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 co ntents were carried out using X-ray powder diffraction. The results sh ow a transition of crystalline type from A through C to B, accompanyin g a decrease in degree of crystallinity from 41.8% to 17.2% across a r ange of apparent amylose content from 0% to 84%. Hydration induces an increase in degree of granule crystallinity, but does not change the t ransition of crystal type. Progressively from A-type to C-type, crysta llinity decreases rapidly with an increase in amylose content. From C- type to B-type, overall crystallinity decreases more slowly. The cryst al type is strongly dependent on amylose content and on average chain length of the respective amylopectin. Waxy A-types have an average cha in length of about 20, while in high amylose B-types this rises to app roximate to 35. The proportion of short chains (similar to 10-13 gluco se units) appears to affect crystal type significantly. Some V-type ma terial was detected at high amylose levels. The proportion of this inc reased after prolonged exposure of the granules to iodine vapour. Impl ications for the arrangement of starch components in the granule are d iscussed. (C) 1998 Elsevier Science Ltd. All rights reserved.