MORPHOLOGY OF STARCH GRANULES IN CEREAL GRAINS AND MALTS

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Abstract
During malting, amylases have limited action on large starch granules of barley endosperm but rapidly degrade the small granules. In contrast, the small starch granules of wheat endosperm are resistant to enzymic attack. High levels of exogenous gibberellic acid increase the production of α-amylase and encourage the appearance of radial channels in the partially-degraded large starch granules. These endo-corroded granules are mainly found in the proximal (embryo) half of the endosperm where levels of α-amylase are much higher than at the distal end. Degradation of malt starch can therefore result from enzymic attack both outside and inside the granules. Malting of barley reduces the population of small starch granules which are slower to gelatinize than large granules at the infusion mashing temperatures of 65° C. During germination of barley multiple starch granules are rapidly synthesized in single amyloplasts in the scutellum. The endosperm of high amylose barley is devoid of small starch granules and the average size of the large granules is reduced. Steeliness in sorghum is related to the close packing of the starch-protein matrix rather than to unequal distribution of protein. The significance of these results is discussed, particularly in relation to the morphology of starch granules, the nature of their outer covering, the distribution of amylopectin and amylose within the granule, and the site of enzymic attack.