Nwh. Cheetham et Lp. Tao, VARIATION IN CRYSTALLINE TYPE WITH AMYLOSE CONTENT IN MAIZE STARCH GRANULES - AN X-RAY-POWDER DIFFRACTION STUDY, Carbohydrate polymers, 36(4), 1998, pp. 277-284
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.