Fine Structures of Amylose and Amylopectin

Amyloses of five commercially important starches (wheat, corn, rice, tapioca and potato) were purified by fractional crystallization of their amylose-alcohol complexes and shown to be free of amylopectin.111,205 All five amyloses showed comparable iodine affinity, blue value and max of their polyiodide complexes (Table 10.5), which is consistent with their being (1 —4)-linked a-glucans with an average DPn of —500 or above.206 The average size of amylose molecules in cereal starches is smaller than 

Commercial sample, Type II, Sigma Chemical Co., St. Louis, Missouri 

Property Corn Wheat Potato Rice (Indica) Rice (Japonica) 

The polymodal distribution of unit chains and their proportions in amylopectin support the cluster model. The cluster model of amylopectin suggests a repeat structure made up of one population of A chains, another of relatively short B chains (Bj), and three populations or more of long B chains (B2, B3, B4, B5). The various populations of chains appear to be separated with a periodicity of 12, so that A chains have been assigned to the group of chains with DPn 6-12, B] chains = DP 13-24, B2 chains = DP 25-36 and (Bn)n 3 to chains 37.225 The weight-percent 

The A plus Bj chains dominate the chain length distributions in amylopectins both by weight (58-74%) and by number (85-93%), compared with the sum of the long B chains (see Table 10.8). The A and Bj chains are believed to occur within one cluster, while the B2 and B3 chains transverse two and three clusters, respectively. The B4 chains and beyond are small in number and were not detected by the end group labeling protocol. However, the weight-percent of B4 chains with DPn above —50 (DPW —80) is measurable.217,224,225 

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All chapters/subjects that were also in the previous edition have been updated. Chapters have been added on the biochemistry and molecular biology of starch biosynthesis, structural transitions and related physical properties of starch, and cyclo-dextrins. There are two chapters on the structural features of starch granules that present not only advances in understanding the organization of starch granules, but also advances in understanding the fine structures of amylose and amylopectin, both of which are based on techniques that have been developed since 1984. 

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