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. Addition￾ally, 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 poly￾mers 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 un￾Pericarp 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