Variation in crystalline type with amylose content in maize starch granules: an X-ray powder diffraction study

Comparative studies of native maize starches with different amylose contents were carried out using X-ray powder diffraction. The results show a transition of crystalline type from A through C to B, accompanying a decrease in degree of crystallinity from 41.8% to 17.2% across a range of apparent amylose content from 0% to 84%. Hydration induces an increase in degree of granule crystallinity, but does not change the transition of crystal type. Progressively from A-type to C-type, crystallinity decreases rapidly with an increase in amylose content. From C-type to B-type, overall crystallinity decreases more slowly. The crystal type is strongly dependent on amylose content and on average chain length of the respective amylopectin. Waxy A-types have an average chain length of about 20, while in high amylose B-types this rises to approximate to 35. The proportion of short chains (similar to 10-13 glucose units) appears to affect crystal type significantly. Some V-type material was detected at high amylose levels. The proportion of this increased after prolonged exposure of the granules to iodine vapour. Implications for the arrangement of starch components in the granule are discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.