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