Chemical Components Carbohydrates importance, both in their contribution to the The terminology surrounding carbohydrates structural and storage components of the grain frequently serves to confuse rather than to clarify and to the behaviour of grains and their pro- Archaic and modern conventions are often inter. ducts during processing. In this context the most mixed and definitions of some terms are incon important monosaccharide, because of its abund sistent with their use. Even the term carbohydrate nce, is the six-carbon polyhydroxyaldehyde itself is not entirely valid It originated in the belief that naturally occurring compounds of this class could be represented formally as hydrates of carbon. i.e. Cx(H2O)y. This definition is too rigid however as the important deoxy sugars like hamnose, the uronic acids and compounds such acid and phloroglucinol would qualify for inch.C as ascorbic acid would be excluded and aceti sion. Nevertheless the term carbohydrate remains to describe those polyhydroxy compounds which reduce Fehlings solution either before or after hydrolysis with mineral acids(Percival, 1962) It is customary to classify carbohydrates ccording to their degree of polymerization Thus: monosaccharides(l unit), oligosaccharides (2-20 units)and polysaccharides(>20 units HOC Monosaccharides are the simplest carbohy drates; most of them are sugars. monosaccharides may have 3-8 carbon atoms but only those with 5 carbons (pentoses) and 6 carbons(hexoses) are common. Both pentoses and ses exist in a number of isomeric forms, they may be polyhydroxyaldehydes or polyhydroxyketones Structurally, they occur in ring form which may be six-membered (pyranose form five-membered (furanose form FIG 3. 1 Structural representations of (1)xylose xylopyranose),(2)arabinose(alpha-L-arabinofurar In mature cereal the mers are of glucose(beta-Dglucopyranose),(4) fructose(beta-D little importance in their own right but, as furanose, (5)D-galacturonic acid,(6) ribose ribofuranose),(7)deoxyribose(beta-D-deoxyribofuranose), and components of polymers, they are of extreme (8)mannose(alpha-D-mannopyrane
3 Chemical Components Carbohydrates The terminology surrounding carbohydrates frequently serves to confuse rather than to clarify. Archaic and modern conventions are often intermixed and definitions of some terms are inconsistent with their use. Even the term carbohydrate itself is not entirely valid. It originated in the belief that naturally occurring compounds of this class could be represented formally as hydrates of carbon. i.e. C,(H,O),,. This definition is too rigid however as the important deoxy sugars like rhamnose, the uronic acids and compounds such importance, both in their contribution to the structural and storage components of the grain, and to the behaviour of grains and their products during processing. In this context the most important monosaccharide, because of its abundance, is the six-carbon polyhydroxyaldehyde: do HbH, HOCH, 4TQHHt HO H@H 1 tQoH CH,OH HfJcH,oH H:”;” HOCH, o HBOH I as ascorbic acid would be excluded and acetic ! ?H a, acid and phloroglucinol would qualify for inclu- (1) (2) sion. Nevertheless the term carbohydrate remains to describe those polyhydroxy compounds which reduce Fehlings solution either before or after It is customary to classify carbohydrates Thus: monosaccharides (1 unit), oligosaccharides (2-20 units) and polysaccharides (>20 units). Monosaccharides are the simplest carbohyb HO,CHz 0 4 hydrolysis with mineral acids (Percival, 1962). ‘;1 YH OH H according to their degree of polymerization. (3) (4) HOCH, o drates; most of them are sugars. Monosaccharides H OH OH OH may have 3-8 carbon atoms but only those with (5) (6) 5 carbons (pentoses) and 6 carbons (hexoses) are common. Both pentoses and hexoses exist in a number of isomeric forms, they may be polyhydroxyaldehydes or polyhydroxyketones. OH H HH H wy HO Structurally, they occur in ring form which (7) (8) may be six-membered (pyranose form) Or five-membered (furanose form). In mature cereal grains the monomers are of components of polymers, they are of extreme FIG 3.1 Structural representations of (1) xylose (beta-Dxylopyranose), (2) arabinose (alpha-L-arabinofuranose), (3) glucose (beta-D-glucopyranose), (4) fructose (beta-D-fructo- furanose, (5) D-galacturonic acid, (6) ribose (beta+ (8) mannose (alpha-D-mannopyranose). little importance in their Own right but, as ribofuranose), (7) deoxyribose (beta-D-deoxyribofurose), and 53
ECHNOLOGY OF CEREALS Olig The smallest oligosaccharide glycosidic link. Although this may appear to be association it is capable of considerable FIG 3. 2 Formation of the glycosidic link according to the configuration of the dic link and the position of the hydroxyl group involved in the bonding. Three important glucose. It is the monomeric unit of starch, variants among disaccharides involving only ellulose and beta-D-glucans a-D-glucopyranose are shown in Fig. 3.3 The most important pentoses are the poly. In these compounds the reducing group of only hydroxyaldehydes D-xylose and L-arabinose, one of the monosaccharide molecules is involved because of their contribution to cell wall polymers. in the glycosidic link and the reducing group of The structures of these compounds and of some the other remains functional other monosaccharides found in cereals are shown In sucrose, another important disaccharide found in plants, fructose and glucose residues are The most abundant derivatives of monosaccha- joined through the reducing groups of both rides are those in which the reducing group forms hence their reducing properties are lost. Sucrose a glycosidic link with the hydroxyl group of is readily hydrolyzed under mildly acid condi another organic compound(as in Fig. 3.2), fre- tions, or enzymically, to yield its component quently another molecule of the same species monomers which of course again behave as reduc- or another monosaccharide. Sugar molecules ing sugars. Sucrose is the main carbon compound may be joined to form short or long chains involved in translocating photosynthate to the by a series of glycosidic links thus producing grain. It features prominently during develop oligosaccharides or polysaccharides ment rather than in the mature grain because it CH2OH B-D-glucopyranosyl-(1-4)-a-D-glucopyranose),(3)isomaltose(a-D-glucopyrand pyrano
54 TECHNOLOGY OF CEREALS Oligosaccharides The smallest oligosaccharide, the disaccharide, comprises two sugar molecules joined by a glycosidic link. Although this may appear to be a simple association it is capable of considerable variation according to the configuration of the glycosidic link and the position of the hydroxyl group involved in the bonding. Three important D-glucose. It is the monomeric unit of starch, variants among disaccharides involving only cellulose and beta-D-ghcans. a-D-glucopyranose are shown in Fig. 3.3. The most important pentoses are the poly- In these compounds the reducing group of only hydroxyaldehydes D-xylose and L-arabinose, one of the monosaccharide molecules is involved because of their contribution to cell wall polymers. in the glycosidic link and the reducing group of The structures of these compounds and of some the other remains functional. other monosaccharides found in cereals are shown In sucrose, another important disaccharide in Fig. 3.1. found in plants, fructose and glucose residues are The most abundant derivatives of monosaccha- joined through the reducing groups of both; rides are those in which the reducing group forms hence their reducing properties are lost. Sucrose a glycosidic link with the hydroxyl group of is readily hydrolyzed under mildly acid condianother organic compound (as in Fig. 3.2), fre- tions, or enzymically, to yield its component quently another molecule of the same species monomers which of course again behave as reducor another monosaccharide. Sugar molecules ing sugars. Sucrose is the main carbon compound may be joined to form short or long chains involved in translocating photosynthate to the by a series of glycosidic links, thus producing grain. It features prominently during developoligosaccharides or polysaccharides. ment rather than in the mature grain because it -0 H -y + HO-I3 T A-I3 OH H20 FIG 3.2 Formation of the glycosidic link. CH20H HO HQOGw HO He0 Ho0 H OH H OH CHZOH H OH (1) (2) HO H OH I HO HQH H OH (3) FIG 3.3 Structural conformation of (1) maltose (a-D-ghcopyronosyl-( 1-+4)-a-D-glucopyranose), (2) cellobiose (~-~-glucopyranosyl-( 1+4)-a-~-glucopyranose), (3) isomaltose (a-D-glucopyranosyl- ( 1+6)-P-D-glucopyranose)
CHEMICAL COMPONENTS is converted during maturation, to structural and TablE 3.3 longer-term storage carbohydrates such as starch. Proportions of Soluble Sugars in Mill Fractions of rice* In sweet corn the sucrose content is higher by a Mill fraction of dry matter factor of 2-4 throughout grain development than in other types of maize at a similar stage, as the Rough 0.5-1-2 rate of conversion is slower(Boyer and Shannon, Hull 0.220.45 1983) Literature values for sugars in cereals va th 5.5-6.9 methods of analysis and with varieties examined Embryo and in consequence tables which bring together Data from Juliano and Bechtel, 1985 esults of different authors can be misleading Henry recently ana\ zu wo varieties of each of Polysaccharides six cereal species. All results were obtained b the same methods and are thus comparable Oligomers and polymers in which glucose values for free glucose and total (including that residues are linked by glycosidic bonds are known in sucrose and trisaccharides)are given in as glucans. The starch polymers, amylose and Table 3. 1 amylopectin, are glucans in which the a-(1-4)- link, as in maltose(Fig. 3.2), features Addition ally, in amylopectin the a-(1-6)link, as in Total Soluble Glucose and Fructose in Two Varieties of Each isomaltose(Fig. 3.3)occurs, giving rise to branch points. The same linkages are present in the other Barley Oat Rice Rye Triticale Wheat main storage carbohydrate found in sweet corn The product is known as phytoglycogen, it is 170.120.14 0090.130.19 0. 25 0.11 highly branched with a-(1-4) unit chain lengths 0.210.11 Fructose2.311.010.84 3.221.73 f 10-14 glucose residues and outer chains of 6-30 units(Marshall and Whelan, 1974). Unlike the true starch polymers phytoglycogen is largely Data from Henry, 1985 soluble in water and as a result the soluble saccharides of sweet corn contribute about 12% Free sugars are not distributed uniformly of the total grain dry weight. The starch polymers throughout the grain. The distribution in the are discussed at greater length in a later section maize grain is shown in Table 3.2 of this chapt The embryo has the highest concentration of In cellulose theβ(1→4) form of linkage,as free sugars in other cereals also. This is reflected present in cellobiose(Fig. 3. 3)occurs. B-Links in the distribution among mill fractions, as are also involved in the other important cell wall illustrated with respect to rice in Table 3.3 mers contribute about a quarter of the oly. components,(1→3,1→4)β-D- glucan. These poly walls of wheat aleurone but they are particularly Proportions of free Sugars in the r ABL52 natomical fractions of the important in oat and barley grains, in the starchy endosperm of which they may contribute as much as 70%(Fincher and Stone, 1986). With water grain part of dry matter they form viscous gums and contribute significantly 0.50.8 dietary fibre. Both the ratio of(1→3)to(1→4) 100-12.5 links and the number of similar bonds in an u 0.20.4 interrupted sequence differ between the species Whole grain 1.6l-2.22 Extraction and analysis of the mixed linkage com pounds are particularly difficult in the presence of Data from Watson, 1987. such large excesses of a-glucan(Wood, 1986)
CHEMICAL COMPONENTS 55 is converted during maturation, to structural and longer-term storage carbohydrates such as starch. In sweet corn the sucrose content is higher by a factor of 2-4 throughout grain development than in other types of maize at a similar stage, as the rate of conversion is slower (Boyer and Shannon, Milled 0.22-0.45 1983). Hull 0.6 TABLE 3.3 Proportions of Soluble Sugars in Mill Fractions of Rice* ill fraction % of dry matter Rough 0.5-1-2 Brown Bran 5.5-6.9 Embryo 0.7-1.3 8-12 Literature values for sugars in cereals vary with methods of analysis and with varieties examined and in consequence tables which bring together results of different authors can be misleading. Henry recently analyzed two varieties of each of Polysaccharides six cereal species. All results were obtained by the same methods and are thus comparable. Oligomers and polymers in which glucose Values for free glucose and total (including that residues are linked by glycosidic bonds are known in sucrose and trisaccharides) are given in as glucans. The starch polymers, amylose and Table 3.1. amylopectin, are glucans in which the ~~(1-4)- link, as in maltose (Fig. 3.2), features. Additionally, in amylopectin the a-(1+6)-linkY as in isomaltose (Fig. 3.3) OCCUrS, giving rise to branch points. The same linkages are present in the other main storage carbohydrate found in sweet corn. The product is known as phytoglycogen, it is Glucose 0.17 0.12 0.14 0.21 0.25 0.11 highly branched with a-( 1-4) unit chain lengths 0.09 0.13 0.19 0.29 o.21 O.ll of 10-14 glucose residues and outer chains of Fructose 2.31 1.01 0.84 5.79 3.22 1.73 1.98 1.00 0.75 5.11 3.05 2.46 6-30 units (Marshall and Whelan, 1974). Unlike the true starch polymers phytoglycogen is largely soluble in water and as a result the soluble saccharides of sweet corn contribute about 12% are discussed at greater length in a later section of this chapter. In cellulose the P-(1+4) form of linkage, as present in cellobiose (Fig- 3.3) occurs. P-Links are also involved in the other important cell wall components, ( 1-3, 1+4)-P-~-glucan. These polymers contribute about a quarter of the cell walls of wheat aleurone but they are particularly TABLE 3.2 important in oat and barley grains, in the starchy Proportions of Free Sugars in the Anatomical Fractions of the Maize Grain* endosperm of which they may contribute as much as 70% (Fincher and Stone, 1986). With water Grain part yoof dry matter they form viscous gums and contribute sigmficantly to dietary fibre. Both the ratio of (1-3) to (1-4) Endosperm 0.5-0.8 Embryo 10.0-12.5 links and the number of similar bonds in an unPericarp 0.2-0.4 interrupted sequence differ between the species. Extraction and analysis of the mixed linkage com- Tip cap 1.6 pounds are particularly difficult in the presence of such large excesses of a-glucan (Wood, 1986). * Data from Juliana and Bechtel, 1985. TABLE 3.1 Total Soluble Glucose and Fmctose in TWO Varieties of Each of Six Cereals* Barley Oat Rice Rye Triticale Wheat * Data from Henry, 1985. Free sugars are not distributed uniformly ofthe total grain dry weight. The starch Polymers throughout the grain. The distribution in the maize grain is shown in Table 3.2. The embryo has the highest concentration of free sugars in other cereals also. This is reflected in the distribution among mill fractions, as illustrated with respect to rice in Table 3.3. Whole grain 1.61-2.22 * Data from Watson, 1987
TECHNOLOGY OF CEREALS 4)-B-D-XYLp)-(-4)-B-D-XYLp)-(-4)-B-D-XYLp)-(-4)-B-D-XYL(p)- FIG 3.4 Structure of arabinoxylan of wheat aleurone and starchy endosperm cell walls. p, represents ne pyranose or 6-membered ring form; f, represents the furanose or 5-membere Pentosans which appear white when seen as a bulk powder While glucans are polymers of a single face. They have a refractive index of about 1.5 sugar species the common pentosans (polymers Specific gravity depends upon moisture content of pentose sugars) comprise two or more different species, each in a different isomeric form. Thus but it is about 1. 5. The mysteries of granule arabinoxylans, found in endosperm walls of wheat structure, development and behaviour have and other cereals, have a xylanopyranosyl back- exercized the minds of scientists for hundreds of bone to which are attached single arabinofuranosyl years and continue to do so. Granules from residues(Fig. 3. 4). different species differ in their properties and there is even variation in form among granules from the same storage organ Shape is determined Starch in part by the way that new starch is added to Starch is the most abundant carbohydrate in existing granules, in part by physicochemical all cereal grains, constituting about 64% of the conditions existing during the period of growth re wheat grain(about 70% and in part by composition of the endosperm), about 73% of the dry matter composition of the dent maize grain and 62% of the proso millet grain. It occurs as discrete granules of up The main way in which composition varies is to 30 um diameter and characteristic of the species the relative proportions of the two macro- In shape molecular species of which granules consist Starch granules are solid, optically clear bodies (Fig. 3.5) CH2OH CH2OH CH2OH OH CH2OH CH, OH FIG 3.5 Structural representation of amylose (i)and amylopectin(ui)
56 TECHNOLOGY OF CEREALS -4)-B-D-XYL(p)-(l-4)-~-D-XYL(p)-(I-4)-~-D-XYL(p)-(I-4)-~-D-XYL (p)- (I- 3 3 I I I I a-L-ARA(f) a-L-ARA(f) FIG 3.4 Structure of arabinoxylan of wheat aleurone and starchy endosperm cell walls. p, represents the pyranose or &membered ring form; f, represents the furanose or 5-membered ring form. Pentosans which appear white when seen as a bulk powder because of light scattering at the starch-air interWhi1e glucans are po1ymers Of a sing1e face. They have a refractive index of about 1.5. sugar species the common pentosans (polymers Specific gravity depends upon moisture content of pentose sugars) comprise two or more different but it is about 1.5. The mysteries of granule species, each in a different isomeric form. Thus structure, development and behaviour have arabinoxylans, found in endosperm walls of wheat exercized the minds of scientists for hundreds of and other cereals, have a xylanopyranosyl backyears and continue to do so. Granules from bone to which are attached single arabinofuranosyl different species differ in their properties and residues (Fig. 3.4). there is even variation in form among granules from the same storage organ. Shape is determined Starch in part by the way that new starch is added to existing granules, in part by physicochemical Starch is the most abundant carbohydrate in conditions existing during the period of growth all cereal grains, constituting about 64% of the and in part by composition. dry matter of the entire wheat grain (about 70% Composition of the endosperm), about 73% of the dry matter of the dent maize grain and 62% of the proso millet grain. It occurs as discrete granules of up The main way in which composition varies is to 30 pm diameter and characteristic of the species in the relative proportions of the two macroin shape. molecular species of which granules consist Starch granules are solid, optically clear bodies (Fig. 3.5). CH20H CH20H CH,OH CH20H ---o p&oQoQoQ O--- H OH H OH (i) H OH H OH CH 20H CH20H --.oJQ0q (Ii) 0 I CH2OH CH20H ---o ~o~o&oJF& 0 --- H OH H OH H OH H OH FIG 3.5 Structural representation of amylose (i) and amylopectin (ii)
CHEMICAL COMPONENTS Amylose comprises linear chains of(1-4) B chains-those to which A chains are attached linked a-D-glucopyranosyl residues. Amylopectin C chains chains which carry the only has, in addition, (1-6)tri-O-substituted residue les reducing group of the molecu acting as branch points. amylose has betwe The amylose contents of most cereal starches 1000 and 4400 residues, giving it a molecular lie between 20 and 35%, but mutants have been weight between 1.6 10 and 7. 1 x 10. In used in breeding programmes to produce culti solution amylose molecules adopt a helical form vars with abnormally high or low amylose con- and may associate with organic acids, alcohols or, tents. It is in diploid species such as maize and more importantly, lipids to form complexes in barley that such breeding has been most success- which a saturated fatty acid chain occupies the ful as polyploid species are more conservative core of the helix. Binding of polyiodide ions in with single mutations having less chance of the core in the same way is responsible for the expression(cf. Ch. 2). High amylopectin types characteristic blue coloration of starch by iodine are generally described as waxy as the appearance The average length of amylopectin branches is of the endosperms of the first mutants discovered 17-26 residues. As their reducing groups are had suggested a waxy composition. Waxy maize involved in bonding, only one is exposed The cultivars have up to 98% amylopectin(100% molecule is generally considered to consist of 3 according to some references). High amylose types of chain(Fig. 3.6) maize starches consist of up to 80% amylose a chains- side chains linked only via their reducing ends to the rest of the molecule Granular form Although some variation exists within species, there are many characteristic features by which TablE 3. 4 Characteristics of Starch Granules of cereals Cereal Shape and diameter Features Wheat Large, lenticular: 15-30 Characteristic 8 chain uatorial groove Triticale Large 已= 1-30 As wheat 10-40 As wheat, often cracks. Visible hilum 560A) Barley Small, spherical: 2-10 As wheat Oats ind, ovoid: granul R Comprising up to Maize in floury endosperm of (potato) amylopectin proposed 6-20; average 15 ternating 2 bands are amorphous Millet Spherical/angular: 4-12; As maize ced by courtesy of American Association of Cere ★ Based on Kent,1983
CHEMICAL COMPONENTS 57 Amylose comprises linear chains of (144) B chains - those to which A chains are attached. linked a-D-glucopyranosyl residues. Amylopectin C chains - chains which carry the only has, in addition, (1-6) tri-0-substituted residues reducing group of the molecule. acting as branch points. Amylose has between The amylose contents of most cereal starches 1000 and 4400 residues, giving it a molecular lie between 20 and 35%, but mutants have been weight between 1.6 x lo5 and 7.1 x lo5. In used in breeding programmes to produce cultisolution amylose molecules adopt a helical form vars with abnormally high or low amylose conand may associate with organic acids, alcohols or, tents. It is in diploid species such as maize and more importantly, lipids to form complexes in barley that such breeding has been most successwhich a saturated fatty acid chain occupies the ful as polyploid species are more conservative, core of the helix. Binding of polyiodide ions in with single mutations having less chance of the core in the same way is responsible for the expression (cf. Ch. 2). High amylopectin types characteristic blue coloration of starch by iodine. are generally described as waxy as the appearance The average length of amylopectin branches is of the endosperms of the first mutants discovered 17-26 residues. As their reducing groups are had suggested a waxy composition. Waxy maize involved in bonding, only one is exposed. The cultivars have up to 98% amylopectin (100% molecule is generally considered to consist of 3 according to some references). High amylose types of chain (Fig. 3.6): maize starches consist of up to 80% amylose. A chains - side chains linked only via their Granular form reducing ends to the rest of the molecule. Although some variation exists within species, there are many characteristic features by which TABLE 3.4 Characteristics of Starch Granules of Cereals* 0 _- - - - - - - - - - - - - - - Cereal Shape and diameter Features Wheat Large, lenticular: 15-30 Characteristic -____ _-__-_ (Pm) equatorial groove Small, spherical: 1-10 Angular where closely packed Triticale Large, lenticular, 1-30 As wheat Small, spherical: 1-10 Large lenticular: 10-40 As wheat, often displaying radial cracks. Visible hilum Barley Small, spherical: 2-10 As wheat Large, lenticular: 10-30 Small, spherical: 1-5 Compound, ovoid: Simple, angular: 2-10 Comprising up to 80 - - __-- up to 60 granuli Rice Compound granules comprising up to 150 angular granuli: 2-10 p Maize Spherical: In floury endosperm Angular: In flinty endosperm Both types 2-30; average 10 As maize FIG 3.6 Structure of (potato) amylopectin proposed by Sorghum Spherica"analar: 16-20; average 15 Robin et al. (1974). Bands marked 1 are considered to be Spherical/angular: 4-12; As maize average 7 crystalline while alternating 2 bands are amorphous. Reproduced by courtesy of American Association of Cereal Chemists. - - - - - -. - - - - - - - - - - - - - - - - - Millet, pearl * Based on Kent, 1983
TECHNOLOGY OF CEREALS FG3.7 electon micrograph of one large starch granule and numerous small starch granules arge granule shows the equatorial groove. From A D. Evers, Starke, 1971, 23: 15 Reproduced with permission of the Editor of Die Starke. an experienced microscopist can identify the cereals are similar in shape to the smaller popula source, either from observation of an aqueous tion of Triticeae granules, but rice and oats have suspension at room temperature or with the some compound granules in which many granuli additional help of observed changes when the fit together to produce large ovoid wholes. Shapes suspension is heated, leading to gelatinization at of high-amylose granules are varied and may be a temperature characteristic of the species and related to their individual composition. The later type (snyder, 1984). The characteristic blue developers tend to be filamentous, some resembling staining reaction with iodine/potassium iodide strings of beads. Characteristics of starch granules solution does not occur with waxy granules, from cereals are shown in Table 3.4 which contain virtually no amylose, they stain Within the endosperm of a species small differ brownish red to yellow. It is characteristic ences in granule shape may arise as a result for amylose percentage to increase during of packing conditions. These can be seen in endosperm development, consequently staining grains as mealy and vitreous(or horny)regions reactions change during growth In mealy regions, packing is loose and Granules of cereals from the Triticeae tribe(see adopt what appears to be their natural form. In Ch. 2)are of two distinct types. The larger ones horny regions close packing causes granules to are biconvex while the smaller ones are nearly become multi-faceted as a result of mutual pres spherical (Fig. 3.7). Granules from most other sure. Small indentations can also arise from other
58 TECHNOLOGY OF CEREALS FIG 3.7 Scanning electon micrograph of one large starch granule and numerous small starch granules of wheat. The large granule shows the equatorial groove. From A.D. Evers, Stiirke, 1971, 23: 157. Copyright by Leica U.K., Reproduced with permission of the Editor of Die Stiirke. cereals are similar in shape to the smaller population of Triticeae granules, but rice and oats have some compound granules in which many granuli fit together to produce large ovoid wholes. Shapes of high-amylose granules are varied and may be related to their individual composition. The later developers tend to be filamentous, some resembling strings of beads. Characteristics of starch granules from cereals are shown in Table 3.4. Within the endosperm of a species small differences in granule shape may arise as a result of packing conditions. These can be seen in grains as mealy and vitreous (or horny) regions. In mealy regions, packing is loose and granules adopt what appears to be their natural form. In horny regions close packing causes granules to become multi-faceted as a result of mutual pressure. Small indentations can also arise from other an experienced microscopist can identify the source, either from observation of an aqueous suspension at room temperature or with the additional help of observed changes when the suspension is heated, leading to gelatinization at a temperature characteristic of the species and type (Snyder, 1984). The characteristic blue staining reaction with iodine/potassium iodide solution does not occur with waxy granules, which contain virtually 'no amylose, they stain brownish red to yellow. It is characteristic for amylose percentage to increase during endosperm development, consequently staining reactions change during growth. Granules of cereals from the Triticeae tribe (see Ch. 2) are of two distinct types. The larger ones are biconvex while the smaller ones are nearly spherical (Fig. 3.7). Granules from most other
CHEMICAL COMPONENTS FIG 3. 8 Scanning electron micrograph of maize starch granules of spherical and angular types. Some angular granules show indentations due to pressure from protein bodies endosperm constituents such as protein bodies. labelled precursors incorporated into growing granules(Badenhuizen, 1969). Such a system Pitting on the surface can be caused by enzymic of growth allows for the change in shape that hydrolysis and it is possible to find such granules occurs in starches of the Triticeae, by preferential in some cereal grains in which germination has deposition on some parts of the surface. As a begun or in which insect damage has occurred. result they change from tiny spheres to larger There is no evidence that these two physical lentil shaped granules(Evers, 1971) modifications to granule form change the chemical Some structures not evident in untreated granules can be revealed or exaggerated by treat As granules are transparent some manifesta- ment with weak acid or amylolytic enzymes. In tions of internal structure can be detected, even cereal starches a lamellate structure results if their significance cannot be fully appreciated. from removal of more susceptible layers and One such internal feature is the hilum exhibited persistence of more resistant layers. Layers may by granules of some species. It is a small air be spaced progressively more closely towards the space, considered to represent the point of initia- outside. The number of rings appears to coincide tion around which growth occurred(Hall and with the number of days for which a granule Sayre, 1969). This assumes that granules grow grows(Buttrose, 1962 ). Lamellae cannot be by deposition of new starch material on the outer revealed in granules from plants grown under surface of existing granules, and indeed this has. conditions of continuous illumination(Evers been demonstrated by detection of radioactively 1979
CHEMICAL COMPONENTS 59 FiG 3.8 Scanning electron micrograph of maize starch granules of spherical and angular types. Some angular granules show indentations due to pressure from protein bodies. endosperm constituents such as protein bodies. (Fig. 3.8). Pitting on the surface can be caused by enzymic hydrolysis and it is possible to find such granules in some cereal grains in which germination has begun or in which insect damage has occurred. There is no evidence that these two physical modifications to granule form change the chemical properties of the granules. As granules are transparent some manifestations of internal structure can be detected, even if their significance cannot be fully appreciated. One such internal feature is the hilum exhibited by granules of some species. It is a small airspace, considered to represent the point of initiation around which growth occurred (Hall and Sayre, 1969). This assumes that granules grow by deposition of new starch material on the outer surface of existing granules, and indeed this has . been demonstrated by detection of radioactively labelled precursors incorporated into growing granules (Badenhuizen, 1969). Such a system of growth allows for the change in shape that occurs in starches of the Triticeae, by preferential deposition on some parts of the surface. As a result they change from tiny spheres to larger lentil shaped granules (Evers, 1971). Some structures not evident in untreated granules can be revealed or exaggerated by treatment with weak acid or amylolytic enzymes. In cereal starches a lamellate structure results from removal of more susceptible layers and persistence of more resistant layers. Layers may be spaced progressively more closely towards the outside. The number of rings appears to coincide with the number of days for which a granule grows (Buttrose, 1962). Lamellae cannot be revealed in granules from plants grown under conditions of continuous illumination (Evers, 1979)
TECHNOLOGY OF CEREALS Size distributions less than half the total starch present. Some 70% The literature contains many tables of granule all the amylose bu size ranges and size distributions of granules from must also include much of the amylopectin. The different botanical sources While such tables evidence of biochemical studies and electron microscopy has pointed to the existence of struc useful guides they do not all accord in detail and tures with a periodicity of 5-10 nm,reflecting the some fail to indicate the nature of the distribution. alternating crystalline and amorphous zones of For example the bimodal distribution of the Triticeae is not always indicated although this is amylopectin an important characteristic by which the source of a starch may be recognized. In wheats the Granule surface and minor components proportional relationship between large biconvex The distribution of amylose and amylopectin and small spherical granules is fairly constant molecules in one starch granule was estimated by (approx 70% large granules w/w), and this is the same for rye and triticale French (1984): for one spherical granule 15 um G In barley there is a wider variation, in part due in diameter, with a mass of 2.65 x 10 g there to the existence of more mutant types( Goering would be about 2.5 x 10% molecules of amylose et al., 1973). Among 29 cultivars, small granules (D P=1000, 25% of total starch)and 7.4 x 10 counted for between 6% and 30%of the total molecules of amylopectin(D P.= 100,000,75% of starch). If the molecular chains are perpend- icular to the surface of the granule there would be about 14 x 10 molecular chains terminating Granule organization at the surface. Of these 3.5 x 10 would be amylose molecules and the rest would be Under crossed polarizers starch granules amylopectin chains exhibit birefringence in the form of a maltese Surface characteristics of granules are also cross. This indicates a high degree of order affected by the minor components of starches within the structure. The positive sign of the Bowler et al.(1985) reviewed developments in birefringence suggests that molecules are organized work on these although they point out that this in a radial direction(French, 1984). Amylomaize is an under-researched area. Non-starch materials starch exhibits only weak birefringence of an present in commercial starch granules can arise unusual type(Gallant and Bouchet, 1986) from two sources. They may be inherent com Starch granules exhibit X-ray patterns, indicat- ponents of the granules in their natural condition ing a degree of crystallinity. Cereal starches or they may arise as deposits of material solubilized give an a pattern, tuber, stem and amylomaize or dispersed during the process by which the starches give a B pattern and bean and root starch is separated arches a c pattern the c pattern is considered The main non-starch components of starch to be a mixture of A and B. It is accepted that granules are protein and lipid amounts vary with the crystallinity is due to the amylopectin as it is starch type: in maize 0. 35% of protein (n X 6.25) shown by waxy granules. Furthermore, amylose is present on average Slightly more is present can be leached from normal granules without in wheat starch(0.4%). The most significant ffecting the X-ray pattern. The a and B patterns proteins in terms of their recognized effects or are thought to indicate crystals formed by double starch behaviour are amylolytic enzymes bound helices in amylopectin. The double helices occur to the surface Even traces of alpha-amylase can in the outer chains of amylopectin molecules, have drastic effects on pasting properties through where they form regions or clusters. The crystal- hydrolyzing starch polymers at temperatures up line parts of starch granules are responsible for to the enzymes inactivation temperatures many of the physical characteristics of the granules' SDS PAGE(sodium dodecyl sulphate, poly- structure and behaviour. Nevertheless they involve acrylamide gel elecrophoresis) showed surface
60 TECHNOLOGY OF CEREALS Size distributions less than half the total starch present. Some 70% is amorphous; this comprises all the amylose but must also include much of the amylopectin. The evidence of biochemical studies and electron microscopy has pointed to the existence of structures with a periodicity of 5-10 nm, reflecting the alternating crystalline and amorphous zones of amylopectin. Granule surface and minor components The distribution of amylose and amylopectin molecules in one starch granule was estimated by French (1984): for one spherical granule 15 pm in diameter, with a mass of 2.65 x lO-9 g there would be about 2.5 x lo9 molecules of amylose (D.P = 1000, 25% of total starch) and 7.4 x lo7 molecules of amylopectin (D.P. = 100,000, 75% of starch). If the molecular chains are perpendicular to the surface of the granule there would be about 14 x 10' molecular chains terminating at the surface. Of these, 3.5 x 10' would be amylose molecules and the rest would be Surface characteristics of granules are also affected by the minor components of starches. Bowler et al. (1985) reviewed developments in work on these although they point out that this is an under-researched area. Non-starch materials present in commercial starch granules can arise from two sources. They may be inherent components of the granules in their natural condition or they may arise as deposits of material solubilized or dispersed during the process by which the starch is separated. The main non-starch components of starch granules are protein and lipid. Amounts vary with starch type: in maize 0.35% of protein (N x 6.25) is present on average. Slightly more is present in wheat starch (0.4%). The most significant proteins in terms of their recognized effects on starch behaviour are amylolytic enzymes bound to the surface. Even traces of alpha-amylase can have drastic effects on pasting properties through hydrolyzing starch polymers at temperatures up to the enzymes' inactivation temperatures. SDS PAGE (sodium dodecyl sulphate, polyacrylamide gel elecrophoresis) showed surface The literature contains many tables of granule size ranges and size distributions of granules from different botanical sources. While such tables are useful guides they do not all accord in detail and some fail to indicate the nature of the distribution. For example the bimodal distribution of the Triticeae is not always indicated although this is an important characteristic by which the source of a starch may be recognized. In wheats the proportional relationship between large biconvex and small spherical granules is fairly constant (approx 70% large granules w/w), and this is the same for rye and triticale. In barley there is a wider variation, in part due to the existence of more mutant types (Goering et al., 1973). Among 29 cultivars, small granules accounted for between 6% and 30% of the total starch mass. Granule organization exhibit birefringence in the form of a maltese cross. This indicates a high degree of order within the structure. The positive sign of the birefringence suggests that molecules are organized in a radial direction (French, 1984). Amylomaize starch exhibits only weak birefringence of an unusual type (Gallant and Bouchet, 1986). Starch granules exhibit X-ray patterns, indicating a degree of crystallinity. Cereal starches give an A pattern, tuber, stem and amylomaize starches give a B pattern and bean and root starches a C pattern. The C pattern is considered to be a mixture of A and B. It is accepted that the crystallinity is due to the amylopectin as it is shown by waxy granules. Furthermore, amylose can be leached from normal granules without affecting the X-ray pattern. The A and B patterns are thought to indicate crystals formed by double helices in amylopectin. The double helices occur in the outer chains of amylopectin molecules, where they form regions or clusters. The crystalline parts of starch granules are responsible for many of the physical characteristics of the granules' structure and behaviour. Nevertheless they involve Under crossed polarizers starch granules amylopectin chains
CHEMICAL COMPONENTS proteins of wheat starch to have molecular masses of water available during cooking. Digestibilit of under 50 k while integral proteins were over in the intestines of single-stomached animals is 50 K. Altogether ten polypeptides have been also increased by gelatinization separated between 5 K and 149 K. The major 59 K polypeptide is probably the enzyme respon- Gelatinization sible for amylose synthesis. It has been shown to be concentrated in concentric shells within This is a phenomenon manifested as several granules. Two other polypeptides of 77 K and changes in properties, including granule swelling 86 K are likely to be involved in amylopectin and progressive loss of organized structure synthesis. Perhaps the most interesting of the(detected as loss of birefringence and crystallinity), surface proteins is that in the 15 K band. increased permeability to water and dissolved This has been found in greater concentration or bstances (including dyes), increased leaching starches from cereals with soft endosperm than of starch components, increased viscosity of the on those from cereals with hard endosperm. The aqueous suspension and increased susceptibility protein has been called friabilin,, because of its to enzymic digestic association with a friable endosperm(cf Ch. 4) At room temperature starch granules are not (Greenwell and Schofield, 1989) totally impermeable to water, in fact water uptake Phosphorus is another important minor con- can be detected microscopically by a small increase stituent of cereal starches. It occurs as a com- in diameter. The swelling is reversible and the onent of lysophospholipids. They consist of 70% wetting and drying can be cycled repeatedly lysophosphatidyl choline, 20% lysophosphatidyl without permanent change. If the temperature of ethanolamine and 10% lysophosphatidyl glycerol. a suspension of starch in excess water is raised The proportion of lysophospholipids to free fatty progressively, a condition is reached, around acids varies with species: in wheat, rye, triticale 60C, at whic ible swelling begins, and and barley over 90%occurs as lysophospholipids, continues with increasing temperature. The in rice and oats 70% and in millets and sorghum change is endothermic and can be quantified by 55%. In maize 60% occurs as free fatty acids thermal analysis techniques Removal of lipids from cereal starches reduces starch are: wheat 19.7, maize 18.0, waxy majay (Morrison, 1985) Typical heats of gelatinization in J per g of he temperatures of gelatinization-related changes 19.7 and high amylose maize 31.79(Maurice et and increases peak viscosity of pastes. In other al. 1983). Swelling involves increased uptake of words they become more like the lipid-free potato water and can thus lead to increased viscosity by reducing the mobile phase surrounding the gran ules; accompanying leaching of starch polymers Technological importance of starch into this phase can further increase viscosity. The swelling behaviour of starch heated in water is Much of the considerable importance of starch often followed using a continuous automatic in foods depends upon its nutritional properties; viscometer, such as the Brabender Amylograph it is a major source of energy for humans and for ( Shuey and Tipples, 1980). Upon heating a slurry domestic herbivorous and omnivorous animals. of 7-10% starch w/w in water at a constant rate In the human diet it is usually consumed in a of 10-5C per min, starch eventually gelatinizes cooked form wherein it confers attractive textural and begins to thicken the mixture The tempera qualities to recipe formulations. These can vary ture at which a rise in consistency is shown is from those of gravies and sauces, custards and called the pasting temperature. The curve then pie fillings to pasta, breads, cakes and biscuits generally rises to a peak, called the peak viscosity (cookies). Much of the variation in texture depends When the temperature reaches 95C, that tem upon the degree of gelatinization, which in turn perature is maintained for 10-30 min and stirring depends upon the temperature, and the amount is continued to determine the shear stability of
CHEMICAL COMPONENTS 61 proteins of wheat starch to have molecular masses of water available during cooking. Digestibility of under 50 K while integral proteins were over in the intestines of single-stomached animals is 50 K. Altogether ten polypeptides have been also increased by gelatinization. separated between 5 K and 149 K. The major Gelatinization 59 K polypeptide is probably the enzyme responsible for amylose synthesis. It has been shown to be concentrated in concentric shells within This is a phenomenon manifested as several granuies. Two other polypeptides of 77 K and changes in properties, including granule swelling 86 K are likely to be involved in amylopectin and progressive loss of organized structure synthesis. Perhaps the most interesting of the (detected as loss of birefringence and crystallinity), surface proteins is that in the 15 K band. increased permeability to water and dissolved This has been found in greater concentration on substances (including dyes), increased leaching starches from cereals with soft endosperm than of starch components, increased viscosity of the on those from cereals with hard endosperm. The aqueous suspension and increased susceptibility protein has been called 'friabilin', because of its to enzymic digestion. association with a friable endosperm (cf Ch. 4) At room temperature starch granules are not (Greenwell and Schofield, 1989). totally impermeable to water, in fact water uptake Phosphorus is another important minor con- can be detected microscopically by a small increase stituent of cereal starches. It occurs as a com- in diameter. The swelling is reversible and the ponent of lysophospholipids. They consist of 70% wetting and drying can be cycled repeatedly lysophosphatidyl choline, 20% lysophosphatidyl without permanent change. If the temperature of ethanolamine and 10% lysophosphatidyl glycerol. a suspension of starch in excess water is raised The proportion of lysophospholipids to free fatty progressively, a condition is reached, around acids varies with species: in wheat, rye, triticale 60"C, at which irreversible swelling begins, and and barley over 90% occurs as lysophospholipids, continues with increasing temperature. The in rice and oats 70% and in millets and sorghum change is endothermic and can be quantified by 55%. In maize 60% occurs as free fatty acids thermal analysis techniques. (Morrison, 1985). Typical heats of gelatinization in J per g of dry Removal of lipids from cereal starches reduces starch are: wheat 19.7, maize 18.0, waxy maize the temperatures of gelatinization-related changes 19.7 and high amylose maize 31.79 (Maurice et and increases peak viscosity of pastes. In other al., 1983). Swelling involves increased uptake of words they become more like the lipid-free potato water and can thus lead to increased viscosity by starch. reducing the mobile phase surrounding the granules; accompanying leaching of starch polymers into this phase can further increase viscosity. The swelling behaviour of starch heated in water is Technological importance of starch Much of the considerable importance of starch often followed using a continuous automatic in foods depends upon its nutritional properties; viscometer, such as the Brabender Amylograph it is a major source of energy for humans and for (Shuey and Tipples, 1980). Upon heating a slurry domestic herbivorous and omnivorous animals. of 7-10% starch w/w in water at a constant rate In the human diet it is usually consumed in a of 1°-5"C per min, starch eventually gelatinizes cooked form wherein it confers attractive textural and begins to thicken the mixture. The temperaqualities to recipe formulations. These can vary ture at which a rise in consistency is shown is from those of gravies and sauces, custards and called the pasting temperature. The curve then pie fillings to pasta, breads, cakes and biscuits generally rises to a peak, called the peak viscosity. (cookies). Much of the variation in texture depends When the temperature reaches 95"C, that temupon the degree of gelatinization, which in turn perature is maintained for 10-30 min and stirring depends upon the temperature, and the amount is continued to determine the shear stability of
TECHNOLOGY OF CEREALS Temperoture, C recently been found that is resistant to enzyme attack. Known as resistant starch, it behaves as Peak viscosity back viscos dietary fibre and is most abundant in autoclaved Setback amylomaize starch suspensions(Berry, 1988) setback Starch damage(see Chs 6 and 8) Granule damage of a pa properties of starch in some ways similar to gelatinization. Defining the exact type of damage is difficult and this accounts for the continued use of the general term. The essential characteris tics associated with damaged starch are somewhat Brabender arm Sogang. characteristics recorded oy the similar to gelatinized granules but there are differences also. Thus mechanical damage results the starch. Finally the paste is cooled to 30oC 1, increased capacity to absorb water, from and the increase in consistency is called set-back (Fig.3.9) 0.5-fold starch dry mass when intact to 3-4fold when damaged (gelatinized granules absorb as much as 20-fold) Retrogradation (see also Ch 8) ncreased susceptibility to amylolysis; Suspensions of gelatinized granules containing 3. loss of organized structure manifested as loss more than 3% starch form a viscous or semi-solid of X-ray pattern, birefringence, differential starch paste which, on cooling, sets to a gel. Three scanning calorimetry gelatinization endotherm dimensional gel networks are formed from the 4. reduced paste viscosidisoiaized granules amylose-containing starches by a mechanism 5. increased solubility, leading to leaching of known as 'entanglement. The relatively long amylose molecules that escape from the swollen amylose is preferentially leached( Craig and granules into the continuous phase become er angled at a concentration of 1-1. 5% in water. At a molecular level the disorganization of On cooling the entangled molecules lose transla- granules appears to be accompanied by fragmenta- tional motion, and the water is trapped in the tion of amylopectin molecules during damage network. Crystallites begin to form eventually at whereas gelatinization achieves loss of organization junction zones in the swollen discontinuous phase, without either polymer being reduced in size causing the gel slowly to increase in rigidity of wheat flour is important as it affects the amount Controlling starch damage level during milling (Osman, 1967). When starch gels are held for prolonged periods, retrogradation sets in. As of water needed to make a dough of the required applied to starch this means a return from a consistency( see ch 7)(Evers and Stevens, 1985) soluble, aggregated or crystalline condition Retrogradation is due largely to crystallization of Cell walls amylose, which is much more rapid than that The older literature describes the components of amylopectin. It is responsible for hardening of of cereal grain cell walls as pentosans and hemi- cooked rice and shrinkage and syneresis of starch celluloses. Pentosans are defined earlier in this gels and possibly firming of bread. Although chapter, but hemicelluloses are more difficult to regarded as crystalline, retrograded gels are define and indeed the term is even now only used susceptible to amylolysis, however a fraction has loosely Hemicelluloses were originally assumed
62 TECHNOLOGY OF CEREALS Temperature, "C recently been found that is resistant to enzyme SVSU I Porta I Hold I cool attack. Known as resistant starch, it behaves as dietary fibre and is most abundant in autoclaved amylomaize starch suspensions (Berry, 1988). 30 55 95 ~ 95 53 Setback viscosity, C Peak viscosity 0 v) > c - Starch damage (see Chs 6 and 8) x f 4J Granule damage of a particular type alters the properties of starch in some ways similar to gelatinization. Defining the exact type of damage is difficult and this accounts for the continued use of the general term. The essential characteristics associated with damaged starch are somewhat similar to gelatinized granules, but there are differences also. Thus mechanical damage results in: 1. increased capacity to absorb water, from 0.5-fold starch dry mass when intact to 34fold when damaged (gelatinized granules absorb as much as 20-fold); E a 2 40 60 90 Time (mid FIG 3.9 Chart showing characteristics recorded by the Brabender Amylograph. the starch. Finally the paste is cooled to 30°C and the increase in consistency is called set-back. (Fig. 3.9) Retrogradation (see also Ch. 8) 2. increased susceptibility to amylolysis; 3. loss of organized structure manifested as loss Suspensions of gelatinized granules containing of X-ray pattern, birefringence, differential more than 3% starch form a viscous or semi-solid scanning calorimetry gelatinization endotherm; starch paste which, on cooling, sets to a gel. Three 4. reduced paste viscosity; dimensional gel networks are formed from the 5. increased solubility, leading to leaching of amylose-containing starches by a mechanism mainly amylopectin. (In gelatinized granules, known as 'entanglement'. The relatively long amylose is preferentially leached (Craig and Stark, 1984).) amylose molecules that escape from the swollen granules into the continuous phase become entangled at a concentration of 1-1.5% in water. At a molecular level the disorganization of On cooling the entangled molecules lose transla- granules appears to be accompanied by fragmentational motion, and the water is trapped in the tion of amylopectin molecules during damage network. Crystallites begin to form eventually at whereas gelatinization achieves loss of organization junction zones in the swollen discontinuous phase, without either polymer being reduced in size. causing the gel slowly to increase in rigidity Controlling starch damage level during milling (Osman, 1967). When starch gels are held for of wheat flour is important as it affects the amount prolonged periods, retrogradation sets in. As of water needed to make a dough of the required applied to starch this means a return from a consistency (see Ch. 7) (Evers and Stevens, 1985). solvated, dispersed, amorphous state to an Cell walls insoluble, aggregated or crystalline condition. Retrogradation is due largely to crystallization of amylose, which is much more rapid than that The older literature describes the components of amylopectin. It is responsible for hardening of of cereal grain cell walls as pentosans and hemicooked rice and shrinkage and syneresis of starch celluloses. Pentosans are defined earlier in this gels and possibly firming of bread. Although chapter, but hemicelluloses are more difficult to regarded as crystalline, retrograded gels are define and indeed the term is even now only used susceptible to amylolysis, however a fraction has loosely. Hemicelluloses were originally assumed