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Molecular Structure and Organization of Starch Granules from Developing Wheat Endosperm

¹Department of Food Science, University of Guelph, Guelph, ON, N1G 2W1, Canada. ²Department of Food Science and Nutrition, University of Minnesota, St. Paul, MN 55108, U.S.A. ³Corresponding author. Phone: +358 (44) 0611530. Fax: (612) 625-5272. E-mail: eric.bertoft@abo.fi ⁴Deceased; formerly Department of Food Science and Nutrition, University of Minnesota, St. Paul, MN, U.S.A.

Wheat starches isolated from seeds harvested between 7 and 49 days after anthesis (DAA) were fractionated into large (>8 μm) and small (<8 μm) granules and studied for starch structure and architecture. Starch granules at 7 DAA possessed unimodal size distribution, whereas it was bimodal at later maturity stages. The apparent amylose fraction of starch granules at early maturity (7 and 14 DAA) consisted of intermediate-type materials, whereas starch at later maturity stages (28 and 49 DAA) contained branched amylose. Wide-angle X-ray scattering (WAXS) revealed a well-developed polymorphic structure already at 7 DAA. Although the presence of a small proportion of B-type crystallites mixed with A-type crystallites was observed in the X-ray diffractogram of starches at early maturation (7 and 14 DAA), it was masked by the A-type crystallites at later maturity stages. However, the large granules had a higher proportion of B-type crystallites and lower relative crystallinity (RC) than their small-granule counterpart. The iodine absorption properties of the starch granules demonstrated different levels of mobility of the starch polymers at different stages of maturity and the mobility of more glucan polymers in the large granule population compared with the small granules at the same maturity stage. Iodine did not change the characteristic A-type crystalline pattern of starch, but it increased RC. Changes in peak width at half height based on WAXS data further suggested the possible interaction of iodine with amylopectin intercluster chain segments and branch chains in formation of inclusion complexes.