concentrate the starch granules and re-move any remaining small particles of fiber andprotein. Although starch granules within pota-toes occur in a wide range of sizes (see Table 3),the larger granules are primarily obtained in thisprocess. Any remaining protein is removed byreslurrying the starch in water and concentratingthe starch using a series of hydroclones in acountercurrent fashion. The starch is then col-lected by filtration and dried to ca. 20%moisture.

In the USA, potato starch is recovered com-mercially as a coproduct of potato processingoperations, viz. slicing. Recovered starch is fur-ther washed and dried.

8.4. Tapioca/Cassava Starch

Tapioca starch is obtained from roots of thecassava plant, which is grown in tropical areasand is variously known as cassava, cassada, yuca,manioca, mandioca, and tapioca, depending onthe region where it is grown [44]. Roots areharvested 10–12 months after planting and pro-cessed as quickly as possible. Processing consistsof washing the roots, then converting them into apulpy slurry that is passed through a series ofscreens to remove fiber. The resulting slurry ofstarch granules is concentrated and dewatered viacentrifugation. Finally, the starch is dried. Theyield of starch is ca. 26% (based on the freshweight of roots of 60–70% moisture content).

8.5. Rice Starch

Most rice starch is prepared by steeping brokenmilled rice kernels in 0.3–0.5%NaOH solution at25 �C to 50 �C for times of up to 24 h [45].Steeping in an alkaline solution softens the ker-nels and dissolves the glutelin protein, whichconstitutes approx. 80% of the total protein inrice. Wet-milling of the steeped kernel producesa slurry of starch. An additional holding of thealkaline slurry for 10–24 h further solubilizes theproteins. Fiber is removed using screens or filters,and the starch is isolated from the slurry, washed,neutralized, and dried. Rice starch may also be

produced by a mechanical process which in-volves physical separation of starch granules andprotein bodies.

9. Modified Starches

The terminology related to modified starches, aswith other terminology used in the starch indus-try, is not always precise. In this article, starchesthat are modified by reaction of their hydroxylgroups with a chemical reagent to convert theminto esters, ethers, or other chemical derivatives,such as graft copolymers, are called derivatizedstarches. Those that are modified by somechange in the basic polymer structure or molec-ular mass (by acid or enzyme treatment or oxi-dation) are called converted or thinned starches(Section 9.1.1). Starches that have undergone aphysical (thermal) treatment are described by thenature of the product, most often being a type ofpregelatinized starch (Section 9.2). Any starchtreated in any way to alter one or more of itsoriginal physical or chemical properties is calleda modified starch. A modified starch may bemultiply-modified, e.g., derivatized, converted/thinned, and/or pregelatinized.

Native starches have low process toleranceand, in the case of food starches, generally pro-duce undesirable textures. Their pastes oftenhave poor stability. Conversion and/or derivati-zation produces higher quality products. Com-panies that produce and market starches makeavailable a number of products that are the resultof various combinations of starch type and mod-ification. Converted, derivatized, and unmodifiedstarches from different sources are often em-ployed in the same general applications by dif-ferent users.

Uses of native starches are limited because, asindicated above, their inherent physiochemicalproperties make them less than optimally desir-able for most applications. Rather, the majorityof starches used in various food and other indus-trial applications aremodified starches. There areseveral reasons for modifying the properties ofstarches, but the two principal ones may be thefollowing: (1) to prevent the formation of pre-cipitates or microgel particles in a starch paste orgelation of the entire paste (Section 5.2.2), and(2) to reduce the viscosity of a starch paste(Section 5.2.3) so that higher solids solutions

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can bemade. Both are needed for starch productsused in the papermaking process. For starchesused in processed food products, additional fac-tors are important. In foods, starch ingredientsprovide bulk, body, and improved texture andmouthfeel, as well as other functionalities. Formost applications, food processors preferstarches with better behavioral characteristicsthan those provided by native starches. Uponbeing cooked into slurries, native (i.e., unmodi-fied), corn (maize) and waxy corn (waxy maize)starches produce weak-bodied, cohesive, rub-bery pastes and undesirable gels. However, theirfunctional properties can be improved dramati-cally by modification. In general, modificationsare made to increase the ability of the pasteproduced by cooking towithstand the heat, shear,and low pH associated with processing condi-tions, to make the starch paste more stable, or tointroduce specific functionalities.

Modifications can be accomplished by chem-ical (the majority) or physical treatments. Typesof modifications include cross-linking of poly-mer chains (Section 9.1.2), non-cross-linkingderivatization (sometimes called stabilization[Section 9.1.3] and depolymerization (Section9.1.1) (all chemical modifications) and pregela-tinization (a physical modification) (Section9.2). Many improvements in the properties ofa starch can be made with these modifications,singly or as combinations of them. Some specificproperty improvements that can be obtained arereduction in the energy required for cooking,modification of cooking characteristics, in-creased solubility, increased or decreased pasteviscosity, increased freeze-thaw stability ofpastes, enhancement of paste clarity, increasedpaste sheen, enhancement or reduction of gelformation and/or gel strength, reduction of gelsyneresis, improvement of interaction with othersubstances, improvement of stabilizing proper-ties, enhancement of film formation, improve-ment in water resistance of films, reduction inpaste cohesiveness, and increased stability of thegranules and pastes to lower pH, heat, and shear.Such imparted characteristics are itemized inTable 4.

The most important commercial derivativesof starch are those in which only a very few of thehydroxyl groups are derivatized. Normally, thehydroxyl groups of the starch polymer moleculesare converted into ester or ether groups at very

low degree of substitution (DS) values, the DSbeing the average number of hydroxyl groups perglucosyl unit that have been derivatized by etheror ester formation. DS values of modified foodstarches are most often< 0.1 and are generally in

Table 4. Characteristics imparted by primary starch modifications

Modification Main Attributes

Hypochlorite-

oxidized

Whiter

Lowered gelatinization temperature

Reduced maximum paste viscosity

(usually)

Softer, cleaner gels

Greater adhesion of pastes

Cross-linked

(food)

Increased stability to heat

Increased cooking temperature (delayed

pasting)

Increased shear resistance

Increased stability to low pH/acid

Decreased setback of cooks

Decreased or increased paste viscosity

Increased body

Stabilized Lowered gelatinization/pasting

temperature (easier cooking)

Improved tolerance of cold storage of

products

Improved freeze-thaw stability of products

Decreased paste setback/retrogradation

Reduced gelation

More easily dispersed when pregelatinized

Greater paste clarity

Increased moisture control

Cross-linked

and stabilized

Lowered gelatinization/pasting temperature,

but increased paste viscosity

Other attributes of cross-linking and

stabilization

A variety of textures and rheological

properties

Octenylsuccinylated Emulsifying properties

Emulsion-stabilizing properties

Ability to encapsulate hydrophobic

materials

Thinned Reduced hot-paste viscosity

Lowered gelatinization/pasting

temperature

Increased solubility

Increased gel strength

Increased paste clarity or opacity

Increased film-forming capability

Dextrinized Imparts crispness (food)

Increased stability (food)

Reduced viscosity

Improved film formation

Emulsifying properties

Increased browning (food)

Increased adhesion of pastes

Remoistenable adhesiveness

Pregelatinizeda Thickening without cooking

a Including cold-water swelling.

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the range 0.002–0.2. Thus, generally, on average,there is one substituent group for every 5 to 500D-glucopyranosyl units.

Starches are chemically modified as an aque-ous slurry of granules. For esterification oretherification, a starch slurry of 30–40% solidsis introduced into a stirred reaction tank. Sodi-um sulfate or sodium chloride is added to a 10–30% concentration to inhibit gelatinization. ThepH is adjusted, generally with sodium hydrox-ide, to pH 8–12, the exact value depending onthe reaction to be conducted, after which thechemical reagent is added to the starch slurry.The alkaline pH activates the starch for reactionby converting some of its hydroxyl groups toalkoxide ions for participation in nucleophilicsubstitution reactions. The reaction temperatureis generally maintained at less than 60 �C, oftento 49 �C, to prevent pasting of the granules sothat the derivatized starch can be recovered ingranule form. Following reaction to the desireddegree of substitution, the starch is recoveredby centrifugation and/or filtration, washed, anddried.

9.1. Chemically Modified Starches

9.1.1. Converted Starches andHydrolyzates

In the starch industry, the term conversion refersto depolymerization. Products that are partiallydepolymerized are called converted starches.There are three basic processes for making con-verted starches: using an acid (with variousmoisture contents and degrees of heating), usingan enzyme(s) [amylase(s)], using an oxidant in analkaline system. Extensive conversion producesmaltodextrins, syrup solids, syrups, anddextrose.

9.1.1.1. Thinned Products

To produce products variously known as acid-modified, acid-converted, converted, thin-boil-ing, fluid, or fluidity starches [46–48], which arethe least converted of the products convertedwith an acid, either hydrochloric acid is sprayedonto well-mixed starch or stirred moist starch istreated with hydrogen chloride gas, and themixture is heated until the desired degree of

hydrolysis (which is very slight in any case) isobtained. The acid is then neutralized, and theproduct is recovered by filtration orcentrifugation.

When to stop a conversion is usually deter-mined empirically by monitoring the viscosity ofa hot paste of the acid-modified starch. In thestarch industry, this is known as determiningfluidity. Acid conversion is practiced on bothnative starches and chemically modifiedstarches, i.e., derivatized starch products (Sec-tions 9.1.2, 9.1.3) may also be thinned, in whichcase, the pH of a stirred slurry of starch granules(36–40% solids) that has just been derivatizedand held at a temperature below the gelatiniza-tion temperature of the derivatized starch (usu-ally 40–60 �C) is lowered by the addition of amineral acid, such as hydrochloric acid, until it isin the acid range. When the desired degree oflimited hydrolysis (conversion) is reached, thesuspension is neutralized and the granules arerecovered by centrifugation and/or filtration,washed, and dried.

Compared to the parent starch, acid-convertedstarches undergo a greater degree of granuledisintegration upon cooking and produce pasteswith lower intrinsic viscosity values, reduced hotpaste viscosity, increased gel strength, and betterfilm-forming capabilities (Table 4). Specificcharacteristics are a function of the parent starchand the conversion conditions.

Many starch products are bleached with smallamounts of an oxidant. Use of higher levels of anoxidant oxidizes the starch and often results inpartial depolymerization. Conversion using anoxidant and an alkaline system occurs becauseoxidation of a hydroxyl group of a D-glucopyr-anosyl unit introduces a carbonyl group and leadsto b-elimination and chain cleavage. Such areaction is usually accomplished by treating aslurry of starch granules with a solution of sodi-um hypochlorite [49]. Use of hydrogen peroxideand cupric ions depolymerizes starch moleculeswithout introducing carbonyl groups. Ammoni-um persulfate is used to depolymerize starchesoxidatively for paper sizing and coating process-es. The decision as towhen to stop an oxidation isgenerally determined in theway previously notedfor an acid-converted starch, i.e., by monitoringhot paste viscosity (fluidity).

Properties of starches converted by oxidativecleavage generally parallel those of starches

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converted by acid-catalyzed hydrolysis, but withtwo differences. Pastes of oxidized starches gen-erally are clearer and more stable than thosemade from acid-modified starches, and there canbe greater variability in products in oxidatively-thinned products due to choice of the parentstarch, oxidizing agent, and conditions of oxida-tion. For example, starches treated with lowlevels of hypochlorite at an acidic pH (ratherthan the usual alkaline pH condition) result inproducts with a greater (rather than reduced) hotpaste viscosity.

Thinning is primarily done so that highersolids solutions can be made without generatingexcessive viscosity.

9.1.1.2. Starch Dextrins

Starch dextrins [9004-53-9] are products result-ing from a greater degree of conversion than arethinned products.Dextrinization is the process ofmaking dextrins.

Starch dextrins (occasionally called pyrodex-trins) are made by heating a starch with orwithout addition of an acid [46–50]. There arethree general types of these dextrins: white dex-trins, yellow dextrins, and British gums. Thevariables involved in making dextrins are thebase starch used, the amount and type of acidused, the percentage of moisture in the starch,the temperature employed, and the time ofconversion. In general, white dextrins are madeusing an acid catalyst, relatively low tempera-tures, and short conversion times. Yellow dex-trins are products of higher temperatures andlonger conversion times. British gums are alsomade using high temperatures and long times,but with less or no added acid. Hydrochloricacid is usually the acid used, particularly forfood-grade dextrins. Starch at 10–22%moistureis either treated with gaseous hydrogen chlorideor a solution of hydrochloric, sulfuric, or phos-phoric acid is sprayed onto the starch as anatomized mist. The starch is then predried toa moisture content of 2–5% by heating it at arelatively low temperature, sometimes underreduced pressure, to keep hydrolysis to a mini-mum. During the actual dextrinization step(pyroconversion), the pre-dried starch is heatedat 100–200 �C. The product is then cooled andrehumidified. Alternatively, the starch is firstdried to a moisture content of 2–5% and is then

treated with HCl (either as a gas or sprayed on inliquid form) before being heated.

Three types of reaction take place duringdextrinization, the relative amounts of which area function of the moisture content of the starchand the treatment temperature. (1) Hydrolysis ofglycosidic bonds is the predominant reactionwhen the moisture content is high and is, there-fore, themain reaction in the preparation ofwhitedextrins with a low degree of conversion. Be-cause of glycosidic bond cleavage and the re-sulting reduction in polymer molecular masses,final products generate lower solution viscosi-ties. (2) Transglycosidation (transglycosyla-tion) results in transfers of portions of glucanchains to hydroxyl groups on the same or dif-ferent chains to create new glycosidic bonds.The result is more highly branched and moresoluble macromolecules. (3) Repolymerization(reversion) occurs with catalytic amounts ofacid at high temperatures and low moisturecontents. Under these conditions, glycosidicbonds are formed from the reducing ends of themaltooligosaccharides (and any glucose) re-leased during hydrolysis. The result is highermolecular mass and more highly branchedmacromolecules than were present before re-version occurred.

9.1.1.3. Dextrose Equivalency

Converted, i.e., depolymerized, products areclassified by their dextrose equivalency (DE)which is defined as 100 � DP (degree of poly-merization or the average number of D-glucosylunits in the polymer or oligomer). The DE valueis based on the facts that (1) a new reducing end-unit is generated with each hydrolytic cleavageof a starch polymer chain and (2) the reducingpower of converted products is something thatcan be measured. Amylose and amylopectinhave no measurable reducing power (DE ¼0). The product of complete hydrolysis, D-glu-cose (dextrose) [50-99-7] (DP ¼ 1), has bydefinition a DE of 100. Therefore, the DE of aproduct of starch hydrolysis is its reducingpower as a percentage of the reducing powerof dextrose and is inversely proportional to theaverage molecular mass of the product mole-cules. (Both DE and DP are average values forpopulations of molecules.)

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9.1.1.4. Maltodextrins

Maltodextrins [9050-36-6] by regulatory defini-tion (USA) are purified, nutritive mixtures ofsaccharide polymers obtained by partial hydro-lysis of edible starch [51–53]. Most molecules inmaltodextrin preparations are maltooligosac-charides of DP 2–20. Because the DE values ofthe products are mostly in the range 5–15, theiraverage DPs are in the 7–20 range.Maltodextrinsare produced by hydrolysis of the starch mole-cules within pastes using either a single amylase,several amylolytic enzymes, or an acid to achieveDE values of less than 20 (average DP >5).Maltodextrins made by acid-catalyzed hydroly-sis alone possess a high quantity of linear chainsthat are able to retrograde. When it follows acid-catalyzed hydrolysis to DE 5–10, a-amylase-catalyzed hydrolysis produces maltodextrins oflow hygroscopicity and high water solubility thatgenerate less haze when dissolved. Maltodex-trins are also produced either by use of an a-amylase alone or with a combination of an a-amylase and a debranching enzyme.

Maltodextrins that have been specially pre-pared so that they associate to formmicrocrystalsthat are about the size of oil or fat droplets may beused as fat mimetics/sparers/replacers. Anotherspecially made maltodextrin, one in which di-gestible carbohydrate has been removed, is em-ployed as soluble dietary fiber.

Maltodextrins are primarily used in processedfood products; their collective attributes arelisted below.

. Bland flavor

. Control moisture migration

. Crystallization inhibition

. Encapsulator of flavors and aromas

. Fat replacers, sparers, and mimetics (speciallyprepared maltodextrins in microcrystallineform)

. Film formers

. Good dispersibility in aqueous systems

. Good solubility in aqueous systems

. Low to no sweetness

. Provide body

. Provide a smooth texture

. Range of susceptibility to Maillard browning

. Readily digestible

. Spray dry easily

9.1.1.5. Syrup Solids

Hydrolysis to DE 20–35 (average DP 3–5), fol-lowed by drying, produces syrup solids.

9.1.1.6. Syrups and Crystalline D-Glucose(! Glucose and Glucose-Containing Syrups)

Continued hydrolysis of starch with an acid and/or enzymes produces mixtures of D-glucose,maltose, and other maltooligosaccharides in aconcentrated form known as glucose syrups (usu-ally called corn syrups in the USA) [54–57].Most often, a glucose syrup is a purified, concen-trated, aqueous solution of nutritive saccharidesderived from food-grade starchwithDEvalues of> 20. (Syrups that are not required to be foodgrade are also produced for use in anti-freeze anddeicing compositions.)

Glucose syrups are made by one of threeprocesses: (1) acid conversion (pH ca. 2, temper-ature > 100 �C) (a minor process); (2) acid-enzyme conversion (an acid-converted hydroly-zate is treated with one or more hydrolyzingenzymes, the choice of which depends on thedesired saccharide profile of the finished syrup);or (3) enzyme-enzyme conversion (the majorprocess) (in a single step, a starch suspension ispasted and the paste is liquefied using a thermo-stable a-amylase. The product is then treatedwith one or more other enzymes depending uponthe desired saccharide profile). Specifically, glu-cose syrups are most often made by passing anaqueous slurry of starch containing a thermallystable a-amylase through a jet cooker, whererapid gelatinization and enzyme-catalyzed hy-drolysis (liquefaction) occur. After the solution iscooled to 55–60 �C, glucoamylase is added andhydrolysis is continued. When hydrolysis iscomplete, the syrup is clarified, decolorized,and concentrated. DE values of syrups mayrange from 25 to 95 with the lower-DE syrupshaving greater proportions of maltooligosac-charides and the higher-DE syrups having agreater proportion of D-glucose. Syrups are sta-ble because the oligosaccharides present inthem prevent crystallization and the high osmal-ality (ca. 70% solids) prevents growth of micro-organisms. When crystalline D-glucose (dex-trose) [50-99-7] or its monohydrate is desired,seed crystals are added to a syrup of DE > 95

130 Starch Vol. 34


[54, 56]. High-maltose syrups are also pro-duced, as well as being used as is; they are thestarting materials for the production of maltitol[585-88-6] [57].

9.1.1.7. High-Fructose Syrups andCrystalline D-Fructose

For the production of D-fructose [57-48-7], asolution of D-glucose is passed through a columncontaining immobilized glucose isomerase, pro-ducing an equilibrium mixture of approximately58% D-glucose and 42% D-fructose [56–58].Higher concentrations of D-fructose are desiredfor many applications of high-fructose syrups(HFS), often called high-fructose corn syrups(HFCS) in the USA. To make HFS/HFCS withmore than 42% D-fructose, the isomerized syrupis subjected to a continuous chromatographicseparation process by passing it through a bedof cation-exchange resin in the calcium salt formthat binds D-fructose; the resin is subsequentlyeluted to provide a fraction enriched in D-fruc-tose. This fraction is added to a 42% fructosesolution to produce syrups with higher concen-trations of D-fructose. Two major types of high-fructose syrups are produced: HFS 42 (ca. 42%fructose) and HFS 55 (ca. 55% fructose). Al-though D-fructose is not easily crystallized, crys-talline D-fructose can be obtained from the en-riched solution [59].

9.1.1.8. Other Starch Conversion Products

Other commercial products of starch conversioninclude b-cyclodextrin [7585-39-9] [60] anda,a-trehalose [99-20-7] [57].

9.1.2. Cross-linked Starches

Cross-linking reactions utilize bi- or multifunc-tional reagents to induce formation of intramo-lecular and/or intermolecular cross-links be-tween adjacent starch chains within starchgranules. Reagents employed for the commer-cial production of cross-linked starches includephosphoryl chloride (phosphorus oxychloride,POCl3), sodium trimetaphosphate (STMP),epichlorohydrin (EPI), and adipic–aceticmixed anhydride [61, 62]. Reaction conditions

and schemes vary according to reagent type.For reaction with STMP, starch granules areimpregnated with solutions containing bothreagent and a catalyzing base, after whichslurry moisture levels are reduced to less than15% and granules are heated to 130 �C in thesemi-dry state to drive the reaction [63]. Forcross-linking with STMP, reaction is favored atpH values above 10. In contrast, POCl3, EPI,and adipic–acetic mixed anhydride reagentsare reacted with starch granules in aqueousslurries at pH values of 11–12, 8, and 10.5,respectively [62]. Products of cross-linkingreactions are in the form Starch-O-X-O-Starch,where X represents �PO�

2 �� for reactions withPOCl3 and STMP, ��CH2CH(OH)CH2�� forreactions with EPI, and ��COCH2CH2CH2

CH2CO�� for reactions with adipic–aceticmixed anhydride.

Starch cross-linking reactions are employedto strengthen the structure of swollen granulesupon gelatinization (Section 5.2.1), impartingresistance to viscosity breakdown (Section5.2.3) in the presence of mechanical shear, acidconditions, and/or high temperatures. Cross-linked starch rheology is ultimately defined bythe nature and concentration of swollen granuleswithin a paste, with the firmness of swollengranules increasingwith increasing cross-linkinglevels (Section 5.2.3). Cross-linked starches typ-ically possess one cross-link for every 100–3000glucosyl units. Properties of the products varywith modification level. Very low levels ofcross-linking usually stabilize the granule struc-ture, allowing the modified starch to attain ahigher degree of granule swelling during heat-ing than would be observed for the unmodifiednative starch. In such cases, higher swellingpowers and paste peak viscosities are generallyobserved (Section 5.2.3). Progressively higherlevels of cross-linking generally lead to reducedgranule swelling, solubility, extent of amyloseleaching, paste clarity, and paste peak viscosity(see Table 4). Those products with significantlyreduced peak viscosities are known as inhibitedstarches. Cross-linked starches generally pos-sess improved paste texture (i.e., are lessstringy), but do not generally exhibit stabilityto retrogradation or freeze-thawing without ad-ditional stabilization (i.e., dual modification)(Section 9.1.3).

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9.1.3. Stabilized Starches

Stabilization (also called substitution) involvesderivatization of a starch with a monofunctionalreagent [64], converting starch hydroxyl groupsto ester or ether groups depending on the reagentused. Both starch esters and ethers are producedunder similar reaction conditions. Derivatizationismost often conducted in the form of an aqueousgranule slurry (30–40% solids) in the presence ofa gelatinization-inhibiting salt (10–30% concen-tration; most often sodium sulfate, sometimessodium chloride) between pH values of 8 and 12,the optimum pH being reagent specific. Thetemperature of the reaction medium is typicallyadjusted (most often to 49 �C) to further promotestarch granule swelling and reactivity. Use of agelatinization-inhibiting salt and a temperaturebelow the gelatinization temperature of both thenative starch and the modified product preventspasting, allowing the starch to be recovered ingranular form. Allowed DS/MS levels of pro-ducts intended for use in foods vary from countryto country (with those within the EU beinguniform), but generally range from 0.002 to0.2, with different maximum values for differentreagents (as examples, maximum allowable de-rivatization levels for food ingredient productsare noted in the ensuing discussion whereappropriate).

Commercial monostarch esters includestarch acetates (Starch-OCOCH3), succinatesðStarch- O-COðCH2Þ2CO�

2 Þ, octenylsuccinatesðStarch-OCOCH ½CH2 CO�2 �CH ¼ CHðCH2Þ5

CH3Þ, and phosphates ðStarch-OPO2�3 Þ. Acety-

lated starch (maximum allowable DS � 0.09) isprepared using acetic anhydride or, in countriesin which possession of acetic anhydride is pro-hibited, vinyl acetate [65]. Starch succinates aremade by reactionwith succinic anhydride, but areof limited commercial significance [66]. Reac-tion of starch with oct-1-enylsuccinic anhydride(OSA) produces the oct-1-enylsuccinic ester(OSA starch), for which the DS may not exceed0.02 [66]. Monostarch phosphate esters are gen-erated from reaction of starch with sodium or-thophosphate or sodium tripolyphosphate(STTP) [67]. A DS maximum of 0.002 is permit-ted for monostarch phosphate esters. A largevariety of other esters can be, and have been,made by selection of the proper reagent [65, 68].These other esters include starch and amylose

triacetate, which are water insoluble, thermo-plastic materials (Section 9.2.4).

Manufactured monostarch ethers includehydroxylethyl (Starch-OCH2CH2OH), hydroxy-propyl (Starch-O-CH2CHOHCH3), carboxy-methyl ðStarch-OCH2CO

�2 Þ and cationic (dis-

cussed in Section 9.1.4) starches. Reaction withethylene oxide produces hydroxylethyl starch[68, 69], reaction with propylene oxide produceshydroxypropyl derivatives (maximum MS0.2) [68, 70], and reaction with sodium mono-chloroacetate produces sodium carboxymethylstarch [71], known as sodium starch glycolatein the pharmaceutical industry. A large variety ofother ethers, including silyl ethers, can be madeby selection of the appropriate reagent [68].

Esterified and etherified products generallyexhibit similar physical properties, althoughethers are stable to acids and bases, while estersare not. In general, stabilized starches are sonamed because they have a reduced tendency toundergo retrogradation (usually a very desirableattribute) due to the incorporation of bulky sub-stituent groups along the starch chains, that blockintermolecular associations. Some incorporatedsubstituent groups also possess a negative charge(e.g., monostarch phosphate esters), further re-ducing interchain associations (through like-charge repulsion) to enhance paste stability.Thus, stabilized starches produce more stablestarch pastes and gels with improved clarity, lesssyneresis, and greater freeze-thaw stabilitythan their unmodified starch counterparts (seeTable 4). Substitution (especially at high levels)can disrupt the native granule structure, resultingin reduced starch gelatinization and pasting tem-peratures, as well as increased starch swelling,paste peak viscosities, and solubilities. In thecase of OSA starch, some hydrophobicity is alsointroduced to starch chains, giving the productemulsifying and emulsion-stabilizing properties(Table 4). Stabilization-type reactions are oftenconducted in combination with other types ofchemical and/or physical modifications to yieldmultifunctional starches (Section 9.4).

9.1.4. Cationic Starch

Cationization reactions are carried out usingmonofunctional reagents that impart a positivecharge to starch chains via derivatization with

132 Starch Vol. 34


reagents that incorporate ether groups containingimino, amino, ammonium (quaternary amino),sulfonium, or phosphonium moieties [72]. Themost industrially significant and commonly uti-lized reagent for starch cationization is 3-chloro-2-hydroxypropyltrimethylammonium chloride(CHPTMA) [72–74], although a multitude ofpotential reagents may be employed. Reactionwith CHPTMA gives the following product:Starch-O-CH2CHOHCH2N

þ(CH3)3.Cationic starch may be prepared batchwise

via granular slurry, semi-dry, and solubilizedpaste processes, and by continuous means suchas reactive extrusion [73–75]. The first two pro-cesses yield a modified starch product with re-tained granule structure, while in the latter twoprocesses starch is reacted in a gelatinized ormolten state [75]. For the granular slurry process,reaction may be conducted in either an aqueousor aqueous alcohol reaction medium in the pres-ence of NaOH at a temperature of 49 �C toachieve reaction efficiencies in the range of80–88% [72–75]. Aqueous slurry reactions gen-erally include sodium sulfate to prevent gelatini-zation of the starch granules during reaction,while use of an aqueous alcohol reactionmediumeliminates the need for a gelatinization-inhibit-ing salt [75]. The semi-dry cationization process,which also maintains starch in a granular form,entails heating of dry granular starch (< 120 �C)in the presence of both catalyst (NaOH) andreagent [75], resulting in reaction efficiencies of90–95% [74]. For cationization via the pasteprocess, a starch slurry is adjusted to alkalinepH (ca. 11.3) and gelatinized via steam injection,prior to reaction at 50 �C [76]. The use of anextruder as a continuous reaction vessel (i.e.,reactive extrusion) for derivatization of starchpolymers in the molten state has been describedfor various types of modification [75]. For starchcationization, the reactive extrusion process isreported to achieve reaction efficiencies in therange of 90% [77–79]. However, the high tem-perature and shear conditions imposed by theextrusion process result in some thermomecha-nical degradation of starch molecules, the degreeof which is dependent on the processing para-meters [80, 81].

For cationic starches modified in granularform, relative starch crystallinities, gelatiniza-tion temperatures and enthalpies, and pastingtemperatures decrease with increasing levels of

cationization [72, 73]. Cationic starch deriva-tives also exhibit increased swelling powers,water sorption properties, peak and breakdownviscosities, stability of pastes and gels to synere-sis, lipid-binding capacity, and emulsion-stabi-lizing properties relative to native (unmodified)starches [72, 73].

The primary applications of cationic starchesare in the wet-end, surface-sizing, and coatingprocesses in the paper industry, as well as indus-trial flocculants in sludge dewatering andmineralore recovery operations [72–74]. Degree of sub-stitution (DS) values for cationic starch deriva-tives used in papermaking applications generallyrange from 0.02–0.07, while those for flocculentapplications are significantly higher (DS ca. 0.2–0.7) [73].

9.1.5. Starch Graft Copolymers

Starch graft copolymers are derivatives in whichsynthetic polymers are covalently bonded tostarch molecules, most often through ether lin-kages. These products are generally obtained bygenerating free radicals from hydroxyl groups ofnative or hydroxyalkylated (generally hydro-xyethylated) starch molecules, followed by reac-tion with an unsaturated monomer, such as anacrylic or vinyl monomer. A wide range ofdifferent products has been made using differentmethods for producing starch free radicals, dif-ferent polymerizable monomers, and differentstarches and modified starches [82, 83].

9.2. Thermally Modified Starches

9.2.1. Instant Starches

Pregelatinized starches are precooked productsthat are cold-water soluble, i.e., they can be usedwithout cooking/heating and will produce dis-persions without lumps when correctly prepared[84, 85]. They may be made from either native orchemically modified (Section 9.1) starches bytwo general methods. In one method, a starch-water slurry is applied to a steam-heated roll orinto the nip between two nearly touching andcounterrotating steam-heated rolls, where thestarch is rapidly gelatinized, pasted, and dried.The dry film is scraped from the roll andmilled to

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a powder. The resulting product should containno intact granules, except when the startingmaterial is a highly cross-linked starch. The otherway to make a pregelatinized product is to pass astarch and some moisture through an extruder, inwhich the heat and shear imparted by the processconditions disrupts the granules. In this processalso, the dried product is ground to the desiredmesh size.

Pregelatinized starch products are used infood and laundry products when (1) no heat forcooking is available, (2) no process step requiressufficient heat to cook the starch, or (3) heatcannot be applied because of the heat liabilityof one or more of the other ingredients. If modi-fied starches are used in the preparation of thepregelatinized starch product, the properties ofthe paste introduced by any modification(s) arealso exhibited by the pregelatinized product.

Products known as cold-water-swelling(CWS) starches contain granules that swell ex-tensively in the presence of ambient temperaturewater [85]. They are made by two differentprocesses. In one case, a slurry of starch granulesin aqueous alcohol is heated to disrupt the inter-nal granule structure, i.e., to gelatinize the gran-ules, without much granule swelling and nodissolution of the polymers. In the other, anaqueous starch slurry is atomized via a specialspray-drying nozzle into a stream of heated air.Granules in cold-water-swelling starches are ge-latinized, but their granular form is maintained(as opposed to pregelatinized products). Bothpregelatinized and CWS starches are categorizedindustrially as instant starches.

Starches that are neither pregelatinized norcold-water-swelling are known as cook-upstarches.

9.2.2. Annealed andHeat-moisture-treated Starches

While preparation of instant starches is widelypracticed, less common are thermal treatmentsknown as heat-moisture treatments (HMT) andannealing [86]. Annealing results from heatinggranular starch in excess water (> 40%) at atemperature above the glass transition tempera-ture but below the onset temperature of gelatini-zation. The resulting changes in properties are afunction of the parent starch, the treatment tem-

perature, and the duration of heating, but themostcommon and consistent effects of annealing areincreased onset and peak gelatinization tempera-tures, an increased enthalpy of gelatinization (notalways observed), a narrowing of the gelatiniza-tion temperature range, and decreased digestibil-ity (Section 5.2 and Chap. 11).

Heat-moisture treatment also involves heatingof starch granules above their glass transitiontemperature, but differs in that it is conducted atreduced levels of moisture (< 35%) and at higherprocessing temperatures (80–140 �C). The vari-ables in this process are the type of starch, themoisture content of the starch, the treatmenttemperature, the heat source, and the durationof the treatment. Common property changes areincreases in onset, peak, and often conclusiongelatinization temperatures; reduced granuleswelling and solubility; reduced leaching of am-ylose; decreased peak viscosity; increased pastestability; reduced enthalpy of gelatinization(though, in some cases, enthalpy may notchange); and decreased digestibility (Section5.2 and Chap. 11). However, due to the diverserange of treatment conditions and starches uti-lized for HMT, the resulting starch characteris-tics and properties can vary considerably.

9.2.3. Dry Heating of Starches

Commercial products are made by heating drystarch (< 15% moisture) at a temperature belowthat at which thermal degradation occurs (90–128 �C) [86]. Products generally exhibit greaterstability to the conditions of heat and shearimposed by cooking processes. An increasedpaste viscosity and a creamy consistency forquick-cooking products are also claimed.

9.2.4. Destructurized and ThermoplasticStarch

Although destructurized and pregelatinizedcould be considered to be synonymous terms(because a pregelatinized starch is destructurizedto mixtures of amorphous amylose and amylo-pectin), the term destructurized starchwas origi-nally (1987) used to describe starch of relativelylow moisture content (10–25%) that was heatedunder pressure in an extruder to a temperature

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above its glass transition and melting tempera-tures to yield a thermoplastic mass that could bemade into amolded article or film. The differencebetween a pregelatinized starch made by extru-sion cooking and a thermoplastic destructurizedstarch is in the amount of moisture used as aplasticizer in the feed and, hence, the temperatureneeded to melt the starch [87, 88]. Addition ofother plasticizers, such as glycerol, allows thestarch to be converted into a thermoplastic ma-terial at lowermoisture contents than are requiredfor traditional starch pasting. Addition of a de-polymerizing catalyst to reduce polymer molec-ular masses gives a product from which shapedarticles with improved characteristics can bemade.

Thermoplastic starch can be blended withnatural or synthetic hydrophilic polymers. Insome cases, the blends can be further mixed withhydrophobic polymers, such as polyethylene, forproduction of water-resistant, blown films [82].Destructurized starch can also be mixed withplasticizers and other materials such as naturaland synthetic polymers, graft copolymers, andlubricants to produce a large family of productsalso known as thermoplastic starch. Thermoplas-tic starch can also be made by esterifying oretherifying (e.g., hydroxypropylating) starch tohigh DS values (DS 2 or more) [82]. Thermo-plastic starch materials can be subjected to vari-ous thermoforming processes such as spinning,extrusion, film blowing, injection molding,and foaming. The products are usually bio-degradable.

9.3. Genetically Modified Starches

The characteristics of native starches can bemodified genetically [89]. A major commercialstarch that is the base starch for many modifiedfood starch products is waxy maize starch, amaize starch devoid of amylose (Chap. 4). Waxy(all-amylopectin) and partial waxy (reduced am-ylose content) wheat starches, as well as all-amylopectin potato starch have been developed.Rice plants produce starches with a range ofamylose contents from less than 1% to at least33%. Commercial hybrids of corn, known ashigh-amylose corn or amylomaize starches, pro-duce starches with ca. 55% and > 70% contentsof amylose.

9.4. MultipleModifications of Starches

Starches are modified for many and diversespecific applications (Chap. 10). Variables in-clude choice of the parent starch, including agenetically modified starch (Section 9.3), type ofchemical modification (Section 9.1), and anythermal treatment (Section 9.2). Many modifiedstarch products are made by combinations oftreatments. As examples, a coating binder starchfor a papermaking process will likely be hydro-xyethylated (stabilized) and thinned, and a foodstarch might be made by cross-linking, stabiliz-ing, thinning, and pregelatinizing waxy maizestarch (a genetic modification). Major commer-cial individual (combinations of these modifica-tion processes are commonly used) starch modi-fication processes and products are listed below:

Chemical reactions

A. Cross-linking1. Esterification (approved for food use)

a. Distarch phosphates (approved forfood use)

b. Distarch adipates (approved for fooduse)

2. Etherificationa. Starch cross-linked by reaction with

epichlorohydrin [106-89-8] (not usedin foods in the USA)

B. Stabilization1. Etherification

a. Hydroxyethylstarchesb. Hydroxypropylstarches (approved for

food use)c. Cationic starchesd. Starch graft copolymers

2. Esterificationa. Starch acetates (approved for food use)b. Starch oct-1-enylsuccinates (approved

for food use)c. Monostarch phosphates (approved for

food use)3. Oxidation

a. With hypochlorite (approved for fooduse)

b. With hydrogen peroxide and Cu(II)ions (approved for food use)

c. With ammonium persulfate

C. Depolymerization1. Acid-catalyzed (approved for food use)

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2. Oxidation, followed by alkaline pH (ap-proved for food use)

D. Transglycosylation plus depolymerization(dextrinization) (approved for food use)

Physical/thermal transformations

A. Pregelatinization (approved for food use)B. Preparation of cold-water-swelling starch

(approved for food use)

Genetic control/plant breeding (approvedfor food use)

10. Examples of Uses of Starches andProducts Derived from Starches

10.1. Uses of Starches

The primary uses of starches, which vary fromcountry to country and region to region (seeTable 5) [90], are to produce sweeteners (dex-trose, glucose syrups, HFS) and related products(maltodextrins, syrup solids), in paper and pa-perboard product manufacture [91], in processedfood preparations [92], and to produce fuel etha-nol, although as alreadymentioned, most ethanolis produced by processes in which the starch isnot isolated as a purematerial. Some examples ofuses of specific products follow, although essen-tially all products employed in paper manufac-ture and as a textile sizing agent have beenthinned, and many of the products used in thefood industry have been pregelatinized and/or

thinned in addition to being both cross-linked andstabilized. Thus, relating a specific modifiedstarch to a specific application is often notpossible.

Selected examples of nonfood uses ofstarches, which term includes both native andmodified (derivatized, oxidized, and acid-thinned products, starch-latex copolymers, dex-trins, and pregelatinized products) starches are asfollows:

. Absorbants

. Briquette binder

. Carpet and rug sizing

. Ceramics (clay binder)

. Fiberglass sizing

. Ink and dye thickeners

. Manufacture of paper, boxboard, corrugatedboard, fiberboard drums, spiral tubes, etc. (wet-end additive, sizing agent, coating binder, ad-hesives [remoistenable, case and carton seal-ing, laminating, tube winding, and bagadhesives])

. Ore refining (both flotation and electrolyticreduction processes)

. Tablet binder and excipient

. Textile finishing and sizing

. Wall-covering pastes

Selected examples of food uses of starches,which term includes both native starches andmodified (derivatized [cross-linked and stabi-lized], oxidized,and acid-thinned products, dex-trins, and pregelatinized) products are as follows:

. Baby foods

. Biscuits, crackers, cookies

. Batters and batter mixes

. Beverage emulsions

. Breadings and breading mixes

. Breakfast cereals

. Cake, pancake, doughnut, muffin, and wafflemixes

. Confections (gum, jelly, and panned products)

. Fillings for pies and other baked goods (creamand fruit)

. Glazes

. Gravies and gravy mixes

. Hot-filled products in jars and cans

. Noodles

. Pet foods

. Processed cheese products

Table 5. End-use demand for starches in some major markets [90]

Approximate percent

of total use

EU USA Japan

To produce glucose syrups,

dextrose, and maltodextrins

28 12 27

To produce HFSa 5 31 27

In processed foods 18 3 21

To produce paper and paperboard 22 10 11

To produce fuel ethanol 2 42 5

Other industrial usesb 26 2 10

aHigh-fructose syrups.b Includes textiles, personal care products, chemicals other than

ethanol, detergents, pharmaceutical, and other industrial

products.

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. Processed meat products

. Pudding, pie filling, and other dessert mixes

. Retorted canned products

. Salad dressings (spoonable and pourable)

. Sauces and sauce mixes

. Snack and other extruded foods

. Soups and soup mixes

. Surimi

. Toppings and syrups

. Yogurt

Approximate utilization of corn/maize starchisolated by the wet-milling process in the USA:

As starch products (ca. 16%) [9]

. In paper and paperboard manufacture (ca.10%)

. In processed food products (ca. 3.0%)

. As a textile size (ca. 2.0%)

As the result of conversion (ca. 85%); ex-cluding that used to produce ethanol [9]

. As high-fructose syrups (ca. 49%)

. As glucose syrups, dextrose, andmaltodextrins(ca. 36%)

Uses of native (i.e., unmodified) starches arelimited. In the food industry, they are used todust molds for jelly-type confections, for mois-ture control (e.g., in salt), to help dissipate heatduring grinding of sugar to produce powderedsugar, and for dusting of marshmallows andchewing gum. Nonfood uses include their usein papermaking, where the papermaker pur-chases a native starch, then subjects it tocontrolled oxidative (ammonium persulfate,potassium persulfate, sodium hypochlorite, orhydrogen peroxide) or enzyme-catalyzed (a-amylase) depolymerization or derivatizationon site. Native starch is used to make porousceramic filters (e.g., for diesel vehicle exhaust)and, with addition of other ingredients, as theadhesive for corrugated board manufacture[93]. Native starches are combined with plasti-cizers and other additives and pregelatinized/destructurized in an extruder to prepare water-soluble, loose-fill packaging material.

Acid-modified and other thinned products areused in the warp sizing of textiles, as adhesives,and in themanufacture of gypsum and corrugatedboard [93].

As mentioned, many derivatized starch pro-ducts are also ‘‘thinned.’’ Those starches thathave been thinned, but otherwise unmodified(i.e., thin-boiling or fluidity native starches), areused to make gum candies (e.g., jelly beans,jujubes, orange slices, spearmint leaves) andprocessed cheese loaves.

Cross-linked products are mostly used in thefood industry, and food processors use bothcross-linked-only products and starches that havebeen both cross-linked and stabilized (i.e., duallymodified). Cross-linked starches are used asthickeners in canned (retorted) and hot-filledproducts such as sauces, soups, gravies, pie fill-ings, puddings, spoonable salad dressings, andbaby foods and in batter mixes. Cross-linked andstabilized starches are used in preparation acidicfood products such as tomato-based sauces, saladdressings, and fruit pie fillings; retorted productssuch as sauces, soups, and gravies; aseptically-filled products such as puddings and cheesesauces; frozen foods such as pot pies, fruit pies,and gravies; and baby foods.

Stabilized-only starches are used in both foodand nonfood applications. Starch acetates andhydroxypropylated starches are used as thick-eners of food products when low-temperaturestability is required (e.g., in chilled and frozenproducts) and in general when retrogradation/setback is undesirable. The most widely usedstabilized starch for nonfood applications is hy-droxylethyl starch, which is used in papermakingas a furnish additive, a surface sizing agent, and asa coating and pigment binder [91]. Hydroxyethylstarch also finds use in textile sizing [94]. Acety-lated starches are used in surface sizing of certainpaper products and for warp sizing of textiles.

Cationic starches are used in the papermakingprocess as a wet-end additive to balance fiberflocculation and retention in order to promotesheet dewatering and to attain good sheet forma-tion and strength [91]. It is also used in surfacesizing in certain paper products, and even lessoften as a coating binder. Cationic starch alsofinds use as a flocculant for anionic materials andin fabric softeners.

Monostarch phosphates find use in both foodand nonfood applications. They can be used withalum as a wet-end additive in making paper atlow pH. They are also used as flocculants andsedimentation aids for various cationic materialsand in textile sizing and printing pastes.

Vol. 34 Starch 137


The amphophilic octenylsuccinylatedstarches are special types of stabilized starchproducts. These starches are used in emulsionpreparation and stabilization in products such aspourable salad dressings and flavored beverages.They are also used to encapsulate certain flavors,aromas and drugs and as fat sparers.

Carboxymethylated starches are used as siz-ing agents for textiles [94]. Higher-DS productsare used as additives to control fluid loss and tomodify the rheology of drilling muds in oil andnatural gas production.

Uses and potential uses of starch graft copo-lymers have been reviewed [82, 95]. Graft co-polymers of starch with a latex are useful aspaper-coating materials. Some polyacrylatestarch copolymers are employed as super-absorbants.

Pregelatinized starches are widely used. Non-food uses include as a spray-on laundry starch, asan adhesive in foundry cores for metal casting, asa binder, and as an excipient in solid oral dosageforms of pharmaceuticals. Their use in foods isvaried and widespread since cross-linked and/orstabilized starches can also be pregelatinized. Acommon use is in dry mixes for such products asinstant puddings and soups. They are also used inpizza toppings, extruded snacks, soft cookies,bakery fillings, and low-fat salad dressings.Cold-water-swelling starches are used to thickensugar solutions and glucose syrups in the prepa-ration of gum candies, in certain desserts, and inmuffin mixes.

Thermoplastic starch can be used to preparefood, lawn, and leaf bags for composting, food-service ware, pharmaceutical capsules, personalhygiene products, and packaging material and asfillers for tires.

10.2. Uses of Products derived fromStarches via ExtensiveDepolymerization

The primary uses of dextrins are to prepareadhesives for paper and paperboard products[50], where they are used as bag-seam pastes(white dextrins, British gums), in tube winding(white and yellow dextrins, British gums), forcase and carton sealing (white and yellow dex-trins), in laminating (white and yellow dextrins,

British gums), in preparation of remoistenablegummed tape and labels (white and yellow dex-trins), and in envelope manufacture (white andyellow dextrins).

In the food industry, white dextrins are used inpan coating of confections, as a gloss for bakeryproducts, and as carriers for flavorings, spices,and colorants.

Syrup solids and maltodextrins are utilized infoods in many of the same applications, whichinclude use as agglomerating agents, as binders,as bulking agents, in coatings, as coffee white-ners, in frozen foods, in infant formulas, in meatproducts, in processed cheese products, and inwhipped toppings. Maltodextrins are used toencapsulate flavors and aromas.

Glucose syrups are produced in large quanti-ties and their use is widespread in food products.(Syrup viscosity is important as it relates to thehandling characteristics of the syrup and thetexture of the food product). They are usedlargely as humectants and to provide bulk andbodywith various degrees of sweetness. Productswith low sweetness provide body to ice cream,canned fruits and vegetables, bakery products,and some confections. Products with moderatesweetness are used in foods requiring body butnot high sweetness (e.g., in sauces, toppings,and some confections) and sweet bakery pro-ducts. Glucose syrups are used to reduce wateractivity, lower the freezing point, increase theosmotic pressure, and increase the chewinessof a food product. They also inhibit crystalli-zation of sucrose. In ice cream, they provideresistance to meltdown. They increase the shelflife of hard candies and peanut brittle by pre-venting crystallization and providing resis-tance to changes in moisture. They providefermentable substrates and contribute tobrowning of bakery products, and are used inmany other applications from bakery fillings tosalad dressings.

Glucose syrups are the carbon source for avariety of fermentation processes that are used toproduce a range of products from lactic acid toantibiotics, and as has already been mentioned,various salts are added to them to produce anti-freeze and deicing solutions.

High-glucose syrups are the starting materialsfor the production of sorbitol [50-70-4], ascorbicacid (vitamin C) [50-81-7] via sorbitol, glucono-d-lactone (D-glucono-1, 5-lactone) [52153-09-0],

138 Starch Vol. 34


and polydextrose [68424-04-4], all of which arewidely used in their own rights.

Crystalline D-glucose is used in many of thesame products that utilize glucose syrups when adry form of sugar is desired. In addition, when itis used as part of the sugar coating of doughnutsand in sandwich cookie and sugar wafer fillings,it imparts a desirable cooling sensation in themouth because of its slight negative heat ofsolution.

Most often, hydrogenated starch hydrolysates(HSH) are used to make food products for dia-betics. Some is also used to make sugarlesscandies and chewing gum.

High-fructose syrup containing ca. 42% fruc-tose (HFS 42) is used as the sweetener in manyfruit-flavored beverages, in doughs used in pro-duction of yeast-raised breads and rolls, and inbatters used in production of cakes. HFS 42 isblended with sucrose and glucose syrups in fruitcanning. Dairy products, namely, chocolatemilk, yogurts, milk-based nutrition drinks, icecream, frozen novelties, and frozen desserts, aswell as nondairy products such as jams, fruitfillings, barbecue sauces, and cake frostings, mayalso contain HFS 42. Most cola drinks are madewith HFS 55.

b-Cyclodextrin (! Cyclodextrins) is primar-ily used to prepare powdered essential oils, whichare thereby protected from oxygen and light, butwhich are readily released when the dry powderis added to an aqueous system. Such dry com-plexes are used in dry mixes for baked goods,beverages, and soups; flavored coffee and tea;savory snacks and crackers; breakfast cereals;chewing gum; tabletted candies; processedcheese products; gelatin desserts; and puddings.It is also used to protect other aromas, flavors, andcertain pharmaceuticals from oxidation, volatili-zation, light-induced degradation, and thermallyinduced decomposition.

11. Starch Digestibility

Not all starch is equally digestible. In a foodproduct containing starch, there will be fractionstermed rapidly digestible (or rapidly digesting)starch (RDS), slowly digestible (or slowly digest-ing) starch (SDS), and resistant starch (RS).Rapidly digesting starch is the fraction which,in a laboratory analysis designed to mimic diges-

tion in the stomach and small intestine, is con-verted into D-glucose within 20 min. Slowlydigestible starch is the fraction that is convertedinto D-glucose in the time period between 20 and120 min. Resistant starch is the portion of starchthat does not undergo digestion in the smallintestine (is not converted into D-glucose within120 min in the laboratory test). It is fermented bythe microflora in the large intestine (colon) and,therefore, falls under the definition of dietaryfiber. There are four general classes of resistantstarch: RS1 (starch that is physically inaccessibleto the digestive enzyme a-amylase because it isencased within plant cell walls, nondigesteddenatured protein, or something else that entrapsit); RS2 (uncooked, i.e., ungelatinized granularstarch); RS3 (retrograded starch in which starchpolymer molecules [primarily amylose mole-cules] are highly associated and attacked by a-amylase only slowly); and RS4 (granules that areresistant to the action of a-amylase because ofchemical modification). The glycemic index ofthe fractions is RDS > SDS > RS.

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140 Starch Vol. 34


Further Reading

J. N. BeMiller, R. L. Whistler (eds.): Starch, 3rd ed.,

Academic Press, London 2009.

A. C. Bertolini (ed.): Starches, CRC Press, Boca Raton 2010.

A.-C. Eliasson (ed.): Starch in Food, CRC Press, Boca Raton

2004.

L. P. B.M. Janssen, L.Moscicki (eds.):Thermoplastic Starch,

Wiley-VCH, Weinheim 2009.

Vol. 34 Starch 141


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