Avebe Starch Brochure

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Contents Page1 Introduction 42 Sources of starches 5Biosynthesis of starch 5Occurrence and manufacture of starches 6Varieties of starch 83 Composition and properties of starches 10Chemical composition of starch granules 10Amylose and amylopectin 12Structure and organization of starch granules 14Swelling and gelatinization 15Starch pastes 19Retrogradation 21Starch films 234 Starch modifications 24General aspects 24Pregelatinized starches 25Low-viscosity starches 26Crosslinked starches 30Stabilized starches (esters and ethers) 32Combination of treatments 37Starch sugars 385 Applications of starches 41General aspects 41Foods 42Paper 44Adhesives 46Textiles 47Miscellaneous uses 48


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1 IntroductionStarch is a polymeric carbohydrate composed ofanhydroglucose units and is extracted in granularform from the organs of certain plants. The word"starch" may be derived from the Anglo-Saxon"stearc" and has the meaning of strength orstiffness. Starch granules are deposited in theseeds, tubers, roots and stem piths of plants, as areserve food supply for periods of dormancy,germination and growth. The microscope revealsthat starch is composed of tiny, white granules,ranging from about 1 to 100 microns (= micrometer)in diameter. Size and shape of the granules arepeculiar to each variety of starch.Figure 1 shows schematically the appearance ofvarious starches. After cellulose, starch is the nextmost abundant compound synthesized by plantcells. It is a "renewable" substance; a new supply ofstarch is grown annually.Figure 1:Microscopic appearance of various starches

Starch is not a uniform material. Most starchescontain two types of glucose polymers: (1) a linearchain molecule termed amylose and (2) a branchedpolymer of glucose termed amylopectin. These twofractions occur in differing amounts in starches fromvarious botanical sources. Amylose comprises15-30 % of the common starches.Starch granules are insoluble in water below 50C.When a suspension of starch in water is heatedbeyond a critical temperature, the granules absorbwater and swell to many times their original size.The critical temperature at which this occurs, isknown as the pasting or gelatinization temperature(about 55 to 80 DC ; depending on the type of starch).When heating is continued, the swollen starchgranules begin to desintegrate into swollen starchaggregates. The viscous mass, resulting from the

o 0 @ ) 9@ ( 00 0 Q 0 0 0000 '@ t 00 00 oD 0 0: O g O o 0 0 000000 a .0 0 0 a0000 E P 000 0 0Maize Potato Wheat Tapioca RiceThe commercial sources of starch are the seeds ofcereal grains (maize, wheat, sorghum, rice), tubers(potato), roots (tapioca, sweet potato, arrowroot)and the pith of the sago palm. Each starch isdescribed according to its plant source as maizestarch, potato starch, tapioca starch, wheat starch,etc. The properties of the starch vary with the plantsource from which it is derived. The starch industryuses a combination of grinding and wet purificationtechniques to manufacture starch with a purity ofabout 98-99.5 %. In the manufacturing process,starch is separated from the other constituents of themilled raw material such as fibres, proteins, sugarsand salts.Starch can be considered to be a condensationpolymer of glucose. The glucose units in the starchpolymer are present as anhydroglucose units(AGU). If starch is treated with acids or certainenzymes, it is broken down into its constituentglucose molecules. Similarly, glucose is generatedwhen starch is used as a food by animal and man.

swelling and the colloidal dispersion of starch in anaqueous medium, is termed starch paste. Theprocess, involving the transformation of a starchinto a starch paste, is termed gelatinization. Truesolubilization of all the starch molecules occurswhen the paste is cooked at temperatures of 100-160C. When cooked starch pastes are allowed tostand, the phenomenon known as retrogradation(synonym: set-back) can take place and this ismanifested in the formation of a gel or precipitate.Native starch can be modified by physical, chemicaland/or enzyme treatment to alter its properties or toimpart new ones.The ability of starch products to produce a viscouspaste when heated in water is its most importantpractical property. The hydrocolloidal properties ofstarch make it suitable for a great variety ofapplications. Starch and its derivatives are widelyused in the manufacture of foods, paper, textiles,adhesives, pharmaceuticals and buildingmaterials.


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2Sources of starchesBiosynthesis of starchGreen leaves of plants contain chlorophyll, that isable to absorb light quanta and utilize the energy tocatalyze the formation of glucose and oxygen fromcarbon dioxide and water. This process is known asphotosynthesis and may be written as follows(simplified):

lightchlorophyll

water + carbon dioxide _----.. glucose + oxygen

Starch is formed in the leaves of plants by conden-sation polymerization of glucose with the aid ofstarch synthesizing enzymes. This process may bewritten as follows (simplified):

enzymes glucose starch + waterDuring active photosynthesis (during the day) thestarch is accumulated in the leaves in the form of tinygranules of about 1 micron in diameter (leaf ortransistory starch). During the night, this leaf starchis partly broken down by enzymes and transported inthe form of sugars (mainly sucrose) to other parts ofthe plant. Some of these sugars are reconverted intostarch in the seeds, tubers and roots of variousplants (storage starch). It isfrom these sources thatthe commercial starch is obtained.Starch molecules are synthesized in plants fromsugars. The true mechanism for the biosynthesis ofamylose and amylopectin is not entirely clear. Theenzymes phosphorylase (P-enzyme), starchsynthetase and a branching enzyme (Q-enzyme)are, or may be, involved in starch biosynthesis.Nowadays most starch scientists believe that starchsynthetase isthe true chain-lengthening enzyme innormal starch biosynthesis. The branching enzymeis responsible for the synthesis of the branchingpoints in the amylopectin molecules. The way inwhich amylose escapes from branching in theobvious presence of branching enzyme is still aquestion.Development of starch granules commences withthe accumulation of poorly organized material ofunknown chemical composition. At a certain mo-ment there is the deposition of a minute amount ofinsoluble polysaccharide, which acts as a nucleus

for further starch deposition. This nucleus is thebotanical center (hilum) of the granule, aroundwhich the granule is grown. Initial growth givesnearly spherical granules. As the granules areenlarged they often become elongated or flattened.The starch molecular chains grow in an orientationperpendicular to the growing surface of the starchgranule. As the dissolved glucose units are linked tothe growing starch polymer, they simultaneouslysolidify. During the growing of the starch granulethere is an increase in the proportion of amylose andan increase in molecular size of both amylose andamylopectin.


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6 Chapter 2

Occurrence and manufacture ofstarchesStarch occurs in practically every type of tissue ofgreen plants: leaves, roots, tubers, seeds and fruits.Storage of starch takes place in the undergroundorgans of various plants, for example potato, sweetpotato, tapioca, arrowroot and canna. The trunk ofthe sago palm becomes filled with a starch bearingpith during its growth. The seeds of many plantscontain starch as a reserve nutrient, for examplegrasses, rice, wheat, maize, sorghum, barley andoat. Starch constitutes the major portion of thecarbohydrates of legume seeds (peas, beans,lentils). Starch occurs also as a component of manyfruits, for example unripe apples, bananas andgreen tomatoes.Certafn mutants of maize, rice and sorghum arecultivated for their special starch characteristics. Thewaxy cereals (waxy maize, waxy sorghum, waxyrice) contain starch with no amylose fraction (100 %amylopectin). Plant breeders have developedvarieties of maize with 50-80 % amylose in thestarch granules (amylomaize).Despite the great variety and wide distribution ofstarch in nature, the number of major sources forindustrial production of starch is comparativelysmall. The major sources of commercial starch are:maize, potato, tapioca and wheat. Smaller quantitiesof starch are also produced from waxy maize,sorghum, waxy sorghum, rice, sago, arrowroot,sweet potato and mung beans.The composition of the raw materials varies accor-ding to such factors as age, soil, variety and climate.A typical analysis of the various raw materials isshown in Table 1.Table 1:Composition of raw mater ials (in % by weight)

In addition to the components mentioned in Table 1,the raw materials contain other compounds such assugars, salts, acids and pentosans. Figure 2 showsthe composition of potatoes.Figure 2:Composition of the potato (in percent by weight)

( : : " " ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' 1 water 78 %r : : : : : : ; : : : : : ; ] protein 1 %!III] amino acids + amides+ sugars + organic acids+ salts 3 %

c : : : : = J sta rc h 17 %I> } > I fibres 1 %

(dissolved in potato juice)

(dissolved in potato ju ice)(solid component)(solid component)

The potato starch manufacturing process is simple(see Fig. 3). The potatoes are washed and groundby a rasping machine. The rasped potato is thenpassed through rotating sieves. The fibres areretained and are discharged as potato pulp (potatofibres). The remaining starch slurry contains solublecompounds (sugars, proteins, acids, salts) and finefibres. These are separated by further treatmentthrough continuous centrifugal separators orhydrocyclones and fine sieves. The purified starchslurry is used for the production of potato starch

Source Starch Moisture Protein as Lipids Fibre Starch onN x6.251) dry substance

Potato 17 78 2 0.1 77Maize 60 16 9 4 2 71Wheat 64 14 13 2 3 74Tapioca 26 66 0.3 1 77Waxy Maize 57 20 11 5 2 711. N = Nitrogen content

The roots and tubers differ from the cereals in thatthey have a much higher moisture content, but alower lipid content. The starch content, calculated ona dry basis, is for all raw materials about 70-80 %.

derivatives or is dewatered and dried. Themanufacture of the root starches (tapioca, sweetpotato, arrowroot) is similar to the manufacture ofpotato starch.


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Sources of starches 7

Figure 3: Potato starch manufacturing process

Potato Potato Wet Cleansupply juice fibres waterIt t t ashing Grinding Juice Fibre Starch Drying~ ~ . . . ~ refining _ .equipment equipment extraction separation equipment equipment iStarch


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8 Chapter 2

Varieties of starchIrrespective of the source, all starches occur innature as minute granules, each having its inherentcharacteristics, size and shape. The source of astarch can be identified from its microscopicappearance (see Figure 1). Table 2 shows the sizeand shape of starches of different origin.Table 2: Starch Granule Properties

Potato starch granules are oval in shape, withpronounced oystershell-like striations around aneccentrically placed hilum (see Figure 4).Potato starch has the largest granules of anycommercial starch. Figure 5 gives the particle sizedistribution (by number and by weight) of potatostarch (schematic).

Shapeize; Size; Size;diameter; diameter; diameter;range number weight(micron) average average

(micron) (micron)5-100 28 402-30 10 151-45 8 254-35 15 253-26 10 15

Starch Type

Potato tuberMaize cerealWheat cerealTapioca rootWaxy Maize cereal

oval, sphericalround, polygonalround, lenticularoval, truncatedround, polygonal

The commercial starches can be divided into threegroups. The first group comprises the tuber (potato),root (tapioca, arrowroot, sweet potato) and pith(sago) starches. The second group comprises thecommon cereal starches (maize, wheat, sorghum,rice). These two groups are distinctly different fromeach other with respect to chemical composition andphysical properties. The third group comprises thewaxy starches (waxy maize, waxy sorghum, waxyrice). These starches are obtained from cereals, butthe physical properties of the waxy starches aresimilar to those of tapioca starch.Potato starch(Synonym: farina)

Figure 4:Potato starch (polarized light)

Figure 5:Size distribution of potato starch granules

/ distribution by weight. .

o 10 20 30 40 50 60 70 80- Diameter of starch granule (micrometer)

Special strains of potato have been developed,which give a high starch yield. About 3 % of theworld crop of potatoes are used for the productionof potato starch. The world production of potatostarch is approximately 2 mil lion tons, of which500.000 tons are produced in the Netherlands.Potato starch products are a.o. used in themanufacture of food and feed products, paper,textiles, for the production of adhesives and asspecial additives in drilling muds.


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Sources of starches 9

Maize starch(Synonyms: corn starch; regular corn starch; in theUSA and some other countries the term corn starchis commonly used)Maize starch granules are medium-sized and roundor polygonal in shape. One gram of maize starchcontains about 1,000,000,000 granules. One starchgranule contains about 10,000,000,000,000 starchmolecules. The specific area of maize starch isabout 300 m2/kg. Only about 6 % of the annual worldmaize crop is used for the manufacture of starch.The world production of maize starch is about 16million tons, of which about 8 million tons areproduced in the United States. Maize starch ac-counts for over 75 % of the total world production ofstarch. About 70 % of the produced maize starch isconverted into glucose syrup, glucose-fructosesyrup and dextrose. Large quantities of maize starchare also used in the production of corrugated boardand paper.Wheat starchWheat starch consists of two types of granules: thesmaller spherical granules (1-10 micron) and thelarger, lenticular granules (20-45 micron). The largestarch granules in wheat flour average about 10% ofthe total granule number while accounting for about90 % of the starch granule weight.Wheat starch is produced in many countries as a by-product in the manufacture of wheat gluten. Only 0.4% of the world crop of wheat is processed to starchand gluten. The world production of wheat starch isabout 1 million tons. Wheat starch is used in thebaking industry, in the production of adhesives andfor conversion into starch sugars.Tapioca starch(Synonyms: cassava starch; manioc starch)Tapioca starch granules are round or truncated atone end to form kettle drum shapes. Tapioca starchis manufactured from the root of a tropical plantcalled cassava, tapioca or manioc. The worldproduction of tapioca starch is about 1 million tons. Itis produced in Thailand, Brazil, the Philippines,Nigeria, Malaysia and Angola. Tapioca starch is atypical root starch and is used in the production offood products and adhesives.

Waxy maize starch(Synonyms: waxy corn starch; amioca)Maize starch and waxy maize starch are identicalunder the microscope. Waxy maize starch isproduced from waxy maize, a special botanical typeof maize. To prevent cross-pollution with regularmaize, waxy maize must be grown in relativelyisolated fields.Starch from waxy maize consists solely ofamylopectin. It is produced mainly in the UnitedStates. The rheological properties of waxy maizestarch resemble most closely those of tapiocastarch. Waxy maize starch products are used in themanufacture of adhesives and as thickeners invarious food products.


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3 Composition and properties of starchesChemical composition of starchgranulesStarch granules contain usually 10-20 % moistureand small amounts of proteins, lipids and traces ofinorganic materials. Table 3 gives the averagechemical composition of various commercialstarches.Table 3:Average Chemical Composition of Starch Granules

of palmitic, linoleic and oleic acid.The presence of lipids in the common cerealstarches has a profound effect on the physicalproperties of these starches. The lipids exist as anamylose-lipid inclusion complex inthe granules.The linear fraction of the starch molecules(amylose) forms helical clathrates with polar fattysubstances, such as the higher fatty acids. The

Starch Moisture at Lipids % Proteins % Phosphorus Amount of taste and65%RH1) on d.s." on c.s. % on d.s. odour substancesand 20C N3) x 6,25 (relative)

Potato 19 0.1 0.1 0.08 lowMaize 13 0.8 0.35 0.02 highWheat 13 0.9 0.4 0.06 highTapioca 13 0.1 0.1 0.01 very lowWaxy Maize 13 0.2 0.25 0.01 medium1. RH =Relative Humidity of the atmosphere2. d.s. = Dry substance3. N = Nitrogen content

MoistureThe moisture content of starch products depends onthe relative humidity (RH) of the atmosphere inwhich they have been stored. If this humiditydecreases, the starches will give up moisture; if theRH increases, they will absorb moisture. Theequilibrium moisture content of starch is alsodependent on the type of starch product. Undernormal atmospheric conditions, most commercialnative starches contain 10 to 20 % moisture. Theequilibrium moisture content of all starches is low ata low relative humidity of the atmosphere. At a RH ofzero, the moisture content of the starches nears tozero. At a RH of 20 %, the moisture content of allstarches is about 5-6 %.Lipids(fatty substances)The tuber (potato) and root (tapioca) starchescontain only a very small percentage of l ipids (about0.1 %), compared with the common cereal starches(maize, wheat, rice, sorghum), which contain 0.8-1.0% l ipids. The fatty substances in the cereal starchesare predominantly free fatty acids (in maize andwaxy maize starch) or phospholipids (in wheatstarch). The free fatty acids consist mainly

amylose-lipid complexes are insoluble, butdissociate when heated in water above a giventemperature. The dissociation temperature isindicative of the strength of bonding and depends onthe type of complexing agent. The amylose-lipidcomplexes tend to repress the swell ing andsolubilization of the cereal starch granules. Elevatedtemperatures (above 125C) are required to disruptthe organized native amylose-lipid structure in thecereal starch granules and to solubilize the amylosefraction. The presence of fatty substances cancreate problems in the use of maize and wheatstarch products, because of the tendency to becomerancid on storage.ProteinsThe amount of proteins as shown in Table 3(calculated as N x 6.25) includes real proteins, butalso peptides, amides, amino acids, nucleic acidsand enzymes which may be present in the starchgranules. The tuber (potato) and root (tapioca)starches contain only a small amount of proteins(about 0.1 %), compared with the cereal starches(maize, wheat, waxy maize), which contain0.2-0.4 % proteins. Because of the residual protein,the cereal starches may have a mealy flavour andodour and also a tendency to foam. The small


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Composition and properties of starches 11

granules of wheat starch contain much more protein(1.6 %) than the large granules (0.1 %).PhosphorusThe phosphorus in the cereal starches is mainlypresent as phospholipids. The root starches(tapioca) contain a very low amount of phosphoruscompounds. Potato starch is the only commercialstarch which contains an appreciable amount ofchemically bound phosphate ester groups. Theester phosphate groups are bound to the C-6position of glucose-units of the amylopectin mole-cules in potato starch (see Figure 6).Figure 6:Ester phosphate groups in potato starch and monostarch phosphate

~COH

- +ONaI

H:2C-O-P=OIOH.J--- 0

n

~COH

m

The amount of phosphate groups in potato starchranges from 1 phosphate group per 200 to 400glucose units. This corresponds with a degree ofsubstitution (DS) of about 0.003 to 0.005. Thephosphate substituent confers the properties of apolyelectrolyte on potato starch amylopectin whendispersed into aqueous solutions. The mutualrepulsion of the charged groups forces the moleculeto expand. The phosphate groups can be conside-red as ion exchanging groups.Flavour and odour substancesThe pregelatinized common cereal starches (maize,wheat) have a relatively raw cereal flavour. Thesestarches impart cereal-type flavours to the foods inwhich they are incorporated. Potato and tapiocastarches contain only a low amount of flavoursubstances and this may be due to their low lipid andprotein content.


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12 Chapter 3

Amylose and amylopecti nStarch is a carbohydrate, composed of carbon,hydrogen and oxygen atoms in the ratio 6 : 10: 5,(C6H1Ps)n Starch can be considered tobe a condensation polymer of glucose, consisting ofanhydroglucose units. The glucose units are linkedto one another through the C-1 oxygen, known asglucoside bond. The glucoside linkage is stableunder alkaline conditions and hydrolyzable underacid conditions. The glucose unit at the end of thepolymeric chain has a latent aldehyde group and isknown as the reducing end group. Most starches area mixture of amylose and amylopectin, each havinga wide range of molecular sizes. Starches ofdifferent origin have different amylose toamylopectin ratios, as Table 4 shows. Table 4 showsalso the average degree of polymerization (DP) ofboth fractions in various starches.Table 4: Amylose and Amylopectin contents and Degreeof Polymerization (OP) of various starchesStarch Amy- Amylo- Average DP Average DP

lose pectin amylose amylopectinPotato 21 79 4000 2,000,000Maize 28 72 1000 2,000,000Wheat 26 74 1000 2,000,000Tapioca 17 83 4000 2,000,000Waxy Maize 0 100 2,000,000

AmyloseAmylose is a linear polymer containing up to 6000glucose units, connected by 1,4 linkages (seeFigure 7).Enzyme studies indicate perhaps a trace amount ofbranching in the amylose molecules or in a portionthereof. The ratio amylose: amylopectin is fairlyconstant for a given species of starch. Maize andsorghum starch have a much higher amyloseFigure 7:Linear chain structure of amylose molecules

content (about 28 %), compared with the tuber androot starches (potato, tapioca, arrow-root), whichcontain only about 20 % amylose. The waxy star-ches contain no amylose fraction. Amylo-maizestarch may contain up to 80 % amylose.Amylose covers a range of degrees ofpolymerization, depending upon the source of thestarch. The amylose molecules of potato andtapioca starch have a substantially higher molecularweight than maize and wheat starch amylose. Theamylose fraction of potato starch has a degree ofpolymerization (DP) ranging from 840 to 22,000glucose units. The amylose fraction of maize starchhas a DP-range of about 400 to 15,000 glucoseunits.Amylose forms inclusion complexes with iodineand various organic compounds such as butanol,fatty acids, various surfactants, phenols andhydrocarbons. These complexes are essentiallyinsoluble in water. It is believed that amylosecomplexes by forming a helix coil around thecomplexing agent. The complex of amylose withiodine gives a characteristic blue colour, which isused to establish the presence ofamylose-containing starch.Amylopectin

Amylopectin has a highly branched structure,consisting of short linear chains with a DP rangingfrom 10 to 60 glucose units. The average DP ofthese chains is about 22. They are connected toeach other by alpha-1 ,6-linkages (see Figure 8).Figure 8: Structure of amylopectin branching points

CH,OH CH,OH CH,OH0 0 0

H H H , H, ,0 0

H OH H OH H OH,,, _ _ : N- -


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The glucose units with an alpha-1 ,6-linkage are thebranching points of the amylopectin molecule andmake up about 5 % of the total glucose units inamylopectin. The average length of the outerchains of amylopectin before reaching a branchpoint is about 14 glucose units. The average chainlength of the inner branches is about 8 glucoseunits. A portion of the branches are separated byonly one glucose unit. This suggests the presenceof regions of dense branching. Probablyamylopectin has a cluster-type structure. Thebranch points are arranged in tiers of some kind(see Figure 9).Figure 9: Model of amylopectin molecule.R = reducing end-group

Amylopectin is one of the largest molecules innature with an average degree of polymerization ofabout 2 million (corresponding with an averagemolecular weight of about 400 million). Apparently,there are no substantial differences between theaverage molecular weight of the amylopectinmolecules of various kinds of starch (see Table 4).The molecular weight of amylopectin is about 1000times as high as the molecular weight of amylose.Glucose unitThe glucose units of the starch molecules containa primary hydroxyl group on carbon-6 and asecondary hydroxyl group on carbon-2 andcarbon-3 (see Figure 10).Figure 10: Glucose unit

o

Starch molecules have a multitude of hydroxylgroups, which impart hydrophilic properties to thestarch and lead to the dispersibility of starch onheating with water. However, these hydroxylgroups also tend to attract each other, forminghydrogen bonds between adjacent starchmolecules and thus preventing the dissolution ofstarch granules in cold water.


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14 Chapter 3

Structure and organization ofstarch granulesViewed under polarized light, the starch granulesusually show a strong interference cross (maltesecross), centered through the hilum. This suggestsa radial organization of some sort. Thecharacteristic X-ray diffraction patterns of thestarch granules prove that there are areas ofcrystallinity in the starch granules.The starch molecules are arranged in the granulesin a radial fashion toward the hilum (see Figure 11).Figure 11:Micellar organization within starch granules

Whenever linear segments of the starch moleculesparallel one another, hydrogen bonding forces pullthe chains together into associated crystallinebundles or micelles (see Figure 12).Figure 12:Micelle within the starch granule

It appears that crystallinity and structural integrity ofthe starch granule are essentially due to theamylopectin component. The large amylopectinmolecule participates in both the crystalline micellesas well as in the less organized regions.The starch granules are organized into more or lesscrystall ine regions and amorphous regions, thetransition between these regions is a gradual one.

The regions of micellar crystallinity hold thegranules together. Because of their radialorientation, they are responsible for both theoptical polarization and the X-ray spectrum. Theorientation of the crystalline micelles perpendicularto the granule surface implies that the molecularaxes of the starch molecules are also arranged inthis fashion.Figure 13 shows a schematic model of theorganization of a starch granule.Figure 13: Schematic model.oi the structure of a starchgranule

The amorphous regions are those where chainfolding or multiple branching occur, preventing theformation of ordered polymer structures.The areas of crystallinity in the various nativestarches comprise about 25-50 % of the totalvolume of the starch granules. In the tuber androot starches, solely the amylopectin moleculesconstitute the crystal line structure. The amylose inthese starches is present in the amorphous stateand can be readily leached out preferentially fromthe granule. In the cereal starches, theamylopectin fraction is the most important elementof the crystalline structure.A portion of the amylose molecules in the commoncereal starches is present as a complex with lipidmolecules. This complex forms a weak crystallinestructure and could be involved in the structuralnetwork of the granules. The amylose-lipidcomplex provides some reinforcement of thecereal starch granules that could retard granuleswelling.


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Swelling and gelatinizationGeneral observationsNative starches are insoluble in water below theirgelatinization temperature. This is a very importantproperty, which enables an easy extraction of thestarch granules from their plant source in aqueoussystems. In addition, the native starches can bechemically modified in suspension in water andrecovered in purified form by filtration, washingwith water and drying.Starch granules are insoluble in cold water due tothe hydrogen bonds, formed either directly vianeighbouring alcoholic OH-groups of the individualstarch molecules or indirectly via water bridges.The hydrogen bonding forces are weak, but thereare so many hydrogen bonds in a starch granulethat it does not dissolve in cold water.When starch granules are heated in water toprogressively higher temperatures, a point isreached where the polarization cross starts to fadeat the hilum, and this phenomenon rapidly extendsto the periphery of the granule. Simultaneously, thegranule starts to swell irreversibly. The term"gelatinization" is applied to this loss of polarizationand concurrent initiation of swelling. The initialswelling takes place in the amorphous regions ofthe granule, disrupting the weak bonding betweenthe starch molecules and hydrating them. Thetangential swell ing disrupts the orderly radialorganization of the micelles and the granule losesits polarization. As the temperature of the aqueousstarch mixture rises, more hydration occurs in theamorphous regions and the hydrogen bonds in thecrystalline regions begin to be disrupted. Thegranules continue to expand to a greatly swollenreticulated network, still held together by persistantmicelles which have not been disrupted (seeFigure 14).

A portion of the amylose molecules leach out intothe aqueous substrate. The viscosity increases toa maximum that corresponds to the largesthydrated swollen volume of the granules.Extensive swelling is associated with disruption ofthe crystalline areas. In the first stages ofgelatinization the shorter micelles dissociate, whilethe longer micelles will persist to highertemperatures. As the heating and agitation of themixture continues, the swollen starch granulesbegin to rupture and collapse, yielding a viscouscolloidal dispersion of swollen granule fragments,hydrated starch aggregates and dissolvedmolecules. Figure 15 gives an impression of theswelling, disruption and colloidal dispersion of astarch granule during gelatinization.

Figure 14:Micellar organization within swollen starchgranules

Figure 15: Swelling, disruption and dispersion of a starchgranule during gelatinization

D - pasting temperature

0))peakviscosity ()Oo complete

dispersion

oo

I60 100

Teperature roC)

Upon raising the temperature of a starchsuspension, the first granules will start to gelatinizeat a certain temperature. Other granules in thesame suspension, usually smaller in size than thefirst ones, start to gelatinize at highertemperatures. This implies that the gelatinizationprocess of a starch suspension cannot be definedas to take place at a certain temperature, butrather during a certain temperature range.


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16 Chapter 3

Brabender visco-amylographThe ability of the starch granules to swell andsubsequently disrupt is of great technologicalimportance.According to a well-known method the viscositychanges during cooking of a starch paste can befollowed with the help of a Brabender visco-amylo-graph. This apparatus measures the viscosity ofstarch-water dispersions that are stirred andheated at a uniform rate, held at any desiredtemperature for a specific time and then cooled atuniform rate. A suspension of starch in water istransferred to the sample cup of the Brabendervisco-amylograph. The instrument is started andthe temperature of the sample is increased at arate of 1.5 C per minute. Heating is continued untilthe sample reaches a temperature of 90C or 95C and the sample is maintained at thistemperature for 20, 30 or 60 minutes, while stirringand recording the viscosity continuously. The pasteis then cooled to 50C or 25 C at a rate of 1.5 Cper minute and held for e.g. 1 h. at thistemperature while stirring.Brabender viscosity curves are characteristic anddifferent for each type of starch as shown in Fig 16.Initially, no viscosity effect is noted as thesuspension of starch is heated until the pastingtemperature is reached. The temperature at whichthe viscosity begins to rise is termed pastingtemperature. Appreciable granule swelling mustoccur before the viscosity is sufficiently high to berecorded by the Brabender instrument.Figure 16: Brabender viscosity curves of native starches

_ Time (minutes)30 90 120 180

3000"I I

I III 5% starch by weightIIIIII!Potato,(\~,,, ........... . . . . . . ., "

V Tapi;;;; - . . - - - ----, ,, . . ,. . . . . . . -.- . _ . _ . _'" '. .,. .: MS/~6" .'..' /". . . , . , . . r-'-'_ _".",'I ~eat

2500

2000

1 500o 50 70 95 95 50 50I Heat I Hold Cool I Hold I_ Temperature rC)

As shown in Table 5 and Figure 16, potato andtapioca starch have a lower pasting temperaturethan maize and wheat starch.As the temperature of the starch sample isincreased, the granules swell to impinge on eachother and increase the viscosity of the starchpaste. This process continues until the "peakviscosity" is reached. The peak viscosity is thehighest viscosity that the user may encounterduring preparation of a starch paste. Tuber androot starches show a sharper rise in viscosityduring cooking and a higher peak viscosity than thecommon cereal starches do. Typical values for thepeak viscosity (in Brabender Units = BU) areshown in Table 5 (starch concentration 5 %).Potato starch shows the highest peak viscosity.Maize starch gives a relatively low peak viscositybecause the granules are only moderately swollen.Tapioca starch and the waxy starches swell to agreater extent than maize starch. Wheat starchshows a very low peak viscosity. The peak viscosi-ty is a measure of the thickening power of a starch.On further cooking and stirring at elevatedtemperatures, the cohesive forces in the swollengranules become excessively weakened and thestructure of the paste collapses. The fragileswollen granules break down and thin out as aresult of granule fragmentation under shear. Thetuber and waxy starches break down in viscositymore rapidly and drastically than the commoncereal starches do. The next part of the Brabendercurve shows the behaviour of starch pastes uponcooling from 95C to 50 DC.The increase inviscosity during cooling is a measure ofretrogradation (set-back) due to association ofamylose molecules.Swelling power and solubilityIf an aqueous suspension of starch is heatedabove the gelatinization temperature, the granulesundergo a progressive swelling. The granules swellin a pattern peculiar to the particular starch. Theextent of swelling can be determined bysuspending a weighed starch sample in water,heating for 30 minutes in a thermostated bath, thencentrifuging, decanting the aqueous supernatantsolution and weighing the precipitate of swollengranules. Swelling power, at the pastingtemperature used, is calculated as the weight ofsedimented swollen granules per gram of drystarch. The swelling power can be determined overthe entire pasting temperature range (about50-95C) at intervals of 5C. The swelling poweris then plotted against the temperature of pasting,to give characteristic curves as shown in Figure 17.


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Table 5:Gelatinization Characterist ics of Native StarchesStarch Pasting Peak Viscosity Peak Viscosity Swelling Solubility ( %)

temperature Range average power at 95 DCDC 5 % starch conc. 5 % starch conc. at 95 DCBrabender Units Brabender Units

Potato 60-65 1000-5000 3000 1153 82Maize 75-80 300-1000 600 24 25Wheat 80-85 200- 500 300 21 41Tapioca 60-65 500-1500 1000 71 48Waxy Maize 65-70 600-1000 800 64 23

Figure 17:Swelling patterns of native starches

r~-----+------~----+------r--~50 60 70 95C0 90-emperatureThe swelling power at 95 DCof various nativestarches is shown in Table 5.The solubility can be determined by evaporation todryness of the supernatant solution above-mentio-ned. The solubility is expressed as the percentage(by weight) of the starch sample that is dissolvedmolecularly after heating in water at 95 DCduring30 min. Potato and tapioca starch show the highestsolubilization. The lipids in the cereal starchesreduce the solubilization.On heating in water, the granules of potato, tapiocaand waxy maize starch desintegrate more rapidlythan the granules of maize and wheat starch andconsequently they more quickly reach thehomogeneous condition necessary for many uses.Maize starch shows a relatively slow, restrictedtwo-stage swelling property. This suggests two

unequal sets of bonding forces within the granule,the weaker relaxing below 75 DC,the strongerabove 85 DC.The amylose-lipid complexes inmaize starch inhibit granule swell ing. Defattedmaize starch swells more freely and uniformly.Potato starch undergoes a very rapid andexceptionally high swelling at relatively lowtemperatures, indicating weak internal bonding.This is partly due to the presence of ionizableesterified phosphate groups, which assist swellingby reason of mutual electrical repulsion. The rapidSingle-stage swelling at relatively low temperatureis a typical starch polyelectrolyte behaviour. It isconceivable that hydrogen bonding in potato starchgranules partly occurs through hydrate waterbridges instead o-fby strong direct association ofstarch molecules. Although the bonding forces inthe potato starch are weak, they are comparativelyextensive, immobilizing the starch substance withinthe granule even at high levels of swelling. Potatostarch granules swell several hundredfold beforeextensive solubilization of starch molecules into theaqueous phase occurs.Root starches swell at lower temperatures and to agreater extent than the common cereal starches.This shows that the degree of association in rootstarches is less than in the common cereal star-ches. Waxy maize starch swells much more freelythan regular maize starch, since waxy maize starchcontains no amylose-lipid fraction to reinforce themolecular network within the granule.Molecular solubilization of starchThe organized micelles of the starch granules arequite persistent, and starches cooked at about 95DCfor one hour may still contain highly swollen,hydrated starch aggregates. True solubilization ofall the starch substance does not occur normallyunless the paste is cooked at temperatures of 100-160 DC(dependent upon the type of starch),


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18 Chapter 3

Tuber, root and waxy starches can be cooked to acompletely dissolved state at about 100C; maizestarch requires about 125 DC,and amylomaizestarch about 150C. By super-temperature heating(100-160 DC),as in an autoclave or in acommercial steam-jet cooker, the micelles disso-ciate to give a low-viscosity starch solution.Jet-cooking of starchesCooking starch slurries (suspensions) by the batchmethod was commonly used until about 1950 (seeFigure 18).Figure 18:Batch system for the preparation of cookedstarch

water

starch

product dischargewater

storage tank

pump

At that time continuous cooking systems wereintroduced. Jet-cookers utilize direct steaminjection (see Figure 19).

Jet-cooking is a continuous method of pastingstarch in which steam under pressure mixescompletely with a starch slurry, rapidly heats it andcooks it within a few seconds. The cooked starchpaste is then delivered directly to the point of use(see Figure 20).Jet-cookers are used in the starch industry and invarious starch-consuming industries (paper, foodand adhesives industry). By cooking starch slurriesat high temperatures (100-175 "C) and pressuresin jet-cookers, the starch granules are fragmentedand dispersed more extensively than by cooking atconventional temperatures in batch cookers (below100C).

Figure 19: Schematic drawing of a jet cooker

~ slurryt ? 2 2 Z Z I steam~ product discharge

The first types of jet-cookers were described in1937-1938. Only after 1950 jet-cookers were usedon an industrial scale. Various forms of apparatusmay be used. In special jet-cooker systems starchis simultaneously cooked and oxidized, or cookedand enzyme-modified.Figure 20: Flow diagram of jet-cooking of starches

water diluting water steam

starch

[et cookerslurry tank


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Starch pastesThe properties of a starch paste are determined bythe variety of starch, the type of modification, thecooking procedure and the presence of othermaterials. Careful attention must be given tocontrolling the conditions of cooking in order toobtain the desired properties consistently. Thefactors involved are cooking equipment, starchconcentration, temperature, time, intensity ofagitation and pH.Starch pastes may contain unswollen granules,partly swollen granules, swollen granules,fragments of swollen granules, swollen starchaggregates, dissolved starch molecules andretrogradated starch precipitates. Some propertiesof starch pastes, which are obvious important forthe user are: the viscosity, the texture, the pastetransparency and the resistance to shear. Thepaste properties of starches from differentbotanical sources differ from each other, as isshown in Table 6.Table 6 Properties of pastes of native starches

Texture of starch pastesGenerally, maize starch pastes consist of granulesthat are not very swollen, in contrast to the potatostarch pastes in which the granules are fullystretched and swollen. The swollen granules ofpotato starch are weakly bonded internally andshear will cause the paste to elongate to "strings",snapping back when released. The texture ofpotato starch pastes can be described as cohesive,visco-elastic, long bodied, stringy, fluid and rubbe-ry. The texture of pastes of the root and waxystarches is similar to that of potato starch, butgenerally less cohesive and less visco-elastic. Thetexture of pastes of the common cereal starchescan be described as non-cohesive, short, soft,heavy bodied or salve-l ike (see Table 6).Clarity of starch pastesOn thermal gelatinization, the initial opacity of anaqueous suspension of starch decreases and the

Starch Paste Paste Paste Resistance Rate ofviscosity texture clarity to shear retrogadation

Potato very high long nearly clear medium-low mediumMaize medium short opaque medium highWheat medium-low short cloudy medium highTapioca high long quite clear low lowWaxy Maize medium-high long fairly clear low very low

Paste viscosityThe viscosity of a starch paste reflects the workrequired to displace swollen starch particles pastone another. The viscosity is determined by thetype of starch, the cooking procedure and thestarch concentration. The highest viscosity of astarch paste is shown by the peak viscosity of theBrabender curve (see Figure 16 and Table 5).Generally, potato starch shows a higher pasteviscosity than the other starches (compared atthe same conditions). This may be explained bythe influence of the phosphate groups in potatostarch. A higher phosphate content in potatostarch results in a higher viscosity. The root andwaxy starches have a higher paste viscosity thanthe common cereal starches (as is shown inTable 6). In orderto obtain a starch paste with agiven viscosity, a lower amount of potato starch isneeded than with tapioca starch, maize starch orwheat starch.

paste becomes progressively more transparent.The clarity of the final paste is dependent upon thetype of starch (see Table 6). Pastes of tuber, rootand waxy starches are much clearer than those ofthe ordinary cereal starches; potato starch un-doubtedly gives the clearest paste. In general,paste clarity is directly related to the state ofdispersion and the retrogradation tendency of thestarch. The pastes of the common cereal starches(maize, wheat, sorghum, rice) are described asopaque, cloudy, dull or flat. The pastes of the tuber(potato), root (tapioca, arrowroot, sweet potato)and waxy starches are described as translucent,clear or transparent.


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20 Chapter 3

Resistance of starch pastes to shearThe viscosity of a starch paste may be reduced bymechanical shear (agitation, mixing, stirring), whichtears apart the swollen granules. Root, tuber andwaxy starches cook readily to give greatly swollen,fragile granules, which easily break down and thinout during stirring. The common cereal starchescook more slowly, swell to a lesser degree, andshow a moderate resistance against viscositybreakdown by agitation (see Table 6). The resistan-ce under shear can be tested by stirring starchpastes for e.g. 20 minutes and then determining theviscosity. After stirring, tapioca and waxy maizestarch show the lowest viscosity, potato starch isintermediate and maize starch shows the highestremaining viscosity. The introduction of a very fewchemical cross-linkages within the granule tightensup the molecular network, restricts granule swellingand hence stabilizes the viscosity of starch pastesagainst breakdown by agitation.


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RetrogradationIf a dilute starch solution stands for a prolongedtime, it gradually becomes cloudy and eventuallydeposits an insoluble white precipitate. If a moreconcentrated starch dispersion is allowed to cool,it will rapidly set to an elastic gel. These are bothprocesses of retrogradation, whereby the starchsubstance goes from a dissolved and dissociatedstate to an associated condition. In starch granu-les, mainly the amylopectin molecules constitutethe crystalline micelles. In contrast, retrogradated,crystalline starch material is composed mainly ofamylose molecules. The mechanisms of retrogra-dation are schematically shown in Figure 21.Figure 21:Mechanism of starch retrogradation(amylose)

Solution

= ? T ; = = -Precipitate

Retrogradation of starch pastes or starch solutionsmay have the following effects: Increase in viscosity Development of opacity and turbidity Formation of insoluble "skins" on hot pastes Precipitation of insoluble starch particles Formation of gels Syneresis of water from the paste (weeping).Retrogradation is a complex process and dependson many factors such as type of starch, starchconcentration, cooking procedure, temperature,storing time, pH, cooling procedure and thepresence of other compounds. Retrogradation ofstarch dispersions is generally favoured by lowtemperatures and by high starch concentrations.The retrogradation rate is fastest at pH 5-7,decreasing at higher and lower pH. Retrogradationdoes not occur at pH above 10 and is slow below

pH 2. Retrogradation is retarded by the salts ofmonovalent anions and cations and by urea.The role of the amylose fractionAmylose is considered primarily responsible forretrogradation processes, and this is the mostsignificant property of this fraction. Dissolvedamylose molecules can orient themselves in aparallel alignment, so that a large number ofhydroxyl groups along one chain are in closeproximity to those on adjacent chains, Whenthis occurs, the hydroxyl groups form associa-tions through interchain hydrogen bonds, andthe amylose chains are bound together to formaggregates that are insoluble in water. In dilutesolutions, the aggregated chains of amyloseprecipitate. Inmore concentrated dispersions,the aggregated amylose entraps the aqueousfluid in a network of partially associated starchmolecules, forming a gel. In both instances X-ray diffraction patterns show that a crystallineorganization is formed. A temperature of 100-160C may be required to redissolve retrogra-dated amylose particles in water.There is a relationship between the chain lengthof the amylose molecules and the ease and kindof retrogradation. Amylose exhibits a maximumrate of retrogradation (minimum in solubility) ata degree of polymerization (DP) of about 100-200 glucose units (see Figure 22).Figure 22: Rate of retrogradation of amylose(schematic)

10 100 1000 10,000-egree of polymerization of amylose


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22 Chapter 3

The rate of retrogradation decreases with longerand shorter amylose molecules. Long amylosemolecules do not readily move into tight associationwith other chains and have difficulty in lining up withtheir neighbours over long intervals. Amylosemolecules smaller than the optimum length do notassociate as completely and are too short to give agel.The role of amylopectinAmylopectin is much less prone to retrogradationthan amylose. The association of dissolvedamylopectin molecules is strongly inhibited by theirhighly branched structure. Therefore, amylopectintends to be soluble, forming solutions that do not gelunder normal conditions. Under extreme conditionsof high starch concentrations or freezingtemperatures, the branched fraction may undergoretrogradation effects. These effects are attributedto the association of the outer chains of theamylopectin molecules. These associatedbranches may contain about 20-30 glucose units. orexample, the staling of bread has been traced toassociative bonding between the outer branchesof the amylopectin fraction. In pastes or solutions ofthe common starches (containing amylose andamylopectin) the presence of the branchedamylopectin fraction has a moderating influence onthe retrogradation of the linear amylose fraction,slowing down its precipitation and diminishing its geltendencies.The role of lipid substancesThe 'normal' retrogradation occurs generally oncooling and storage of starch pastes attemperatures of 70C and below. There is,however, another form of retrogradation, whichoccurs during storage of maize starch solutions atrelatively high temperatures (75-95 "C) and takesthe form of a precipitate of regularly sized particles.High temperature retrogradation is observed whenmaize starch is gelatinized at temperatures of 120-160C with a jet-cooker and the resultant paste isstored at 75-95 C. The precipitated particles areformed from inclusion complexes of maize starchamylose with free fatty acids which occur naturally inmaize starch. Precipitation, caused by hightemperature retrogradation, did not occur whendefatted maize starch, waxy maize starch or potatostarch were gelatinized above 120C and stored at70-95 C. The complex of amylose with the higherfatty acids is not formed above 95C, which wouldindicate that dissociation of such a complex takesplace above 95 C.

Retrogradation of various native starchesThe rate of retrogradation of different nativestarches is shown in Table 6. A measure of theretrogradation isthe increase in viscosity, aftercooling starch pastes from 95C to 50 C, asshown in the Brabender curves (see Figure 16).The common cereal starches (maize, wheat,sorghum, rice) retrogradate more quickly than thetuber (potato) and root (tapioca, arrowroot, sweetpotato) starches. The waxy starches show thelowest rate of retrogradation.Maize starch pastes and solutions retrogradaterelatively quickly. The high amylose content (28%), the relatively small molecular size of the maizeamylose molecules (DP between 400 and 15,000),and the high lipid content (0.8 %) are importantfactors which promote retrogradation. Asubstantial portion of the starch molecules inmaize starch pastes is usually present as anassociated amylose-lipid complex with reducedhydration capacity. This relatively inert portiondoes not contribute to the binding force orthickening power of the maize starch paste. Theother common cereal starches (wheat, sorghum,rice) show similar retrogradation effects as maizestarch.Potato starch dispersions show only a moderatetendency towards retrogradation. This is attributedto the low amylose content (21 %), the great lengthof the potato amylose molecules (DP between 840and 22,000) and the low lipid content (0.1). Theroot starches have a low (tapioca) to moderate(sweet potato, arrowroot) tendency towardsretrog radation.The most practicable method for preventingretrogradation effects is to derivatize the starchmolecules with a small amount of ether or estergroups. The introduction of only two or three ofsuch groups per 100 glucose units (degree ofsubstitution 0.02-0.03) prevents side-by-sidealignment of linear chains.


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Starch filmsIn foods, starch products are generally used in theform of a paste, usually for thickening purposes.Most of the industrial, non-food uses of starchproducts involve cooking of starch in water andapplication of the resulting paste to a solidsurface, followed by drying of the film or coating.The function of the starch may be as an adhesive(to cause two surfaces to stick together), or thecooked starch paste may be used to give asmooth film or coating over paper sheets or textilefabrics. The starch film must have certain specificproperties to qualify it for the intended use. Theseproperties include plasticity, internal strength,water solubility, response to humidity, film trans-parency and gloss.Films of potato and tapioca starch products havea greater flexibility, a higher tensile strength, ahigher elongation before rupture and a highertoughness than fi lms of maize and wheat starchproducts.For a number of applications it is necessary thatthe applied starch film dissolves fast and at lowtemperatures in water. This is the case whenremoistenable starch films of gummed papers arewetted with water and when sized textile yarnsare desized.Starch films obtained from different types ofstarch vary both in solubility and rate 0f retrogradation. Films of starch productsobtained from potato, tapioca or waxy starchesredisperse in water more readily and morecompletely than films of starch products obtainedfrom the common cereal starches (maize, wheat,sorghum, rice). The low solubil ity of films from thecommon cereal starches is attributed primarily tothe influence of the amylose fraction, in combination with a high content of lipids. The smallamylose molecules of the cereal starches willretrogradate during drying of the fi lm, not onlybecoming insoluble itself, but likewise entanglingthe amylopectin molecules in an insoluble net-work. The root, tuber and waxy starches aremuch less prone to retrogradation and, therefore,the films of these starches dissolve readily inwater.Starch products are used as remOisteningadhesives on gummed papers, like envelopeflaps, gummed labels, postage stamps andgummed tape. The starch film should retain itswater solubility for considerable periods of time.Starch films obtained from maize starch show anexcessive loss of adhesive properties on ageing.This appears to be due to retrogradation of starch

molecules, which slowly develops in the film,thereby lowering its solubility and hence itsadhesiveness. Tuber, root and waxy starchproducts give starch films which retain theiradhesiveness for long periods of time.Films of starch products made from potato,tapioca or waxy starches have greater clarityand higher gloss than films of starch productsmade from maize or wheat starch products.During drying and ageing, films of maize starchand wheat starch undergo retrogradation,whereby the amylose molecules associate intoinsoluble micelles which impart both opacityand fragility to the film.


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4 Starch modificationsGeneral aspectsWhy modified starches?Very often native starch is not the best product in aparticular application or process. Modifications ofnative starches are carried out to provide starchproducts with the properties needed for specificuses.Modification of native starch properties is animportant factor in the continued and increased useof starch products to provide thickening, binding,gelling, adhesive and/or film-forming functionality.The various ways of modification of native starch aredesigned to change one or more of the followingproperties:- pasting temperature- solids-viscosity relationships- gelatinization and cooking characteristics- resistance of starch pastes to breakdown inviscosity by acids, heat and/or mechanical shear- retrogradation tendencies- ionic character- hydrophilic character.Methods of modificationThe modification of native starch may involve achange in physical form, a controlled degradationTable 7: Modification of starch

and/or the introduction of chemical groups. Table 7gives the various methods of starch modification.Reaction techniques for chemical starchmodificationThe industrial reaction techniques forthe chemicalmodification of starch can be divided into threegroups: Reactions in aqueous gelatinized starch 'pastes Reactions with dry or semi-dry starch granules Reactions in suspensions of ungelatinized starchgranules in water.Properties of native starches retained inmodifiesdstarchesBy chemical and/or physical modification, theproperties of the native starches are altered. Therelative differences between the native starches(mentioned in the chapters 'Sources of Starches',page 6 and 'Composition and properties ofstarches', page 10) are however more or lessretained in the corresponding modified starches.The distinctive composition and properties of thevarious native starches are clearly present in themodified starches produced from these nativestarches (chemical composition, pastingtemperature, paste viscosity, paste clarity,retrogradation tendencies, starch film properties,solubility, adhesiveness and application ability).

No Type of modification Main objectives TreatmentPregelatinized starch Cold water dispersibility Drum-drying Extrusion

2 Low-viscosity starches Lower viscosity a, b, corda. Dextrins Lower viscosity Dry heat treatment with acidRange of viscosity stabilityb. Acid-modified starch Lower viscosity Acid hydrolysis (suspension)High gel tendencyc. Oxidized starch Lower viscosity Oxidation (in suspension orImproved viscosity stability paste)d. Enzymatically modified starch Lower viscosity Alpha-amylase (paste)

3 Crosslinked starch Modification of cooking Crosslinking in suspensioncharacteristics4 Stabilized starch Improved viscosity stability EsterificationEtherification5 Combinations of modifications Combinations of objectives Combinatons of treatments1 ,2, 3 and/or 4 1,2, 3 and/or 4 1,2,3 and/or 46 Starch sugars Sweet saccharides Acid and/or enzymes


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Starch modifications 25

Pregelatinized starchesPregelatinized (precooked) starches are usedwhen a starch product that is swellable (soluble;dispersible) in cold liquids without cooking isrequired. Pregelatinized starches aremanufactured by the starch producer by means ofdrum-drying (starch suspensions or starch pastes)or extrusion (semi-dry starch). Pregelatinizedstarches are mostly produced on steam-heatedroller drums of the single or double type. Extrusionis used to a lesser degree.In fact, in the manufacture of pregelatinizedstarches the starch is precooked for the user.Precooked starches have practical importance forusers who do not have the facil ities for cookingstarch. Typical applications of pregelatinizedstarches are convenience foods (instantpuddings), wet-end additives in paper making,wallpaper adhesives and drilling muds.Pregelatinized starches can be made from anynative or modified starch. Except for their coldwater solubility, slightly lower viscosity and lesstendency to gel, the properties of the pregelatinized starches are similar to those of theparent starch.The common cereal starches (maize, sorghum,wheat) contain unsaturated lipids. In the intactgranule, these lipids are protected from oxidation.In the pregelatinized cereal starches, lipids maydevelop rancid flavours on storage (due to oxida-tion).


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26 Chapter 4

Low-viscosity starchesLow-viscosity starches are produced by controlleddegradation of native starches. Low-viscositystarches can be gelatinized (cooked) in water athigher concentrations (10-65 % by weight) thannative starches (maximum about 10% by weight).Low-viscosity starches are needed in applicationswhere a high-solids starch paste with a pumpableand workable viscosity is required. The highviscosity of a cooked dispersion of a native starchrequires a large amount of water (low starchconcentration) to provide a workable viscosity forpumping, mixing and applying the paste to asubstrate (e.g. paper, textiles). This low-solidsstarch paste would require large quantities of energyand a longer drying time to obtain a dried product.Low-viscosity starches are obtained when the starchis subjected to a treatment which results in rupture ofsome of the glucosidic bonds in the starchmolecules. As a consequence, the viscosity of thestarch product wil l decrease. The hydrolysis of aglucosidic bond is shown in Figure 23.

Figure 23: Hydrolysis of aglucosidic bond of astarch molecule

Table 8:Viscosity ranges of native and low-viscositystarches (expressed asparts of water per part of drystarch to give about the same hot viscosity after cooking)Starch product ViSCOSityrangePotato starchWaxy maize starchTapioca starchMaize starchWheat starch

23-2421-2319-2114-1612-14

Oxidized starchesAcid-modified starchesEnzymatically modified starchesBritish gumsWhite dextrinsYellow dextrins

3-122-112-111-111-51-2

Later (in 1833), the roasting of starch in thepresence of sulphuric acid was further investigatedby Biot and Persoz, who examined the productsobtained and gave the name "dextrin" to the gummymaterial they separated because of the direction ofits optical rotation.

OH + HO

The commercial conversion processes for theproduction of low-viscosity starches are carried outby acids and heat (dextrins), acids (acid-modifiedstarches), oxidizing agents (oxidized starches) oralpha-amylase (enzymatically modified starches).The comparative viscosity ranges of the native andlow-viscosity starches, in terms of parts of water perpart of dry starch to give roughly the same hotviscosity, are shown in Table 8.Dextrins(= roasted starch =pyrodextrins)It was a purely accidental discovery that was to playthe biggest role in the development of dextrin. InSeptember 1821 a fire broke out in a textile mill nearDublin in Ireland. Potato starch was stored in anadjacent building which was partially destroyed bythe fire. The brown-coloured powder which was leftafter the fire was found to be soluble in water and togive a sticky solution. It was then discovered that thesame result could be obtained by heating starch inan iron pan. If this story is true, and it seems wellauthenticated, it is slightly ironic that the roastedstarch became known as British gum.

Dextrinization is the roasting of dry starch, mostly inthe presence of small quantities of acid. Nativestarch (10-20 % moisture) is mixed with the requiredquantity of acid (usually hydrochloric acid). The nextstep is a drying process to reduce the moisturecontent of the starch down to about 5-12 % (whitedextrins) or below 5 % (yellow dextrins). Thesubsequent dextrinization process is carried out inrotating roasting kilns or in fluidized bed systems.When the reaction is complete the dextrin isdropped into a vessel and cooled. Finally theproduct is remoistened (to about 10% moisture),sieved and bagged.During dextrinization the starch molecules are firstrandomly hydrolyzed to short fragments, theso-called "white dextrin" stage (see Figure 24).Thereafterthe fragments recombine giving abush-like structure (yellow dextrins).


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Figure 24:Mechanism of dextrinization


Acidhydrolysis

Amylopectin heat+acid+water

starch -------1~~

Polymerization

heat+acid

whitedextrin

yellowdextrin

Dextrins can be made from all the commercial nativestarches; however, the ease of conversion and thequality of the dextrins vary with the type of nativestarch. The manufacture of a high-quality dextrinrequires a high-quality base starch with a low level ofproteins, lipids and other impurities. Potato starchand tapioca starch convert easily to yield dextrinsthat give dispersions of excellent clarity, stability andadhesiveness. Dispersions of maize dextrins do notshow the clarity of potato and tapioca dextrins andtend to thicken rapidly on storage.The yellow dextrins are the most highly depolymerized low-viscosity starches. Their degree ofpolymerization (DP) is approximately in the range of20 to 50. Because of their low DP it is possible tomake fairly free flowing solutions or pastes, whichcontain up to 65 % solids.Acid-modified starchest= acid-converted starch = thin-boiling starch = acid-fluidity starch)C.J.Lintner (Germany) was the first (in 1886) whodescribed the suspension treatment of starch withacids. He treated a starch suspension with a 7.5 percent aqueous solution of hydrochloric acid for 7 daysat room temperature. Then the starch was washedfree of acid with water and dried. The material hasreferred to ever since as Lintner's starch. It is usedas substrate in enzyme assays and as indicator foriodometric titrations.The suspension process is widely used in the starchindustry. Starch is suspended in a dilute acid solu-tion (such as hydrochloric or sulphuric acid) andmaintained at a temperature varying from room

temperature to just below the pasting temperature.The suspension is stirred until the potential viscosityof the suspended starch is reduced to the desiredlevel. The suspension is then neutralized (e.g. withalkali or sodium carbonate), washed and dried.PropertiesThe acid-modified starch (suspension process)differs from its parent native starch in the followingrespects: lower hot-paste viscosity (lower average molecularweight); the acid degradation involves mainly a scission ofthe starch molecules to lower-molecular-weightfragments. There is an increase of the numberof linear molecules, which are smaller than theamylose molecules of the native starch. The resultis an increased tendency of the gelatinized starchsolution to increase in viscosity and form a gelupon cooling and standing; less granule swelling during gelatinization in hotwater.Oxidized starch(=chlorinated starch =oxystarch = thin-boilingstarch)Starch can be oxidized by a number of oxidizingagents (oxidants) such as sodium hypochlorite(NaOCI), calcium hypochlorite, ammoniumpersulfate, potassium persulfate,hydrogen peroxide, peracetic acid, potassiumpermanganate, sodium chlorite, perborates and


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28 Chapter 4

chlorine (in water: hypochlorous acid). The oxidationreaction may be carried out in an aqueous starchsuspension or in a gelatinized starch paste.Depending on the type of oxidant and the conditionsof the reaction, carboxyl (COO H) and carbonyl(C=O) groups are introduced, while at the same timedepolymerization occurs. Oxidation is the onlytreatment in which one modification reagent causestwo important chemical modifications (depolymerization + introduction of carboxyl groups).Although the oxidation affects both amylose andamylopectin molecules, the carboxyl and carbonylgroups formed on the amylose chains have the maininfluence on the reduced tendency to retrogradationand gelling of these products in solution.Bleaching may be considered as a very lightoxidation. The products indicated as oxidizedstarches contain more than 0,1 % added carboxylgroups, while bleached starches contain 0,1 %added carboxyl groups or less.The degree of substitution (OS) of commercialoxidized starches is in the range of 0.01 to 0.04 forcarboxyl groups (1 to 4 carboxyl groups per 100glucose units) and 0.005 to 0.01 for carbonyl groups(0.5 to 1 carbonyl group per 100 AGU).The commercial production of oxidized starches bythe starch industry (suspension-reaction) involvestreating an aqueous starch suspension with asodium hypochlorite solution. Essentially, sodiumhypochlorite (NaOCI) is made by diffusing chlorineinto a cool solution of sodium hydroxide. Thereaction proceeds as follows:2 NaOH+ CI2 --- NaOCI + NaCI+ HpThe reaction of sodium hypochlorite with starch maytake place according to the following equation:St-CHpH + NaOCI --- St-COOHStarch + Sodium Oxidized starch + Salt

hypochlorite+ NaCI

Oxidized starches are sometimes called chlorinatedstarches because of the reagent used, although nochlorine is introduced into the starch.Since about 1870 Elliot & Crabtree Ltd. ofManchester were using hypochlorite solutions inorder to bleach imported sago starch, which wasused in the Lancashire textile industry for sizingpurposes. Since about 1930 the industrialproduction of oxidized starches has increasedrapidly.Today, hypochlorite oxidized starch is probably themost important starch derivative (starch sugars not

included). The production of oxidized starches in thestarch plant involves treating an aqueous starchsuspension (35-45 % solids) with sodiumhypochlorite solution (containing 5 to 10 % availablechlorine) at pH 8 to 10 and 15 to 38C. When therequired level of oxidation (degradation) is reached,the reaction mixture is neutralized to pH 5-6.5. Toremove impurities, solubilized starch and by-products of the reaction, the reaction product iswashed on continuous vacuum filters or inhydrocyclones. Finally the product is recovered byfiltration and dried.Oxidative conversion by users of starch (especiallyin the paper industry) can be realized by adding anoxidant to a starch slurry and then cooking thestarch in continuous systems. The starch isgelatinized and simultaneously OXidized. Hydrogenperoxide and potassium or ammonium persulfatesare the preferred oxidants in the continuousthermochemical conversion processes.Thermochemical conversion of starch involvescontinuously forming of cooked oxidized starchsolutions by subjecting a starch suspension totemperatures of 100-175 C in the presence of anoxidizing reagent for a short period of time (0-2minutes). Jet-cookers or other apparatus for thecontinuous cooking of starch under pressure maybe used (see Figure 36 on page ... ).Properties of oxidized starches (suspension-reaction)Oxidized starches differ from their parent nativestarches in the following respects:- lower hot-paste-viscosity (lower averagemolecular weight)-Iower rate of retrogradation of cooked pastes(because of carboxyl groups introduced in theamylose molecules)-Iower pasting temperature; faster rate ofgelatinization; lower peak viscosity- higher clarity of pastes, solutions and films


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bleached starches are whiter because of thebleaching treatment (removal of most proteina-ceous impurities) improved flavour and odour bleached starches may have a very low bacterialcount.Enzymatically modified starchIn the continuous enzyme conversion process astarch slurry containing a heat-stable alpha-amylase is gelatinized instantaneously by directsteam injection (jet-cooker) and dischargedcontinuously to a holding column where the enzymethinning reaction takes place. Then the thinnedpaste is pumped from the column and the enzyme isdeactivated by heating (in a jet-cooker) to an elevated temperature. The resulting paste can directlybe pumped to the point of use.


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30 Chapter 4

Crosslinked starches(= crossbonded starches = inhibited starches)One of the most important types of starchmodification is that resulting from the action of bi- orpolyfunctional reagents capable of reacting withmore than one hydroxyl group, thus formingcrosslinks or bridges from one starch molecule toanother. Treatment of ungelatinized starch granuleswith crosslinking reagents has had particularlyvaluable use. What appears to take place is acrosslinking of starch molecules in the starchgranule, with a resulting tanning or toughening of thegranule. At increasing degrees of crosslinking theungelatinized starch granules become more andmore resistant to gelatinization. In most cases arelatively low degree of crosslinking of about onecrosslink per 100 to 2000 anhydroglucose units isused. With an average degree of crosslinkinqcorresponding to about one on every 20 glucoseunits, starches are obtained which show extremelylimited ability to swell on cooking.Distarch phosphateIn the method of Felton and Schopmeyer (from1940),0.15 to 0.25 % phosphorus oxychloride(based on starch) is added slowly to a slurry of about40 % starch in water at a pH of 10 and a temperatureof about 25 D C . Upon completion of the reaction, thepH is adjusted to neutral, and the granular starchproduct is filtered, washed and dried. The reactiontakes place according to the equation:oNaOH I I

Kerr and Cleveland described in 1954 the crosslin-king of ungelatinized starch in an aqueous alkalinesuspension with sodium trimetaphosphate. Esterifi-cation with trimetaphosphate salts requires morerigorous conditions than does esterification withphosphorus oxychloride. Thus, a distarch phospha-te may be prepared by heating a starch slurry(adjusted to a pH of 10 to 11) containing 2 % sodiumtrimetaphosphate (based on the weight of thestarch) to 50 D C for 1 hr. The pH is adjusted toneutral and the granular distarch phosphate isfiltered, washed and dried.The reaction takes place according to equation:

NaOHoI IStO - P- OSt + Na2H2pp7IONa

Starch+ Sodiumtrimeta-phosphate

Distarchphosphate

+ Sodiumdihydrogenpyrophosphate

Epichlorohydrin-treated starch(= distarch glycerol)M. Konigsberg discovered in 1945 thatepichlorohydrin could react with ungelatinizedstarch granules in aqueous systems to givecrossbonding to the molecules. The obtainedmodified starch has properties similar to thoseobtained with phosphorus oxychloride. The reactionwith epichlorohydrin takes place according to

StO- P - OSt + Nacl equation on the next page.The structure of distarch glycerol is given in Figure

ONa 26, demonstrating the crosslin king of two starchchains by means of a glycerol bridge.

2 StOH+ POCI3

Starch+ Phosphorus ---oxychloride

Distarchphosphate

+ Salt

The structure of distarch phosphate is given inFigure 25, demonstrating the crosslinking of twostarch chains by means of a phosphate bridge.Figure 25: Structure of distarch phosphate

Starch chainoI

0= P-ONaIoI Starch chain

Figure 26: Structure of distarch glycerol

Starch chain

Starch chain


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St-O-CH -CH-CH -O-St + NaCI2 I 2OH

Starch + Epichlorohydrin -- Distarch glycerol + SaltOther crosslinking reagentsAdipic acid anhydride was found to givecrossbonded starch derivatives with propertiesanalogue to those described for phosphorusoxychloride and epichlorohydrin treated starches.The action of borax in starch pastes (increasedviscosity and stability) is probably also a case ofweak crosslinks. Borax is the most important singlesubstance to modify pastes of starch products. Theunknown man who first discovered the effect ofborax on starch adhesives has made the mostoutstanding contribution in this field.Properties of crosslinked starchesCrosslinked starch differs from its parent nativestarch in the following respects: the starch granules have an increased resistanceto swelling and gelatinization a higher viscosity of the cooked paste is attained increased resistance of starch paste viscosities tothe thinning effect of prolonged agitation, heat orlowpH reduction of the cohesive, rubbery, elastic characteristic of starch pastes from starches such aspotato, tapioca and waxy maize to give smooth,salvelike, creamy pastes.


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32 Chapter 4

Stabilized starches(Starch esters; Starch ethers)General aspects

Stabilized starches are usually made by reactingstarch with etherifying or esterifying reagents in thepresence of an alkaline catalyst. The number ofintroduced substituents is indicated as degree ofsubstitution (OS). TheDS is defined as the averagenumber of substituents per anhydroglucose unit(moles substituent per mole of AGU). Thus, a starchderivative with a OS of 0.05 contains 5 substituentsper 100 anhydroglucose units (average value).Generally, the commercial starch esters and starchethers have a OS below 0.2 (lower than 20substituents per 100 AGU).The substituent groups of a partially substitutedstarch derivative are distributed among the threehydroxyl groups (C-2, C-3 and C-6) of the anhydroglucose units of the starch molecule (seeFigure 10 and Figure 30). The distribution isdetermined by the relative reactivity of the hydroxylgroups and the nature of the substitution reaction.The substituents in the commercial starch acetates,hydroxyalkyl starches, carboxymethyl starches andcationic starches are predominantly attached to theC-2 of the anhydroglucose units. The remainingsubstituents are located mainly at the C-6 positionwith only negligible substitution at C-3. Figure 27gives the structure of these stabilized starches with alow OS. In contrast, cyanoethyl starch andmonostarch phosphate are substituted predomi-nantly on the C-6 position.The reaction of starch with monofunctionalesterifying or etherifying reagents is a method off introducing side-chains, in other words: addingirregularities to the starch chains (see Figure 28).These irregularities on the starch chains (and inparticular on the linear amylose molecules) inhibitthe formation of ordered structures in the starchpaste, so retrogradation is retarded (increasedviscosity stability). Therefore starch ethers andstarch esters are often designated as stabilizedstarches. Retrogradation decreases when moresubstituent is introduced in the starch chains. Theobject of crosslinking starch is mainly thecrosslinking of amylopectin molecules in ordertoreduce the rate of swelling of the starch granules. Incontrast, the object of monofunctional esterificationor etherification of starch is mainly theintroduction of side-chains in the linear amylosemolecules to reduce the rate of retrogradation ofstarch solutions.

Figure 27:Structure of stabi lized starches with low OS

oR = -C acetyl group

'CH 3= - CH3- CH2- OH hydroxyethyl group

= - CH2- CH- CH2 hydroxypropyl group1

OH0= -CH2-C e carboxymethyl group'0

CH


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Starch estersMonostarch phosphateIn monostarch phosphate only one of the threeacidic functions of phosphoric acid is esterified witha starch hydroxyl group (predominantly the C-6hydroxyl group; see Figure 6). The monoesters havebeen referred to asphosphate, starch phosphatemonoesters, or simply as starch phosphate, but aremore properly described as starch dihydrogen (ordisodium etc.) phosphate.Monostarch phosphates are prepared by semidryreactions of starch with phosphoric acid or water-soluble salts of ortho-, pyro- and tripolyphosphoricacid. This technique was first used in 1943 byJOlicher and Appelt. They heated a semidry mixtureof starch, urea (e.g. 12 % on starch) and phosphoricacid (e.g. 5 % on starch) during 2-3 hours at 140 DC.The reaction of starch with phosphating reagentsappears to proceed faster and easier in thepresence of urea. Therefore, the reaction may becarried out at lower temperatures and/or during ashorter time and/or with a lower amount ofphosphating reagents. The monostarch phosphatesobtained (with urea) have a lighter colour andcontain less free phosphates as impurity.

In the Neukom-processthe starch is impregnatedwith the phosphate salt solution by suspending inthe solutionand then filtering;by spraying fhesolutionon the dry starch; or by mixing the solutionwith the wet filter cake. The pH values of thephosphate solutions vary from pH 5 to 8.5depending upon the particular phosphate used. Thephosphate impregnated starch is dried (to 8-15 %moisture) and then heated (roasted) attemperatures of 120-175 DCfor 1-15 hours. Thereaction takes place according to the equation:

NaOHoI I

StO- P - Q-Na'+ H 0I 2OH

Starch + Sodiumortho-phosphate

Monostarch +Waterphosphate

The phosphation reaction generally runs to a OS of0.02 to 0.15. The resulting products containbetween 1 and 5 % phosphorus. The industrialproduction of monostarch phosphates may becarried out in the same kind of apparatus as is usedin the production of dextrins, for example in

rotating kilns or fluid-bed systems.Properties of monostarch phosphatesMonostarch phosphate differs from its parent nativestarch in the following respects: The monostarch phosphates tend to become cold-water-dispersible at OS greater than about 0.07.Products with a OS of 0.01-0.07 have a loweredpasting temperature. Undepolymerized, purified monostarch phospha-tes form dispersions with a higher viscosity thanthat of the native parent starch (because of theionic nature of the phosphate group). For example,a 1 .8 % paste of monostarch phosphate has aboutthe same hot viscosity (as measured by the stan-dard Scott test) as a 4.3 % paste of the parentstarch. When during the phosphation reactionsome hydrolysis of the starch occurs (acid condi-tions) the resulting monostarch phosphates(dextrin-phosphates) will have a lower viscosity ascompared with the parent starch. Monostarch phosphates are anionic polyelectrolytes. The viscosity of solutions of these starches isreduced in the presence of salts. Monostarch phosphates may be precipitated fromsolutions by treatment with soluble salts of aluminium etc. When gelatinized by cooking in water, the monostarch phosphate forms a viscous, clear, non-gell ing paste which has long, cohesive flowcharacteristics.Starch acetate (= acetylated starch)Encouraged by the easy reaction of phosphorusoxychloride on granule starch in aqueous suspen-sions (discovered in 1940), it was only natural tolook at other water reactive reagents. Acetic anhy-dride was one of the first reagents to be tried. Itreacts with starch according to equationon the nextpage.In 1945 Caldwell described the preparation ofstarch acetates in granular form by reaction of starchin aqueous suspension with acetic anhydride underalkaline conditions. Successful acetylation dependsupon maintenance of conditions that favouracetyla-tion over acetic anhydride hydrolysis without appre-ciable hydrolysis of starch acetate. The reaction iscarried out at a pH controlled at 7.5 to 9.0 at roomtemperature with slow addition of acetic anhydride.Reaction efficiency is about 75 %. The products are


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34 Chapter 4

St-OH NaOH--Starch + Acetic

anhydride --recovered by neutralization to pH 5 with dilute acids,filtration, washing and drying. Ungelatinized starch acetates containing up to 5 %acetyl groups (OS about 0.2) can be obtained.Beyond this level the granules begin to swell andfiltration becomes difficult.Another reagent, patented by Smith and Tuschhoffin 1957, is vinyl acetate. It reacts with starch according to equation:

St-OHoI I+CH2 = CH - 0 -C - CH3

Starch + Vinyl acetate

Starch acetates are manufactured by treatment ofaqueous starch suspensions with vinyl acetate. Thereaction is run at about 35-40 C and pH 9-10 duringabout 60 minutes. Sodium carbonate is used ascatalyst and buffer. Reaction efficiency is about 70%. The products are recovered by neutralization,filtration, washing and drying. The reaction appearsto be a transesterification with acetaldehyde as aby-product.Properties of granular starch acetatesStarch acetate differs from its parent native starch inthe following respects:-lower rate of retrogradation of cooked pastes. Atrelatively low OS the parallel orientation of amylosemolecules and of the outer branches of amylopec-tin is hindered;- the pasting temperature is progressively loweredas the OS is increased;- pastes are clearer and have a longer, morecohesive texture;- films (formed by drying thin layers of colloidalsolutions) have greater clarity, higher gloss, moreflexibility, larger elongation before rupture, lesscracking tendency and easier solubility;

oI ISt-O-C-CHa + CH3COOH

Starch acetate Acetic acid

- starch acetates are subject to cleavage byrelatively rapid alkaline hydrolysis (saponification).Other starch estersGranular starch succinates may be obtained bytreating ungelatinized starch granules in anaqueous alkaline suspension with succinicanhydride.

o 0I I I ISt - 0 - C - CH3 + CH3 - CHStarchacetate

+ Acet-aldehyde

Octenylsuccinate half-esters of starch are made bytreatment of aqueous starch suspensions withoctenyl succinic anhydride. The introduction ofhydrophobic groups at low OS-levels (0.01-0.1)imparts some hydrophobic properties to the starchwithout destroying the water-dispersibility. Thehydrophobic-hydrophilic balance imparts usefulemulsifying and emulsion stabilization properties.If an intimate blend of urea and a starch containing5-10 % moisture is heated at 90-120 DC,a low OS,nitrogen-containing starch derivative is produced.The product is assumed to be a starch carbamate(St-O-CO-NH2). The reaction is promoted by theaddition of potassium acetate.

Starch ethersVarious reaction techniques are used in theproduction of starch ethers.In general, etherifying agents are relatively slower toreact with ungelatinized starch granules as compa-red to esterifying agents. The high efficiency of thealkaline suspension reaction is caused by theconcentration of alkali on the starch granules(adsorption). The low-OS starch ethers are the mostimportant substituted starch derivatives of today.


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Hydroxyalkyl ethersSince about 1945 considerable attention has beengiven to the preparation of purified granular starchethers with low OS. Low-OS (0.03-0.10) granularhydroxyalkyl starch ethers are prepared by treatingaqueous starch suspensions (about 40 % d.s.) withalkylene oxides (ethylene oxide, propylene oxide) inthe presence of alkalies (sodium hydroxide, calciumhydroxide) at temperatures up to 50C. The reac-tions take place according to equations:

0/" - NaOHSt-OH + CH2 -CH2 - - St -O-CH2 -CH2 -OHStarch + Ethylene oxide - - Hydroxyethyl starch

01 NaOHSt-OH + CH3-CH -CH2 -- St-O-CH -


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36 Chapter 4

Reduced retrogradation of cooked pastes. Not allsubstituents are equally effective in decreasing retrogradation phenomena. A long side-chainis more effective than a short one. A side-chainwith an ionic end-group is more effective thana non ionic side-chain of about the same length.

The gelatinization temperature is lowered. Cold-water-swelling of hydroxyethyl starch granulesbegins at a DS of about 0.3. Granules ofquaternary ammonium alkyl ethers swell in coldwater at a DS of about 0.07.

Pastes have a longer, more cohesive texture. Improved properties of dried starch films(increased solubility, clearer films, more flexiblefilms, higher gloss, more continuous films).

Cooked pastes, films and coatings are moreresistant to biological spoilage. The substituent groups of starch ethers areresistant to cleavage by acids, alkalies and mildoxidizing agents.Specific properties of carboxymethyl starch The carboxymethyl anionic substituents convertthe starch to a polyelectrolyte which shows in-creased solubility and viscosity in the absence ofelectrolytes (salts, acids).

Carboxymethyl starch can be insolubilized byreaction with polyvalent ions (such as aluminium,ferric and cupric ions) leading to precipitation orgelling of dispersions or insolubilization of films.Specific property of cationic starch ethersAs cationic macromolecules cationic starches areattracted to and retained by oppositely chargedparticles or surfaces (anionic materials) such ascellulose fibres and glass.Specific property of non ionic starch ethersThe starch acetates and the hydroxyethyl,hydroxypropyl and cyanoethyl ethers of starch arenonionic and their dispersions (pastes, solutions)are therefore not subject to the solubility andviscosity effects which dissolved electrolytes andwater hardness have on polyelectrolyte polymers.


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Starch modifications

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