Factors influencing the physico-chemical, morphological, thermal and rheological properties of some...

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FOODHYDROCOLLOIDS

Food Hydrocolloids 21 (2007) 1–22

Review

Factors influencing the physico-chemical, morphological, thermal andrheological properties of some chemically modified starches for food

applications—A review

Jaspreet Singha,�, Lovedeep Kaurb, O.J. McCarthyb

aRiddet Centre, Massey University, Palmerston North, New ZealandbInstitute of Food, Nutrition and Human Health, Massey University, Palmerston North, New Zealand

Received 12 August 2005; accepted 20 February 2006

Abstract

Effect of some common chemical modifications such as acetylation, hydroxypropylation and cross-linking on the physico-chemical,

morphological, thermal and rheological properties of starches from different botanical sources have been reviewed. The distinguishing

factors that affect the efficiency of modification are the starch source, amylose to amylopectin ratio, granule morphology, and type and

concentration of the modifying reagent. The extent of alteration in the starch properties reflects the resistance or the susceptibility of a

starch towards different chemical modifications. Modified starches with desirable properties and degree of substitution can be prepared

by critically selecting a suitable modifying agent and a native starch source.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Starch; Potato; Maize; Wheat; Rice; Chemical modification; Cross-linking; Hydroxypropylation; Acetylation; Physico-chemical;

Morphological; Thermal; Rheological

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. Common types of chemical starch modifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

3. Extent of chemical modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4. Physico-chemical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

5. Morphological properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

6. Thermal properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

7. Rheological/pasting properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

8. Nutritional and toxicological aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

9. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

www.elsevier.com/locate/foodhyd

0268-005X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.foodhyd.2006.02.006

�Corresponding author. Tel.: +646 3505799 Ext. 2534.

E-mail address: [email protected] (J. Singh).

1. Introduction

Among carbohydrate polymers, starch is currentlyenjoying increased attention owing to its usefulness indifferent food products. Starch contributes greatly to thetextural properties of many foods and is widely used infood and industrial applications as a thickener, colloidalstabilizer, gelling agent, bulking agent and water retentionagent. The physico-chemical properties and functionalcharacteristics of starch systems and their uniqueness invarious food products vary with starch biological origin(Svegmark & Hermansson, 1993). Starches from variousplant sources, such as wheat, maize, rice and potato havereceived extensive attention in relation to structural andphysico-chemical properties (Takeda & Priess, 1993).Native starch is a good texture stabilizer and regulator infood systems (Cousidine, 1982), but limitations such as lowshear resistance, thermal resistance, thermal decompositionand high tendency towards retrogradation limit its use insome industrial food applications. Starch modification,which involves the alteration of the physical and chemicalcharacteristics of the native starch to improve its functionalcharacteristics, can be used to tailor starch to specific foodapplications (Hermansson & Svegmark, 1996). Starch

modification is generally achieved through derivatizationsuch as etherification, esterification, cross-linking andgrafting of starch; decomposition (acid or enzymatichydrolysis and oxidization of starch) or physical treatmentof starch using heat or moisture, etc. (Table 1). Chemicalmodification involves the introduction of functional groupsinto the starch molecule, resulting in markedly alteredphysico-chemical properties. Such modification of nativegranular starches profoundly alters their gelatinization,pasting and retrogradation behaviour (Choi & Kerr, 2003;Kim, Muhrbeck, & Eliasson, 1993; Liu, Ramsden, &Corke, 1999a, 1999b; Liu et al., 1999a, 1999b; Perera,Hoover, & Martin, 1997; Seow & Thevamalar, 1993). Themajority of the reviewed work has been focused on nativestarch properties, with only few literature surveys coveringthe synthesis and physico-chemical properties of chemicallymodified starches (Hung & Morita, 2005; Kim, 1988;Tharanathan, 2005; Tomasik & Schilling, 2004). In thisreview, we have re-examined the information on thevarious factors that influence the extent of chemicalmodification of starches from different plant sources, withan emphasis on the post-modification changes in theirphysico-chemical, morphological, thermal and rheologicalproperties.

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Table 1

Different starch modification types and preparation techniques

Modification Types Preparation

Physical Heat/moisture

treatment

Heat–moisture treatment—Heating starch at a temperature above its gelatinization point with

insufficient moisture to cause gelatinization

Annealing—Heating a slurry of granular starch at a temperature below its gelatinization point for

prolonged periods of time

Pregelatinization Pregels/instant/cold-water swelling starches prepared using drum drying/spray cooking/extrusion/

solvent-based processing

Conversion Partial acid

hydrolysis

Treatment with hydrochloric acid or ortho-phosphoric acid or sulphuric acid

Partial enzymatic

hydrolysis

Treatment in an aqueous solution at a temperature below the gelatinization point with one or more

food-grade amylolytic enzymes

Alkali treatment Treatment with sodium hydroxide or potassium hydroxide

Oxidation/bleaching Treatment with peracetic acid and/or hydrogen peroxide, or sodium hypochlorite or sodium

chlorite, or sulphur dioxide, or potassium permanganate or ammonium persulphate

Pyroconversion

(dextrinization)

Pyrodextrins—Prepared by dry roasting acidified starch

Derivatization Etherification Hydroxypropyl starch—Esterification with propylene oxide

Esterification Starch acetate—Esterification with acetic anhydride or vinyl acetate

Acetylated distarch adipate—Esterification with acetic anhydride and adipic anhydride

Starch sodium octenylsuccinate—Esterification by octenylsuccinic anhydride

Cross-linking Monostarch phosphate—Esterification with ortho-phosphoric acid, or sodium or potassium ortho-

phosphate, or sodium tripolyphosphate

Distarch phosphate—Esterification with sodium trimetaphosphate or phosphorus oxychloride

Phosphated distarch phosphate—Combination of treatments for monostarch phosphate and

Distarch phosphate

Dual modification Acetylated distarch phosphate—Esterification by sodium trimetaphosphate or phosphorus

oxychloride combined with esterification by acetic anhydride or vinyl acetate

Hydroxypropyl distarch phosphate—Esterification by sodium trimetaphosphate or phosphorus

oxychloride combined with etherification by propylene oxide

J. Singh et al. / Food Hydrocolloids 21 (2007) 1–222

2. Common types of chemical starch modifications

Food grade starches are chemically modified mainly toincrease paste consistency, smoothness and clarity, and toimpart freeze-thaw and cold storage stabilities (Jane, 1997;Liu et al., 1999a, 1999b; Perera et al., 1997; Shi & BeMiller,2000; Tuschhoff, 1986, Chapter 6; Wu & Seib, 1990; Xu &Seib, 1997). The properties of some modified starches andtheir applications are presented in Table 2. The chemicaland functional properties achieved when modifying starchby chemical substitution depend, inter alia, on starchsource, reaction conditions (reactant concentration, reac-tion time, pH and the presence of catalyst), type ofsubstituent, extent of substitution (degree of substitution,DS1; or molar substitution, MS2), and the distribution ofthe substitutent in the starch molecule (Hirsch & Kokini,2002; Kavitha & BeMiller, 1998; Lim & Seib, 1993;Richardson, Nilsson, Bergquist, Gorton, & Mischnick,2000; Rutenberg & Solarek, 1984; Steeneken & Woortman,1994; Takahashi, Fujimoto, Miyamoto, & Inagaki, 1987;Wang & Wang, 2002). Some chemical modificationreactions of starch are presented in Table 3. Acetylatedstarch with a low DS is commonly obtained by theesterification of native starch with acetic anhydride in thepresence of an alkaline catalyst, where as the food grade

hydroxypropylated starches are generally prepared byetherification of native starch with propylene oxide in thepresence of an alkaline catalyst. The hydroxypropyl groupsintroduced into the starch chains are capable of disruptingthe inter- and intra-molecular hydrogen bonds, therebyweakening the granular structure of starch, leading to anincrease in motional freedom of starch chains in amor-phous regions (Seow & Thevamalar, 1993; Wootton &Manatsathit, 1983).Cross-linking treatment is intended to add intra- and

inter-molecular bonds at random locations in the starchgranule that stabilize and strengthen the granule (Acquar-one & Rao, 2003). Starch pastes from cross-linked starchesare less likely to break down with extended cooking times,increased acidity or severe shear (Hirsch & Kokini, 2002;Langan, 1986; Wurzburg, 1986a, 1986b). Cross-linkingminimizes granule rupture, loss of viscosity and theformation of stringy paste during cooking (Woo & Seib,1997), yielding starch that is suitable for canned foods andother food applications (Hirsch & Kokini, 2002; Rutenberg& Solarek, 1984). Nutritional benefits of cross-linkedstarch as a new source of dietary fiber have also beenreported (Woo, 1999; Wurzburg, 1986a, 1986b). Cross-linking is generally performed by treatment of granularstarch with multifunctional reagents capable of formingeither ether or ester inter-molecular linkages betweenhydroxyl groups on starch molecules (Rutenberg &Solarek, 1984; Wurzburg, 1986a, 1986b). Sodium trimeta-phosphate (STMP), monosodium phosphate (SOP),sodium tripolyphosphate (STPP), epichlorohydrin (EPI),phosphoryl chloride (POCl3), a mixture of adipic acid andacetic anhydride, and vinyl chloride are the main agentsused to cross-link food grade starches (Wattanchant,Muhammad, Hashim, & Rahman, 2003; Woo & Seib,1997; Wu & Seib, 1990; Yeh & Yeh, 1993; Yook, Pek, &Park, 1993). POCl3 is an efficient cross-linking agent in

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Table 2

Some properties and applications of modified starches

Types Properties Applications

Pregelatinization Cold water dispersibility Useful in instant convenience foods

Partial acid or

enzymatic hydrolysis

Reduced molecular weight polymers, exhibit reduced

viscosity, increased retrogradation and setback

Useful in confectionery, Batters and food coatings

Oxidation/bleaching Low viscosity, high clarity, and low temperature stability Used in batters and breading for coating various food

stuffs, in confectionery as binders and film formers, in

dairy as texturizers

Pyroconversion

(dextrinization)

Low to high solubility depending on conversion, low

viscosity, high reducing sugar content

Used as coating materials for various foods, good film

forming ability and as fat replacers in bakery and dairy

products

Etherification Improved clarity of starch paste, greater viscosity, reduced

syneresis and freeze-thaw stability

Used in wide range of food applications such as gravies,

dips, sauces, fruit pie fillings and puddings

Esterification Lower gelatinization temperature and retrogradation,

lower tendency to form gels and higher paste clarity

Used in refrigerated and frozen foods, as emulsion

stabilizers and for encapsulation

Cross-linking Higher stability of granules towards swelling, high

temperature, high shear and acidic conditions

Used as viscosifiers and texturizers in soups, sauces,

gravies, bakery and dairy products

Dual modification Stability against acid, thermal and mechanical degradation

and delayed retrogradation during storage

Used in canned foods, refrigerated and frozen foods, salad

dressings, puddings and gravies

1DS represents the average number of hydroxyl groups on each

anhydroglucose unit which are derivitised by substituent groups. DS is

expressed as average number of moles of substituent per anhydroglucose

unit. As each anhydroglucose unit has three hydroxyl groups available for

substitution the maximum possible DS is 3.2The substituent moiety on some starch esters and ethers can react

further with the modifying reagent during the modification reaction,

resulting in the formation of an oligomeric or possibly polymeric

substituent. In these cases molar substitution is preferred, which represents

the level of substitution as moles of monomeric substituent per mole of

anhydroglucose unit. Thus, in contrast to degree of substitution, the value

of MS can be greater than three.

J. Singh et al. / Food Hydrocolloids 21 (2007) 1–22 3

aqueous slurry at pH411 in the presence of a neutral salt(Felton & Schopmeyer, 1943). STMP is reported to be anefficient cross-linking agent at high temperature with semi-dry starch and at warm temperature with hydrated starchin aqueous slurry (Kerr & Cleveland, 1962). EPI is poorlysoluble in water and partly decomposes to glycerol, thuswater-soluble cross-linking agents such as POCl3 andSTMP are preferred. Moreover, it has also been reportedthat EPI cross-links are likely to be less uniformlydistributed than STMP ones (Shiftan, Ravanelle, Alex-andre Mateescu, & Marchessault, 2000). So, the type ofcross-linking agent greatly determines the change infunctional properties of the treated starches. Starchphosphates, which are conventionally prepared have beenreported to give clear pastes of high consistency with goodfreeze-thaw stability and emulsifying properties, and maybe grouped into two classes: monostarch phosphates anddistarch phosphates (cross-linked starches). In general,monostarch phosphates (monoesters) can have a higher DS

than distarch phosphates (diesters) because even a very fewcross-links (in the case of diesters) can drastically alter thepaste and gel properties of the starch. Starch phosphatesare conventionally prepared through the reaction of starchwith salts of ortho-, meta-, pyro-, and tripolyphosphoricacids and phosphorus oxychloride (Nierle, 1969; Paschall,1964).Dual modification, a combination of substitution and

cross-linking, has been demonstrated to provide stabilityagainst acid, thermal and mechanical degradation of starchand to delay retrogradation during storage. Dual modifiedstarches are used widely in salad dressings, canned foods,frozen foods and puddings (Jane, 1997; Wurzburg, 1986a,1986b). The control of reaction conditions is importantduring preparation of dual-modified cross-linked/hydro-xypropylated starches using different cross-linking reagentswith different starch bases such as maize, tapioca, wheat,waxy maize, waxy barley, rice and sago (Tessler, 1975;Wattanchant et al., 2003; Wu & Seib, 1990; Yeh & Yeh,

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Table 3

Some common starch chemical modification reactions


1993; Yook et al., 1993). The quantity of cross-linkingreagent required to prepare a dual modified starch (withdesirable properties) vary with the source of starch, thetype of cross-linking reagent, the efficiency of the cross-linking reaction, the degree of substitution required and thespecified range of final modified-starch properties (Wattan-chant et al., 2003). The effects of different reactionconditions such as starch base concentration, temperature,pH and the concentration of the catalyst salt plays animportant role during preparation of duly modifiedhydroxypropylated-crosslinked starch (Lim & Seib, 1993;Smolka & Alexander, 1985; Tessler, 1975; Wu & Seib,1990; Yeh & Yeh, 1993). The effects of chemical modifica-tions on thermal, morphological and pasting/rheologicalbehaviour of starches may be quantified using instrumen-tation such as differential scanning calorimetry (DSC),scanning electron microscopy (SEM), Viscoamylograph/Rapid Visco Analyser (RVA) and dynamic rheometer,respectively (Kaur, Singh, & Singh, 2004; Kaur et al.,2005a; Kim, Hermansson, & Eriksson, 1992; Liu et al.,1999a, 1999b; Shi & BeMiller, 2000; Singh & Singh, 2003;Singh, Kaur, & Singh, 2004; Singh, Chawla, & Singh, 2004;Yeh & Yeh, 1993).

3. Extent of chemical modification

The rate and efficiency of the chemical modificationprocess depends on the reagent type, botanical origin of thestarch and on the size and structure of its granules (Huber

& BeMiller, 2001). This also includes the surface structureof the starch granules, which encompasses the outer andinner surface, depending on the pores and channels, leadsto the development of the so-called specific surface(Juszczak, 2003). Channels that open to the granuleexterior provide a much larger surface area accessible bychemical reagents, and provide easier access by thereagents to the granule interior. However, the reagentmay diffuse through the external surface to granule matrixin the absence of channels (BeMiller, 1997). Althoughstarches from various sources exhibit fundamental struc-tural similarities, they differ in the specific details of theirmicrostructure and ultrastructure. These structural differ-ences have the potential to affect the chemical modificationprocess (Huber & BeMiller, 2001).The DS and MS of some chemically modified starches

prepared from different sources are presented in Table 4.Potato, maize and rice starches show significant variationin their DS when acetylated under similar reactionconditions (Singh, Kaur et al., 2004; Singh, Chawla et al.,2004; Sodhi & Singh, 2005). Factors such as amylose toamylopectin ratio, intragranule packing and the presenceof lipids mainly govern the degree of substitution duringacetylation of starches from different sources (Phillips, Liu,Pan, & Corke, 1999; Singh, Kaur et al., 2004; Singh,Chawla et al., 2004). Starches with low amylose contenthave been observed to exhibit a higher degree of substitu-tion after acetylation. The CQO bond of the acetyl groupexperiences a different molecular environment depending

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Table 4

DS* and MS** of some modified starches from different botanical sources

Starch source DS (acetylated) MS (hydroxypropylated) DS (Cross-linked)

Normal potato 0.115–0.238a 0.098–0.122h 0.07–0.26l

Normal maize 0.104–0.184b 0.091–0.092i 0.09–0.25l

Normal maize ND*** 0.061–0.094j ND

Waxy maize 0.081c 0.067–0.127j ND

Hi amylose maize ND 0.078–0.119j ND

Hybrid normal maize 0.030–0.040d ND ND

Normal wheat 0.035–0.131e 0.117–0.123i 0.004–0.020m

Normal rice 0.087–0.118f 40.03k 0.025–0.035n

Normal rice 0.018g ND ND

Waxy rice 0.016g ND ND

*DS ¼ degree of substitution.

**MS ¼ molar substitution.

***ND ¼ Not detected.a,bSingh, Kaur et al. (2004); Singh, Chawla et al. (2004) (different levels of acetylation and starches from different potato cultivars).cWang and Wang (2002).dLawal (2004) (two levels of acetylation).ePhillips et al. (1999) (different levels of acetylation used).fSodhi and Singh (2005) (starches from different rice cultivars).gLiu et al (1999b).hKaur et al. (2004) (starches from different potato cultivars).iStapley and BeMiller (2003) (low MS values from two populations of starch granules).jLiu et al. (1999a) (two levels of hydroxypropylation).kYeh and Yeh (1993) (two levels of hydroxypropylation).lSitohi and Ramadan (2001) (DS of starches phosphorylated using a mixture of monosodium and disodium phosphate).mBertolini et al. (2003) (molar substitution values; three levels of cross-linking performed).nYeh and Yeh (1993) (different levels of cross-linking).


on whether it is a substitutent on amylose or onamylopectin (Phillips et al., 1999). It has been reportedthat during acetylation, the acetyl groups are introducedexclusively in the outer lamellae of the granules and highsubstitution occurs only in certain parts of the amylopectinfraction of starch (Biliaderis, 1982; Phillips et al., 1999).However, Chen, Schols, and Voragen (2004) reported thatacetylation occurs in all the amorphous regions and also atthe outer lamellae of crystalline regions, rather thanthroughout the crystalline regions of the whole starchgranule. This has been suggested to be due to the poorpenetrating ability of acetic anhydride in starch granules(Chen et al., 2004). Studies (Singh, Kaur et al., 2004) onacetylated potato starches suggest that the small sizegranule population with lower amylose content favoursthe introduction of acetyl groups and hence results inhigher DS. The size and shape of potato starch granules ofdifferent cultivars have been found to be significantly co-related with their amylose content (Kaur, Singh, & Sodhi,2002; Singh, Singh, Kaur, Sodhi, & Gill, 2003; Singh &Singh, 2003; Singh & Kaur, 2004).

During hydroxypropylation, the hydroxypropyl groupsare primarily introduced into the starch chains in theamorphous regions composed mainly of amylose (Blan-shard, 1987; Hood & Mercier, 1978; Steeneken & Smith,1991). Kavitha and BeMiller (1998) and Shi and BeMiller(2000) have confirmed using acid/enzymatic hydrolysis thatamylose is modified to a greater extent than amylopectin inhydroxypropylated maize and potato starches, and that themodification of amylopectin occurs close to the branchpoints, presumably because amorphous regions are moreaccessible to the modifying reagent. However, Richardsonand Lo Gorton (2001), after investigating the substituentdistribution in hydroxypropylated potato amylopectinstarch (PAP) suggested that the hydroxypropyl groupsare homogeneously distributed on the amylopectin mole-cule. The modification reaction conditions and starchsource may also affect the distribution of hydroxypropylgroups along the starch chain (Kaur et al., 2004; Steeneken& Woortman, 1994). Azemi and Wootton (1995) reportedthat hydroxypropylated common maize, waxy maize, andhigh-amylose maize starches differed with regard to thedistribution of substituent groups on the starch polymermolecules. Investigations carried out on hydroxypropy-lated PAP prepared in granular slurry or solution suggeststhat more substituents were located in close vicinity tobranching points, which constitute the amorphous areas inthe semicrystalline granule, than elsewhere Starch hydro-xypropylated in granule slurry had a more heterogeneoussubstituent distribution compared with starch modified in apolymer ‘solution’ of dissolved starch (Richardson & LoGorton, 2001). The reactivity and concentration ofreagents have been reported to influence the degreeof substitution of cross-linked starches. Also, the type ofreagent used and the reaction conditions determine theratio of mono- and di-type bonds (esters with phosphorusbased agents, and glycerols with EPI) during cross-linking

(Koch, Bommer, & Koppers, 1982). Studies on waxy maizestarches using different reagents (POCl3 and propyleneoxide) reported variations in the relative reaction patterns(Huber & BeMiller, 2001). Due to the higher reactivity ofPOCl3, the cross-links predominate on the granulesurfaces, when using highly reactive POCl3 (Gluck-Hirsch& Kokini, 1997; Huber & BeMiller, 2001) while the lessreactive propylene oxide generally diffuses into the granulematrix prior to reaction (Huber & BeMiller, 2001; Kauret al., 2004).Knowledge about the structural changes in starch

granules caused by modification with chemical reagentscan be of importance for understanding the alteredfunctional properties, and for developing chemicallymodified starches with desired properties (Kim et al.,1992). Shiftan et al. (2000) reported that EPI cross-linkingis not homogeneous and is concentrated in the non-crystalline domain of starch granules. Jane, Radosavljevic,and Seib (1992) found that cross-linking of starch chainsoccurred mainly in amylopectin. Another factor that mayinfluence the extent of cross-linking is the size distributionof starch granule population (Hung & Morita, 2005).During cross-linking small size granules have been reportedto be derivatized to a greater extent than the large sizegranules (Bertolini, Souza, Nelson, & Huber, 2003).Characterization of substitution upon modification is

important at both monomer and polymer levels (Richard-son et al., 2003). The distribution of substituents at bothmonomeric and polymeric levels may be affected by thepresence of granule pores and channels; the proportions ofamylose and amylopectin and their arrangement; thenature of the granule surface and granule swelling(BeMiller, 1997; Kavitha & BeMiller, 1998). Techniquessuch as nuclear magnetic resonance (NMR) spectroscopy(Heins, Kulicke, Kauper, & Thielking, 1998; Xu & Seib,1997) or gas chromatography/mass spectrometry (Richard-son et al., 2000; Wilke & Mischnick, 1997) may be helpfulfor the determination of the distribution of the substituentgroups at the monomeric level. The methods used for theanalysis of the substituent distribution along the polymerchains and the homogeneity/heterogeneity of substitutionare based on partial degradation of the polymer by acidhydrolysis (Arisz, Kauw, & Boon, 1995; Mischnick &Kuhn, 1996) or by enzymatic degradation (Kavitha& BeMiller, 1998; Wilke & Mischnick, 1997; Steeneken &Woortman, 1994; van der Burgt et al., 1998).

4. Physico-chemical properties

The physico-chemical properties of starches such asswelling, solubility, and light transmittance have beenreported to be affected significantly by chemical modifica-tion (Table 5). The change in these properties uponmodification depends on the type of chemical modification.Chemical modifications such as acetylation and hydro-xypropylation increase, while cross-linking has been obser-ved to decrease (depending on the type of cross-linking

ARTICLE IN PRESSJ. Singh et al. / Food Hydrocolloids 21 (2007) 1–226

agent and degree of cross-linking) the swelling power andsolubility of starches from various sources. The introduc-tion of (bulky) acetyl groups into starch molecules byacetylation leads to structural reorganization owing tosteric hindrance; this results in repulsion between starchmolecules, thus facilitating an increase in water percolationwithin the amorphous regions of granules and a conse-quent increase in swelling capacity (Lawal, 2004). Studiesconducted on acetylated maize, potato and rice starches(Gonzalez & Perez, 2002; Singh, Kaur et al., 2004; Singh,Chawla et al., 2004) suggest a significant increase inswelling power and solubility upon acetylation in all thesestarch types. The extent of this increase was observed to behigher for potato starches. The degree of substitutionintroduced after acetylation mainly affects the intensity ofchange in the swelling power and solubility of starches. Thedifferences in the granule size distribution, physico-chemical composition and granule rigidity among thestarches may also be responsible for the alteration inswelling power and solubility after acetylation. Thestructural disintegration probably weakens the starchgranules after acetylation, and this enhances amyloseleaching from the granule, thus increasing starch solubility(Lawal, 2004). The extent of the increase in swelling powerhas been observed to be higher in starches with a lowamylose content and small size granule population (Singh,Kaur et al., 2004; Sodhi & Singh, 2005). Liu et al. (1999b)reported that the waxy starches show an increased swellingpower upon acetylation because of the presence of mainlyamylopectin with a more open structure than in non-waxystarch; this allows rapid water penetration, and increasedswelling power and solubility.

Kaur et al. (2004) studied the swelling power andsolubility of hydroxypropylated potato starches and foundhigher swelling power and solubility in starches with a

higher MS. The decrease in associative forces within thestarch granule due to hydroxypropylation may result in anincreased penetration of water during heating that leads tothe greater swelling (Kaur et al., 2004). Choi and Kerr(2003) studied the effects of hydroxypropylation on watersorption isotherms and molecular mobility of both starchand water molecules in wheat starch gels using pulsed1H NMR techniques. They reported that the waterabsorption capacity of hydroxypropylated wheat starchincreases with an increasing degree of MS in a specificwater activity range (aw40:53). Choi and Kerr (2004) alsoreported that the cross-linked starch granules have higherresistance towards temperature and heating time. Cross-linking strengthens the bonding between the starch chains,causing an increase in the resistance of the granules toswelling with increasing degree of cross-linking. Higherconcentrations of fast acting cross-linking reagents such asPOCI3 result in greater reductions in the swelling potentialas compared with slower acting agents such as EPI (Hirsch& Kokini, 2002). A large concentration of POCl3 cross-links at the surface of the granule causes the formation of ahard outer crust that restricts granule swelling (Gluck-Hirsch & Kokini, 1997; Huber & BeMiller, 2001). Inagakiand Seib (1992) also reported that the swelling power ofcross-linked waxy barley starch declined as the level ofcross-linking increased. Cross-linked starches exhibit lowersolubility than their native equivalents, and solubilitydecreases further with an increase in the concentration ofcross-linking reagent, which may be attributed to anincrease in cross-link density (Kaur et al., 2005a). Cross-linking at low levels, although having a substantial effecton starch properties such as granule swelling (Inagaki &Seib, 1992; Reddy & Seib, 1999; Rutenberg & Solarek,1984) contributes little to water sorption properties(Chilton & Collison, 1974). Starch modifications alter the

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Table 5

Swelling power of some modified starches from different botanical sources

Starch source SP* (g/g) (acetylated) SP* (g/g) (hydroxypropylated) SP* (g/g) (cross-linked)

Normal potato 60–71 (native 58–70)y a 33–39 (native 28–31)e 20–25 (native 28–31)h

Normal potato ND** ND 23–27 (native 28–31)i

Normal maize 38 (native 36)b E20 (native E6)f ND

Waxy maize ND E42 (native E30)f ND

Normal wheat ND 9–16 (native E6–8)g ND

Normal rice 15–19 (native 14–18)c ND E9 (Native E18)d

Waxy rice E31 (native E41)d ND E14 (Native E41)d

*SP ¼ Swelling power (g/g).

** ¼ not detected.y¼ The properties of corresponding native (unmodified) starches are given in brackets.aSingh, Kaur et al. (2004).bSingh, Kaur et al. (2004) (starches from different potato cultivars).cSodhi and Singh (2005) (starches from different rice cultivars).dLiu et al. (1999a, b).eKaur et al. (2004) (starches from different potato cultivars).fLiu et al. (1999a) (two levels of hydroxypropylation)gHung and Morita (2005) (two wheat starch granule populations)hKaur et al. (2005a) (starches from different potato cultivars; cross-linking performed using POCl3).iKaur et al. (2005a) (starches from different potato cultivars; cross-linking performed using EPI).


light transmittance of starch pastes to a considerableextent. Modifications such as acetylation and hydroxypro-pylation have been reported to increase the light transmit-tance of various starches (Kaur et al., 2004; Lawal, 2004;Singh, Kaur et al., 2004; Singh, Chawla et al., 2004). Thechemical substitution of the –OH groups on the starchmolecules by acetyl moieties hampers the formation of anordered structure following gelatinization, and thus retardsretrogradation, resulting in a more fluid paste withimproved long-term clarity (Lawal, 2004). The highretention of water entering the starch granule results in agreater swelling power and favours the clarity of pastes andgels. Starch source, granule size distribution, amylosecontent and degree of substitution introduced uponmodification have been found to be the important factorsaffecting light transmittance of acetylated starches. Singh,Chawla et al. (2004) reported a higher light transmittanceof acetylated potato starches compared to acetylated maizestarches acetylated under similar reaction conditions.Highly cross-linked starch pastes generally show lowerlight transmittance than their counterpart native starches.Incomplete gelatinization and reduced swelling of cross-linked starches is mainly responsible for their reduced pasteclarity (Kaur et al., 2005a; Lim & Seib, 1993; Morikawa &Nishinari, 2000b; Reddy & Seib, 2000; Woo & Seib, 1997;Zheng, Han, & Bhatty, 1999).

The solubility of native, cross-linked and hydroxypropy-lated starches in dimethyl sulphoxide (DMSO) varies to asignificant extent (Table 6). Hydroxypropylation results in asignificant increase in the solubility of starches in DMSO(Kaur et al., 2004; Yeh & Yeh, 1993). Yeh and Yeh (1993)compared the solubilites (in DMSO) of hydroxypropylatedand cross-linked rice starches with the solubility of native ricestarch, and observed higher and lower solubilities forhydroxypropylated and cross-linked rice starches, respec-tively. Differences in granule morphology, amylose contentand degree of substitution of native and modified starches,respectively, have been reported to affect the light transmit-tance in DMSO (Kaur et al., 2004). Sahai and Jackson(1996) reported that the solubility of starch in methylsulphoxide varies significantly with granule size, presumablyreflecting inherent structural hetrogeneity within granules.

5. Morphological properties

Starch modification involves physical, chemical andbiochemical phenomena on the surface of contactingphases. Microscopy (light and SEM) has played animportant role in increasing understanding of granularstructure of modified starches. It has been used to detectstructural changes caused by chemical modifications andthe most substituted regions in starch granules (Kaur et al.,2004; Kim et al., 1992). Most of the structural changesupon hydroxypropylation take place at the relatively lessorganized central core region of the starch granule i.e.where the hydroxypropyl groups are most densely depos-ited (Kim et al., 1992). The ‘pushing apart effect’ exerted bythe bulky hydroxypropyl groups, especially in the centralregion of the granule, might lead to an alteration in granulemorphology upon hydroxypropylation. Another possibleexplanation is that the starch granule itself is notstructurally homogeneous from a physical and chemicalpoint of view, since it has different physical natures(amorphous and crystalline regions) as well as differentchemical compositions in each region (French, 1984). Kauret al. (2004) reported that the treatment of potato starchgranules with propylene oxide (10%, dwb) alters granulemorphology (Fig. 1A and B). Many of the less affectedmodified granules developed a depression that resultedlater in slight fragmentation, indentation, and the forma-tion of a deep groove in the central core region along thelongitudinal axis in highly affected granules (Fig. 2). Thesegranules appeared as folded structures with their outersides drawn inwards, giving the appearance of a doughnut(Fig. 2A and B). Moreover, these altered regions wereobserved to be apparently larger in large size granulescompared with small size granules in all the potato starchesexamined; this may be attributed to differences in thenative granule architecture and fragility. The peripheralregions and also the outer layer of the less affected starchgranules remained unaltered, and the changes remainedconfined to the central core regions. By contrast, highlyaffected starch granules developed blister like appearances,cracks and small protuberances on their surfaces, and adeep groove in the central core region; this suggests that the

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Table 6

Solubility (%) in DMSO of some modified starches from different botanical sources

Starch source SB* (after 4 h) SB (after 8 h) SB (after 16 h) SB (after 24 h)

Normal potato (hydroxypropylated)a y E76 (native E57)y E83 (native E60) E95 (native E65) E99 (native 75)

Normal rice (hydroxypropylated)b E35 (native E10) E80 (native E20) E95 (native E35) E98 (native E55)

Normal rice (cross-linked)c E08 (native E10) E10 (native E20) E20 (native E35) E26 (native E55)

*Solubility in DMSO (%).y¼ values reported as % transmittance in DMSO.

y¼ The properties of corresponding native (unmodified) starches are given in brackets.aKaur et al. (2004) (starches from different potato cultivars used).bYeh and Yeh (1993) (two levels of hydroxypropylation used).cYeh and Yeh (1993) (different levels of cross-linking used).


granule peripheral regions may be the last to be modified(Fig. 2C) (Kaur et al., 2004).

The granule structure is more substantially altered whenthe reaction is carried out with a higher concentration ofpropylene oxide (15% compared with 10%). Highlyaffected granules appear as if gelatinized, having lost their

boundaries and fused together to form a gelatinized mass(Fig. 1C). This effect was again more pronounced in largergranules as compared with smaller granules (Kaur et al.,2004). Huber and BeMiller (2001) also reported that thematerial within the inner regions of potato starch granuleswas more susceptible to reaction (with propylene oxide)

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Fig. 1. Effects of hydroxypropylation on the granule morphology of potato starches I: (A) native potato starch granules, (B) potato starch granules after

hydroxypropylation (at 10% propylene oxide concentration), and (C) effect of increased concentration of propylene oxide (15%) on the starch granule

structure (Kaur et al., 2004).

Fig. 2. Effects of hydroxypropylation on the granule morphology of potato starches II: (A,B) formation of deep groove in the central core region, folding

of starch granules and formation of doughnut like appearance, and (C) formation of blister like appearance and cracks on the starch granules (Kaur,

2004).


than that in the outer granule layers. Also, as potato starchgranules do not possess channels, the reagent diffusesinwards through the exterior granule surface. Propyleneoxide, being less reactive than other modification reagents,may diffuse into the granule matrix prior to reacting(Huber & BeMiller, 2001). However, Biliaderis (1982) in astudy on chemically modified pea and waxy maize starchesfound that hydroxypropylation occurred uniformlythroughout the starch granules. The structural changes inhydroxypropylated starch granules become more evidentwith increasing MS (Kaur et al., 2004; Kim et al., 1992).Hydroxypropylation leads to alteration in the structure ofthe gelatinized potato starch gels also, which may beattributed to a weakened granule structure, leading toincreased granule disruption during gelatinization (Kauret al., 2004). Hydroxypropylated starch gels maintainedwell the planar structure during storage at 4 1C for 10 days,whereas stored gels of native starches showed extensiveaggregation of granule remnants. Similar observationshave been reported by Yeh and Yeh (1993) and Perera andHoover (1999) for hydroxypropylated potato and ricestarch gels respectively. Kaur et al. (2004) have alsoreported the appearance of numerous rod-shaped fuzzyclustered microfibrils in hydroxypropylated starch gels(after 30 days storage at 4 1C) that could be easilydistinguished from the other starchy material (Fig. 3).The extensive phase separation occurring during long-termstorage at 4 1C may be responsible for the formation ofthese rod-shaped microfibrils. Slow cooling has also beenreported to enhance the formation of non-spheruliticmorphologies in starch gels (Nordmark & Ziegler, 2002).

Acetylation treatment has also been found to alter thegranule morphology, although to a lesser extent. Singh,Kaur et al. (2004) carried out acetylation of maize andpotato starches using different concentrations of aceticanhydride, and reported that the acetylation treatmentcaused granule fusion in both maize and potato starches(Fig. 4). The granule fusion was also observed to be morepronounced in potato starches with small size granules(Singh, Kaur et al., 2004; Singh, Chawla et al., 2004).Maize starch granules show a higher resistance towardsacetylation treatment than potato starches. The addition of4% acetic anhydride resulted in the fusion of granules inpotato starches while maize starch granules tended to fuseat concentrations of acetic anhydride of 8% or higher(Singh, Kaur et al., 2004). The granule surface of maizeand potato starch granules was observed to be slightlyrough upon acetylation (Singh, Chawla et al., 2004). Thegranule surface of rice starches becomes rough and thegranules tend to form aggregates upon acetylation (Gon-zalez & Perez, 2002; Jeong, Bae, & Oh, 1993; Sodhi &Singh, 2005). Jae, Jung, and Man (1994) reported thedeformation of rice starch granules after acetylation.However, some granule fusion and deformation, and therough appearance of the granule surface in the acetylatedstarches might also be the result of surface gelatinizationupon addition of NaOH to maintain alkaline conditions

during acetic anhydride addition (Singh, Chawla et al.,2004). Sitohi and Ramadan (2001) studied the granularproperties of phosphorylated normal maize, potato, rice,maize amylopectin and maize amylose starches, preparedusing a mixture of mono- and disodium phosphate. Theyconcluded that the starch granule size is very responsive tothe changes in the DS by phosphate groups, higher the DS,larger the granule size. The average increase in granule sizewas observed to be higher for rice and maize starches thanfor potato starch for a given DS. They suggested that thisincrease may be due to the substitution by phosphategroups inside the modified granules, building certainrepulsive forces that increase the sizes of inter- and intramolecular spaces, allowing more water molecules to beabsorbed. Phosphorylation does not cause any detectablechange in the general granule shape (Sitohi & Ramadan,2001). After being cross-linked using EPI and POCl3,potato starch granules remain smooth and similar to nativestarch granules in morphology when viewed under SEM,

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Fig. 3. (A,B) Rod-shaped microfibrils (showing longitudinal arrangement

of crystalline structures) formed during long term storage of hydro-

xypropylated gelatinized starch pastes (Kaur et al., 2004).


suggesting that the modification does not cause anydetectable morphological change (Kaur et al., 2005a).Some chemical modifications of starch have been reportedto increase granule surface area and granule porosity(Fortuna, Juszczak, & Palasinski, 1999).

6. Thermal properties

Starch gelatinization is the collapse (disruption) ofmolecular orders within the starch granule manifested inirreversible changes in properties such as granular swelling,native crystalline melting, loss of birefringence, and starchsolubilization (Atwell, Hood, Lineback, Varriano-marston,& Zobel, 1988). It is an important starch functionalproperty that varies with respect to the composition(amylose to amylopectin ratio, phosphorus, lipids, proteinsand enzymes, etc.), the molecular structure of amylopectin(unit chain length, extent of branching, molecular weightand granule architecture (crystalline to amorphous ratio),granule morphology and size distribution of starches (Kauret al., 2002, 2004, 2005a, 2005b; Krueger, Knutson, Inglett,& Walker, 1987; Singh & Singh, 2001; Singh et al., 2003;Singh, Kaur et al., 2004; Singh, Chawla et al., 2004; Singh& Kaur, 2004; Tester, 1997). Gelatinization has beenstudied by various means such as thermal analysis(Nakazawa, Noguchi, Takahashi, & Takeda, 1984; Shiot-subo & Takahashi, 1984; Slade & Levine, 1987; Wada,Takahashi, Shirai, & Kawamura, 1979), X-ray diffraction(I’Anson, Miles, Morris, & Ring, 1988; Zobel, Young, &Rocca, 1988) and nuclear magnetic resonance (NMR)(Chinachoti, White, Lo, & Stengle, 1991). DSC is the most

common technique used for detecting both first order(melting) and second order (glass) thermal transitions(Biliaderis, Page, Maurice, & Juliano, 1986; Huang, Chang,Chang, & Lii, 1994; Nakazawa et al., 1984; Russel &Oliver, 1989; Yook et al., 1993).DSC studies have shown that modification alters thermal

transition temperatures (onset (To); peak (Tp); conclusion(Tc) and the overall enthalpy (DHgel) associated withgelatinization (Tables 7 and 8). Upon hydroxypropylation,the reactive groups introduced into the starch chains arecapable of disrupting the inter- and intra-molecularhydrogen bonds, leading to an increase in accessibility bywater that lowers the temperature of gelatinization. Pereraet al. (1997) studied the effect of hydroxypropylation onthe thermal properties of different potato starches. In allstarches, increasing the level of MS resulted in decreases inDHgel, To, Tp and Tc and a widening of the gelatinizationtemperature range (Tc�To). A progressive shift of thebiphasic gelatinization endotherms to lower temperaturesas well as a broadening and shortening of the gelatinizationendotherm with increasing MS have also been observed forhydroxypropylated rice starch, indicating increased inter-nal plasticization and destabilization of the amorphousregions of the granules (Seow & Thevamalar, 1993). Anincrease in gelatinization temperature range upon hydro-xypropylation of different potato starches has also beenreported by Kaur et al. (2004), which could be attributed toincreased inhomogeneity within both the amorphous andcrystalline regions of the starch granules. Decreases havebeen recorded in gelatinization temperatures and gelatini-zation enthalpies of amylose extender (ae) mutant, waxy

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Fig. 4. Effects of acetylation on the granule morphology of maize and potato starches: (A) native maize starch granules, (B) acetylated maize starch

granules, (C) native potato starch granules, and (D) acetylated potato starch granules (Singh et al., 2004b).


mutant (wx) and normal maize starch upon hydroxypro-pylation (Liu et al., 1999a). The decrease was observed tobe greater for high amylose maize starches (66% amylose)as compared with waxy maize starch (3.3% amylose). Thepeak transition temperature (Tp) gives a measure ofcrystallite quality (double helix length), whereas DHgel

gives an overall measure of crystallinity (quantity andquality) and is an indicator of the loss of molecular orderwithin the granule on gelatinization (Cooke & Gidley,1992; Hoover & Vasanthan, 1994; Tester & Morrison,1990). The decrease in DHgel on hydroxypropylationsuggests that hydroxypropyl groups disrupt double helices(owing to the rotation of these flexible groups) within theamorphous regions of the granules. Consequently, thenumber of double helices that unravel and melt duringgelatinization would be lower in hydroxypropylated than

in unmodified starches (Perera et al., 1997). Similar DSCresults have been reported for hydroxypropylated peastarch (Hoover, Hannouz, & Sosulski, 1988), hydroxypro-pylated rice starch (Seow & Thevamalar, 1993) andhydroxypropylated potato starch (Kim & Eliasson, 1993).Substitution on granular starch occurs mainly in theamorphous regions, which promotes swelling in theseregions and thus, disrupts the crystalline phase, whichmelts at a lower temperature than in the case of theunmodified starch.Thermal studies of gelatinization of acetylated Mucuna

bean (Mucuna pruriens) and Jack bean (Canavalia ensifor-

mis) starch revealed that acetylation reduced the To, Tp andTc (Adebowale & Lawal, 2003; Betancur-Ancona, Chel-Guerrero, & Canizares-Hernandez, 1997). Starches withhigher degrees of acetyl substitution have been observed to

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Table 7

Thermal properties of some modified starches from different botanical sources

Starch source T*, 1C (acetylated) T*, 1C (hydroxypropylated) T*, 1C (cross-linked)

Normal potato 48–67 (native 57–69)y a 51–62 (native 57–68)i 60–68 (native 57–68)o

Normal potato 55–64 (native 59–70)b ND 59–68 (native 57–68)p

Normal maize 65–77 (native 69–77)c 59–75 (native 65–81)j ND

Normal maize 66–75 (native 70–78)d ND ND

Waxy maize 64–69 (native 68–72)e 61–79 (native 63–84)j 65–75 (native 62–76)q

Waxy maize ND* ND 67–72 (native 68–73)r

Hybrid normal maize 71–84 (native 60–78)f ND ND

Hi-amylose maize ND 66–95 (native 67–105)j ND

Normal wheat ND 55–71 (native 63–84)k 63–76 (native 63–84)s

Normal wheat ND 46–55 (native 55–67)l 61–72 (native 57–58)t

Waxy wheat ND ND 62–66 (native 61–66)r

Normal rice 60–72 (native 66–78)g 64–83 (native 63–84)m 70–87 (native 71–87)u

Normal rice ND 57–92 (native 63–92)n ND

Waxy rice 60–78 (native 60–78)h ND 61–78 (native 60–78)v

Waxy rice ND ND 55–66 (native 53–67)q

T* ¼ Transition temperatures of different starches (1C).

ND* ¼ not detected, y ¼ The properties of corresponding native (unmodified) starches are given in brackets.

The properties of corresponding native (unmodified) starches are given in brackets.aSingh, Chawla et al. (2004) (different levels of acetylation).bSingh, Kaur et al. (2004) (starches from different potato cultivars).cSingh, Chawla et al.(2004) (different levels of acetylation).dSingh, Kaur et al. (2004) (starches from different potato cultivars).eWang and Wang (2002).fLawal (2004).gSodhi and Singh (2005) (starches from different rice cultivars).hLiu et al. (1999b).iKaur et al. (2004) (starches from different potato cultivars).jLiu et al. (1999a) (two levels of hydroxypropylation).kChoi and Kerr (2004) (three levels of hydroxypropylation).lHung and Morita (2005) (two wheat starch granule populations).mYeh and Yeh (1993) (two levels of hydroxypropylation).nSeow and Thevamalar (1993) (three levels of hydroxypropylation).oKaur et al. (2005a) (starches from different potato cultivars used, cross-linking performed using POCl3).pKaur et al. (2005a) (starches from different potato cultivars used, cross-linking performed using EPI).qTsai et al. (1997) (cross-linking performed using EPI).rReddy and Seib (2000) (cross-linking performed using POCl3).sChoi and Kerr (2004) (cross-linking performed using POCl3).tBertolini et al. (2003) (three levels of cross-linking performed using POCl3; onset transition temperatures only).uYeh and Yeh (1993) (different levels of cross-linking performed using POCl3).vLiu et al. (1999b).


show greater decreases in the transition temperatures andenthalpy of gelatinization. Weakening of the starchgranules by acetylation leads to early rupture of theamylopectin double helices, which accounts for the lowervalues of To, Tp and Tc (Adebowale & Lawal, 2003).Decreases in the thermal parameters are consistent withfewer crystals being present after modification (owing todamage to the crystals during acetylation) and with a co-operative melting process enhanced by added swelling(Singh, Chawla et al., 2004). The extent of the lowering oftransition temperatures and DHgel on acetylation has beenobserved to be different for starches from different sources.Potato starches show a greater decrease in thermalparameters upon acetylation compared with maize andrice starches (Singh, Kaur et al., 2004; Singh, Chawla et al.,2004; Sodhi & Singh, 2005). The differences in granulerigidity, presence/absence of lipids, degree of substitutionand amylose to amylopectin ratio between these starchesaffect the extent of changes in thermal parameters. Cross-linking also alters the thermal transition characteristics ofstarch, the effect depending on the concentration and typeof cross-linking reagent, reaction conditions and thebotanical source of the starch. An increase in gelatinizationtemperature has been observed for cross-linked starches;

these phenomena are related to the reduced mobility ofamorphous chains in the starch granule as a result of theformation of intermolecular bridges. The level of phos-phate cross-links has a strong influence on the DSCproperties of starches. Choi and Kerr (2004) reported thatcross-linked starches prepared using a relatively lowconcentration of the POCl3 had gelatinization parameterssimilar to those of native starches, while cross-linkedstarches prepared using higher reagent concentrationsshowed considerably higher Tc and DHgel values. Thesefindings are consistent with those of Yeh and Yeh (1993)and Liu, Weber, Currie, and Yada (2003), who reportedthat DHgel of starch increased with increasing levels ofcross-linking; however, Tp was not affected significantly.Cross-linking at lower levels reduces the proportion of thestarch that can be gelatinized, resulting in a lower value ofDHgel (Yook et al., 1993). Yoneya, Ishibashi, Hironaka,and Yamamoto (2003) also reported that potato starchestreated with 100 ppm of POCl3 displayed significantlylower gelatinization temperatures and lower DHgel thanother treated samples (with 40–5000 ppm). Decreasing orincreasing the degree of phosphate cross-links caused agradual increase in To, Tp and DHgel of cross-linkedpotato starch samples compared with the values of these

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Table 8

Thermal properties of some modified starches from different botanical sources

Starch source DHgel,* J/g (Acetylated) DHgel, J/g (Hydroxypropylated) DHgel, J/g (Cross-linked)

Normal potato 10.1–11.4 (native 12.1)y a 10–11 (native 11.7–12.9)g 12.7–14.7 (native 11.7–12.9)l

Normal potato 10.1–11.8 (native 12.8–13.2)b ND 12.7–14.4 (Native 11.7–12.9)m

Normal maize 08.52 (native 10.9)a ND ND

Normal maize 08.9–09.7 (native 10.6)b 07.1–8.4 (native 11)h ND

Hybrid normal maize 09.8–10.6 (native 14.3)c ND ND

Hi-amylose maize ND** 6.4–8.4 (native 13.7)h ND

Waxy maize 14.8 (native 15.6)d 08–8.7 (native 13.6)h 15.2 (native 15.3)n

Normal wheat ND 04.7–05 (native 6.8)i 6.6–7.4 (native 06.8)o

Normal wheat ND 02.2 (native 06.2)j —

Waxy wheat ND ND 12.8 (native 13.2)n

Normal rice 08.1–11.4 (native 08.1–11.9)e 08–09 (native 10.4)k 11–13 (native 10.4)p

Waxy rice 08.5 (native 09.8)f ND 10.3 (native 09.8)q

*DHgel ¼ enthalpy of gelatinization of different starches (J/g).

**ND ¼ Not detected.y¼ The properties of corresponding native (unmodified) starches are given in brackets.aSingh, Chawla et al. (2004) (different levels of acetylation).bSingh, Kaur et al. (2004) (starches from different potato cultivars).cLawal (2004).dWang and Wang (2002).eSodhi and Singh (2005) (starches from different rice cultivars).fLiu et al. (1999b).gKaur et al. (2004) (starches from different potato cultivars).hLiu et al. (1999a) (two levels of hydroxypropylation).iChoi and Kerr (2004) (three levels of hydroxypropylation).jHung and Morita (2005) (two wheat starch granule populations).kYeh and Yeh (1993) (two levels of hydroxypropylation).lKaur et al. (2005a) (starches from different potato cultivars; cross-linking performed using POCl3).mKaur et al. (2005a) (starches from different potato cultivars; cross-linking performed using EPI).nReddy and Seib (2000) (cross-linking performed using POCl3).oChoi and Kerr (2004) (three levels of cross-linking performed using POCl3).pYeh and Yeh (1993) (different levels of cross-linking performed using POCl3).qLiu et al. (1999b).


properties for the 100 ppm-treated sample. Chatakanonda,Varavinit, and Chinachoti (2000a) conducted studies oncross-linked rice starches (using as reagent a mixture ofSTMP and STPP) that showed deep and sharp en-dotherms, E30 1C wide. Also, the gelatinization tempera-ture increased significantly (by up to 5 1C) with an increasein the degree of cross-linking, while enthalpy showed nosignificant change (E15% decrease), suggesting completemelting of crystalline regions in spite of cross-linking. Theintroduction of phosphate cross-links into the starch bySTMP alone tightened the molecular structure, leading toan increase in the gelatinization temperature (Chataka-nonda et al., 2000a). The enthalpy of cross-linked normalrice starch was observed to decrease while waxy rice starchshowed an increase in enthalpy after cross-linking (Liu etal., 1999b). These findings suggest that the type andconcentration of the reagent; amylose/amylopectin ratio ofstarch during cross-linking significantly affects the extentof change in thermal properties.

Dual modified (substituted and cross-linked) normalmaize, waxy maize, tapioca, potato and normal wheatstarches are available commercially with varying degrees ofhydroxypropylation and cross-linking. The temperaturesand enthalpies of gelatinization of the dual modified(hydroxypropylated/cross-linked or acetylated/cross-lin-ked) waxy wheat and maize starches are generally lowerthan those of the unmodified starches (Reddy & Seib,2000). The gelatinization temperatures (To; Tp; Tc) ofcross-linked waxy wheat starch and its hydroxypropylated/cross-linked and acetylated/cross-linked forms are 5–7 1Cbelow than those of unmodified forms of waxy maizestarch when measured in excess water (Reddy & Seib,2000).

The majority of the reported work on thermal behaviourof starch has been focused mainly on the melting behaviourof starch crystals, and information on the amorphousphase is limited (Morikawa & Nishinari, 2000b; Tsai, Li, &Lii, 1997; Yeh & Yeh, 1993; Yook et al., 1993; Zheng et al.,1999). Glass transition of rice starch cross-linked using anSTMP-STPP mixture has been studied by Chatakanonda,Varavinit, and Chinachoti (2000b). They claimed that theglass transition temperature (Tg) of the starch was notsignificantly changed by cross-linking or by cold storage ata moisture content of 67%. Chung, Woo, and Lim (2004)reported an increase in the Tg of maize starch by nearly1 1C by cross-linking using an STMP-STPP mixture, whenmeasured with excess water (67%). However, the increasein the Tg by 1 1C may not be of significance because of thedifficulties observed in analysing the Tg of starches(Biliaderis, 1991; Biliaderis et al., 1986; Blond & Simatos,1998; Liu & Lelievre, 1992). The Tg of starch under limitedwater conditions (E15%) is usually measured after thestarch has been transformed to an amorphous statebecause Tg of granular starch containing crystals is difficultto measure due to the low heat capacity change at Tg

(Chung et al., 2002; Zeleznak & Hoseney, 1987). The Tg ofcross-linked starches under limited water conditions dis-

plays a decrease proportional to the amount of cross-linking agent used (Chung et al., 2004). This opposite trendmay be attributed to the internal plasticization by bulkyand ionic mono-substituted phosphate groups that con-tribute to a free volume increase for starch chains thatreduces the glass transition temperature.When a stored starch gel is reheated in a DSC, an

endothermic transition occurs that is not present in theDSC scan of the freshly gelatinized sample. Such atransition is generally attributed to the melting ofrecrystallized amylopectin. The enthalpy change involved,the enthalpy of retrogradation is generally considered tocorrespond to order-disorder transitions of crystallites, i.e.double helices present in extended order arrays and regionsof lesser crystalline order. The retrogradation properties ofstarches are indirectly influenced by the structural arrange-ment of starch chains within the amorphous and crystallineregions of the un-gelatinized granule, which in turn,influence the extent of granule breakdown during gelatini-zation and the interactions that occur between the starchchains during gel storage (Perera & Hoover, 1999). Thecrystallinity of starch granules is disrupted during chemicalmodification (Saroja, Shamala, & Tharanathan, 2000), andthis leads to a greater degree of separation between theouter branches of adjacent amylopectin chain clusters inmodified starches compared with those in native starches.Consequently, double-helix formation (during storage)between adjacent amylopectin chains of the modifiedstarches is much slower and less extensive owing to theintroduction of functional groups upon chemical modifica-tion. Retrogradation of starch has been reported to besuppressed by cross-linking using STMP and STPPmixture, as indicated by their lower enthalpy of retro-gradation after storage which may be because of therestricted mobility of cross-linked amylopectin branchesdue to the presence of phosphate groups (Chung et al.,2004). DSC heating of stored native starch gels has alsobeen reported to be more conspicuous than that ofhydroxypropylated phosphate cross-linked potato starch(HPS) dispersions as the retrogradation phenomenon ofHPS dispersions was barely observed by DSC measure-ments even when a high concentration of starch (33%) wasused (Morikawa & Nishinari, 2000a). Moreover, it hasbeen reported that the effect of phosphate cross-linking onstarch is more pronounced than that of hydroxypropyla-tion with respect to retrogradation. Yook et al. (1993)concluded that the tendency towards retrogradation wasgreatly reduced by both hydroxypropylation and cross-linking, and they showed a synergistic effect of thesetreatments in retarding the retrogradation of gelatinizedrice starch gels. Hydroxypropylated-crosslinked starchexhibited significantly higher onset and peak gelatinizationtemperatures, enthalpy of gelatinization and lower retro-gradation than crosslinked-hydroxypropylated starch, con-firming the previous assumption that the locations ofcross-links are different in the two kinds of modified starch(Yook et al., 1993). Hydroxypropylation followed by


cross-linking provides starch with better storage stability infood applications.

7. Rheological/pasting properties

Rheological properties of a material reflect its structure.During gelatinization, starch granules swell to several timestheir initial volume. Swelling is accompanied by leaching ofgranule constituents, predominantly amylose, and theformation of a three dimensional network (Eliasson,1985; Hennig, Lechert, & Goemann, 1976; Steeneken,1989; Tester & Morrison, 1990). These changes areresponsible for the rheological characteristics exhibited bystarch suspensions during heating and shearing. Rheologi-cal/pasting behaviour of starch is governed by amylosecontent, granule size distribution, granule volume fraction,granule shape, granule-granule interaction and continuousphase viscosity (Kaur et al., 2004; Morikawa & Nishinari,2002; Okechukwu & Rao, 1995; Singh & Kaur, 2004; Singhet al., 2003). Starch exhibits unique viscosity behaviourwith change of temperature, concentration and shear rate(Nurul, Azemi, & Manan, 1999). This can be measured interms of rheological/pasting curves (plots of viscosityversus temperature) obtained with the Brabender Viscoa-mylograph or the Rapid Viscosity Analyzer (RVA) andrheometer. Information obtained from rheology/pastingcurves is vital when considering a starch as a possiblecomponent of a food product (Adebowale & Lawal, 2003).The maximum viscosity at a given concentration reflectsthe ability of the granules to swell freely prior to theirphysical breakdown. Starches that are capable of swellingto a high degree are also less resistant to breakdown oncooking and hence exhibit significant viscosity decreasesafter the maximum viscosity is reached (Adebowale &Lawal, 2003). The increase in viscosity during the coolingperiod is indicative not of only the normal inverserelationship between the viscosity and temperature ofsuspensions but also of the tendency for various constitu-ents present in the hot paste (swollen granules, fragmentsof swollen granules, and colloidally dispersed and dissolvedstarch molecules) to associate or retrograde as thetemperature of the paste decreases.

Chemical modification leads to a considerable change inthe rheological and pasting properties of starches (Table 9).Starch paste viscosity can be increased or reduced byapplying a suitable chemical modification (Agboola,Akingbala, & Oguntimein, 1991). Again, modificationmethod, reaction conditions and starch source are thecritical factors that govern the rheological/pasting beha-viour of starch pastes (Gonzalez & Perez, 2002; Kim et al.,1993; Reddy & Seib, 1999; Singh, Kaur et al., 2004; Singh,Chawla et al., 2004; Woo & Seib, 1997; Yeh & Yeh, 1993).A significant variation in rheological/pasting properties hasbeen reported after acetylation of starch obtained fromdifferent sources. Betancur-Ancona et al. (1997) studied therheological properties of acetylated Canavalia ensiformis

(jack bean) starches and reported a substantial increase in

ARTICLE IN PRESS

Table

9

Pastingproperties

ofsomemodified

starches

from

differentbotanicalsources

Starchsource

Acetylated

Hydroxypropylated

Cross-linked

Peakviscosity

(RVU*)

Breakdown(R

VU)

Peakviscosity

(RVU)

Breakdown(R

VU)

Peakviscosity

(RVU)

Breakdown(R

VU)

Norm

almaize

ND**

ND

414–425(N

ative337)c

286–305(N

ative195)c

ND

ND

Dentmaize

592(N

ative535)y

a252(N

ative175)a

ND

ND

ND

ND

Waxymaize

695(N

ative627)a

352(N

ative387)a

ND

—ND

ND

Hi-amylose

maize

ND

ND

139–144(—

)c9–26(-)c

ND

ND

Waxymaize

ND

ND

350–379(N

ative391)c

213–227(N

ative243)c

ND

ND

Norm

alrice

259(N

ative233)b

146(N

ative136)b

ND

ND

229(N

ative233)b

55(N

ative136)b

Waxyrice

268(N

ative226)b

143(N

ative140)b

ND

ND

346(N

ative226)b

104(N

ative140)b

*RVU¼

Rapid

viscosity

units,where1RVU¼

E10mPas.

**¼

Notdetected,y¼

theproperties

ofcorrespondingnative(unmodified)starches

are

given

inbrackets.

aWilkinset

al.(2003).

bLiu

etal.(1999b).

cLiu

etal.(1999a).


the apparent viscosity upon acetylation. Control ofreaction conditions such as pH during acetylation mayallow reduction of secondary hydrolysis reactions andfacilitate the incorporation of acetyl groups. Such mod-ification enhances the water holding capacity of the starchmatrix and the development of more organized structures,leading to a higher resistance to deformation and thus ahigher peak viscosity can be achieved (Betancur-Anconaet al., 1997). A similar enhancement viscosity has beenreported by Sathe and Salunkhe (1981) for acetylatedPhaseolus vulgaris (haricot bean) starch. Differences inrelative reactivities are observed among native, partialwaxy and waxy starches during acetylation that may affecttheir rheological properties. The amylose/amylopectinratio was also considered to be the prime determinant ofthe change in the paste viscosity upon acetylation. Liu,Ramsden, and Corke (1997) studied the pasting behaviourof acetylated normal, waxy and high amylose maizestarches. They reported a considerable increase in pasteviscosities of high amylose and waxy maize starcheswhereas the increase was not significant for normal maizestarch, after acetylation. Unfortunately, no explanation inthis study is given for this odd order of increase inviscosities of the three types of starches. Another studyconducted by Gonzalez and Perez (2002) on commercialrice starch, reported an increase in pasting viscosity afteracetylation that can be explained through increased waterabsorption. Acetylation influences interactions betweenstarch chains by steric hindrance, altering starch hydro-philicity and hydrogen bonding and resulting in a lowergelatinization temperature and a greater swelling ofgranules, the latter resulting in an increased peak viscosity(Liu et al., 1997; Singh, Kaur et al., 2004; Singh, Chawlaet al., 2004). Adebowale and Lawal (2003) reported thatacetylation leads to a decrease in the RVA breakdown andsetback tendency of mucuna bean starch. The substituentgroups restrict the tendency of the starch molecules torealign after cooling, thus facilitating lower setback values(Betancur-Ancona et al., 1997). Similar results have beenreported for acetylated rice and maize starches (Gonzalez& Perez, 2002; Liu et al., 1997). The hot paste viscosity(HPV) and cold paste viscosity (CPV) of the acetylatedstarches have also been observed to be higher than in thecase of the unmodified starch. This may be attributed tophysical interaction between more swollen but weakergranules (Gonzalez & Perez, 2002). Maize hybrid, growthconditions and milling have also been reported to influencethe reaction efficiency and pasting properties of acetylatedwaxy maize starches (Wilkins et al., 2003).

The changes in viscosity of starches upon hydroxypro-pylation has been studied using the viscoamylograph andRVA (Hsu, Lu, & Huang, 2000; Jeong et al., 1993; Kim etal., 1992; Liu et al., 1999a, 1999b; Reddy & Seib, 1999; Shi& BeMiller, 2000; Tsai et al., 1997). All these studies havesuggested an increase in viscosity after hydroxypropyla-tion. Hydroxypropylation can influence interaction be-tween the starch chains through different possible

mechanisms: (1) by steric hindrance, which prevents theclose association of chains and restricts the formation ofinter-chain hydrogen bonds; and (2) by changing thehydrophilicity of the starch molecules and thus alteringbonding with water molecules. The observed effects ofhydroxypropylation are consistent with an overall reduc-tion in bonding between starch chains and a consequentincrease in the ease of hydration of the starch granule.Gelatinization can thus commence at a lower temperature,and greater swelling of the granule will lead to an increasedpeak viscosity (Liu et al., 1999a). Kim et al. (1992) reportedthat with increase in MS, the pasting temperature of potatohydroxypropylated starches decreased and their peakBrabender viscosity increased. Hoover et al. (1988)reported similar observations on hydroxypropylated peastarches. The peak viscosity of high amylose and normalstarches increased considerably while the waxy starchesshow a little effect on the viscosity upon hydroxypropyla-tion. The higher increase in viscosities of high amylose andnormal starches may be because of more hydroxyl groupsat their amorphous regions, which are mainly composed ofamylose. The extent of change in starch functionalproperties upon hydroxypropylation has also been ob-served to be influenced by the starch granule sizedistribution. Studies carried out by Hung and Morita(2005) on the pasting properties of hydroxypropylatedA- and B-type wheat starch granules showed that, onmodification, considerable increases in the peak, final andbreakdown viscosities occurred. The large A-type wheatgranules showed higher peak, final and breakdownviscosities than did small B-type starch granules.Cross-linking has been reported to increase the shear

stability, viscosity and pasting temperature of waxy ricestarch, and to decrease pasting temperature of normal ricestarch (Liu et al., 1999b). The greater strength of the cross-linked granule limits the breakdown of viscosity undershear, giving a higher HPV, and persistence, or resistanceto breakdown, of the swollen granules and on cooling,results in a higher CPV. Cross-linked waxy maize starchpastes are more viscous and heavy bodied, and are lesslikely to breakdown with extended cooking times, in-creased acidity or severe agitation than their unmodifiedcounterparts (Langan, 1986). Hirsch and Kokini (2002)studied the relative effects of different cross-linking agentson the physical properties of starches and reported thatPOCl3 has the ability to impart a greater viscosity thanother agents. POCl3-treated granules have a more rigidexternal surface than STMP- and EPI- treated granulesowing to the concentration of cross-links at the surface ofthe granule. Cross-linking leads to a higher increase in thepeak viscosity of waxy starches as more amylopectin thanamylose molecules have been reported to become cross-linked (Jane et al., 1992; Liu et al., 1999b).Dynamic rheometry allows continuous measurement of

dynamic moduli during temperature and frequency sweeptesting of a starch suspension. The dynamic storagemodulus (G0) is a measure of the energy stored in the


material and recovered from it per cycle of sinusoidaldeformation while the loss modulus (G00) is a measure ofthe energy dissipated or lost per cycle (Ferry, 1980). Theratio of the energy lost to the energy stored (G00/G0) for eachcycle can be defined as tan d, the loss tangent d is the phasedifference between the applied sinusoidal strain and theresulting sinusoidal stress (Singh & Singh, 2001; Singhet al., 2003). The G0 of starch progressively increases to amaximum (peak G0) at a certain temperature and thendrops with continued heating in a dynamic rheometer. Theinitial increase in G0can be attributed to granule swelling.Granules may swell to fill the entire available volume of thesystem (Eliasson, 1986), and intergranular contact mightthen result in the formation of a three-dimensional networkof swollen granules (Evans & Haisman, 1979; Wong &Lelievre, 1981). With further increases in temperature, G0

decreases, indicating that the gel structure is destroyed(Tsai et al., 1997). This destruction is due to the melting ofcrystalline regions remaining in the swollen granules,allowing the granules to deform (Eliasson, 1986).

The rheological properties of modified starches exhibitsignificant differences from those of native starches whensubjected to temperature sweep testing using heating andcooling cycles on a dynamic rheometer (Kaur et al., 2004;Singh, Chawla et al., 2004). The parameters such as G0 andG00 of acetylated, hydroxypropylated and cross-linkedstarches from different sources increase to a maximumand then drop during heating, confirming that thesestarches follow the same general rheological pattern asnative starches. The temperature of maximum G0 dropssignificantly on acetylation or hydroxypropylation, while itincreases after cross-linking (Kaur et al., 2004; Kaur et al.,2005a; Singh, Chawla et al., 2004). Changes may beexplained on the same basis as for the changes in thermaland pasting properties caused by modification. Acetylationof maize and potato starches results in increased G0 and G00

maxima and a decreased tan d maximum. Acetylatedstarches with higher DS exhibit a correspondingly higherincrease in G0 and G00 maxima during heating. Thesechanges occur for the same reasons that acetylation causesan increase in peak pasting viscosity (see above) (Betancur-Ancona et al., 1997; Singh, Kaur et al., 2004).

Acetylated maize and potato starch gels showed slightlylower G0 and G00 as compared with their native starch gels,during cooling cycle of heated starch pastes on therheometer. This confirms the lowered tendency of thesemodified starches to retrograde (Betancur-Ancona et al.,1997; Singh, Kaur et al., 2004). Similarly, increases in thepeak G0 and G00 of hydroxypropylated starches duringheating occurs, owing to the decrease in associative forceswithin the starch granules caused by the introduction ofhydroxypropyl groups; this introduction results in greaterwater penetration and swelling and a consequent increasein G0. Hydroxypropylated potato starches with a higherMS and large size granules exhibit higher peak G0 and G00

during heating than do the native starches (Kaur et al.,2004). Cross-linking of starches leads to a significant

alteration in their dynamic rheological behaviour. Cross-linking leads to an increase in the peak viscosity of bothnormal and waxy starches (Acquarone & Rao, 2003; Liuet al., 1999a, 1999b). In non-waxy starch, molecules insidegranules are more entangled owing to the presence ofunleached-out amylose and hot cross-linked waxy starchwith adequate granular rigidity can form a gel networkfrom swollen granules alone (Hoover & Hadziyev, 1981;Tsai et al. (1997). Eliasson, Finstad, and Ljunger (1988)observed a more solid like behaviour, higher G0, on the partof cross-linked waxy maize starch as compared to theirnative counterparts. Cross-linked waxy starch pastes hadgreater resistance to cooking shear, temperature, and lowpH compared with native and cross-linked partially waxystarches (Bertolini et al., 2003; Whistler & BeMiller, 1997).A fairly high degree of cross-linking results in a lower peakG0, owing to the lower degree of swelling and consequentlower degree of intergranular interaction. Starches treatedwith a relatively low concentration of POCl3 showed ahigher maximum G0, and lower tan d maximum, than theircounterpart native starches, whereas those treated withhigher concentrations showed an opposite effect (Kauret al., 2005a). Strengthening bonding between starch chainsby cross-linking will increase the resistance of the granuletowards swelling, leading to lower paste viscosity, whichsuggests that the concentration of the cross-linking reagentaffects the structure within the granule, perhaps byaffecting the distribution of the introduced cross-links.Cross-link location has also been reported to have variedeffects on different properties of cross-linked starches(Muhrbeck & Eliasson, 1991; Muhrbeck & Tellier, 1991;Yoneya et al., 2003). The extent of change in therheological properties upon cross-linking also variessignificantly for starches from different sources dependingon the granule size distribution. Potato cultivars with ahigher percentage of large size irregular starch granulesshowed a higher susceptibility towards cross-linking andexhibited greater changes in their functional propertiesupon cross-linking (Kaur et al., 2005a). These character-istics could also be explained on the basis of differences inthe amylose to amylopectin ratio, amylopectin branchchain length, degree of crystallinity between larger andsmaller granules (Noda et al., 2005; Singh & Kaur, 2004).Dual-modification results in starch pastes with higher

peak viscosity and greater stability than those of nativestarch pastes (Wu & Seib, 1990). However, the effect ofdual-modification depends upon the preparation proce-dure. Cross-linking followed by hydroxypropylation hasbeen reported to yield starches that are more shear andheat stable than the native starch. This may be due to thestructural change in the granules after the first modification(cross-linking). Cross-linking reduces the degree of sub-sequent hydroxypropylation, but prior hydroxypropyla-tion increases the degree of subsequent cross-linking.Moreover, the cross-linked and then hydroxypropylated(XL-HP) starch exhibited lower pasting temperature andviscosity than the hydroxypropylated and then cross-linked

ARTICLE IN PRESSJ. Singh et al. / Food Hydrocolloids 21 (2007) 1–22 17

(HP-XL) starch samples, suggesting the locations of thecross-links in the two types of starch are different. Thecross-links in XL-HP starch were also found to be moreresistant to attack by enzymes and chemicals (Reddy &Seib, 2000). The reactivity of the modifying agent duringdual modification may vary towards different starchsources. The cross-linked hydroxypropylated waxy wheatstarch gave pasting curves showing higher viscosities thanthose of cross-linked hydroxypropylated waxy corn starch(Reddy & Seib, 2000). Therefore, by appropriate choice ofthe native starch source (potato, maize, wheat etc.) and ofthe type of chemical modification, concentration of themodifying reagent, modified starches with very usefulrheological properties can be obtained (Dubois, Picton,Muller, Audibert-Hayet, & Doublier, 2001; Kaur et al.,2004; Singh, Kaur et al., 2004; Singh, Chawla et al., 2004).

8. Nutritional and toxicological aspects

Many modified starches made for food use contain onlysmall amounts of substituent groups and have been used assafe food ingredients. During acetylation and hydroxypro-pylation of food grade starches, the level of monosubstitu-tion groups introduced is relatively low. The maximumpermitted levels of substitution for starch acetates, starchphosphates and hydroxypropylated starches are 2.5, 0.4and 10% respectively (FAC (Food additives and con-taminants committee report on modified starches), 1980).Similarly, cross-linked food starches containing one sub-stituent cross-linking group per 1000 or more anhydroglu-cose units are considered safe (Wurzburg, 1986a, 1986b).The legislative approval for the use of novel starchderivatives in processed food formulations is still underdebate, but several tailor-made starch derivatives withmultiple modifications are being prepared and character-ized (Tharanathan, 2005). Some of the starch derivativesare being increasingly used as fat replacers or fatsubstitutes. These derivatives are either partially or totallyundigested, therefore contributes zero calories to the foodon consumption (Tharanathan, 1995). Many studies havesuggested that the physiological effects of chemicallymodified starches are affected by the type of modification(Ebihara, Shiraishi, & Okuma, 1998). The chemicalmodification of starch by acetylation improves the satiat-ing, glycemic and insulinemic properties of the meal(Raben, Andersen, Karberg, Holst, & Astrup, 1997).Phosphorylated/cross-linked starches are slowly digestibleand are thought to provide nutritional benefits for humans(Sang & Seib, 2006). Also the slowly digesting modifiedstarches could be used for the treatment of certain medicalmodalities (e.g. glycogen storage disease and diabetesmellitus) (Wolf, Bauer, & Fahey, 1999).

9. Conclusion

Progress in understanding the high value of chemicallymodified starches has encouraged the starch industry to

produce modified starches using different modificationreagents and starch sources. Some factors such as starchcomposition, concentration and type of reagent, andreaction conditions may affect the reactivity of starchduring chemical modifications like acetylation, hydroxy-propylation and cross-linking. The heterogeneity of thegranule population within a single starch source may alsoaffect the extent of modification. The changes observed inthe physico-chemical, morphological, thermal and rheolo-gical properties of the starches after modification mayprovide a crucial basis for understanding the efficiency ofthe starch modification process at industrial scale.

Acknowledgements

This work was supported by the Foundation forResearch Science and Technology, New Zealand (Contract# MAUX 0402). The authors are greatly indebted to Prof.Harjinder Singh, Director, Riddet Centre for usefuldiscussions.

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