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Retrogradation of dextrins during in vitro digestion of native starch granules
J. HASJIM (1), A. Teng (1), K. Wang (1), T. Witt (1). (1) The University of Queensland, Centre for Nutrition and Food Sciences, Queensland Alliance for Agriculture and Food Innovation, Brisbane, Australia

The molecular and crystalline structures of native starch granules were monitored during an <i>in vitro </i>digestion to understand the changes in starch structure during digestion and the structure that contributes to slow digestion properties. Four types of native starch granules were used, which are waxy maize, normal maize, high-amylose maize, and normal potato starches. Size exclusion chromatography (SEC) was used to characterize the whole (fully branched) starch molecules and their individual (linear) branches after enzymatic debranching. X-ray diffractometry (XRD) and differential scanning calorimetry (DSC) were used to analyze starch crystalline structure. Waxy and normal maize starch granules were hydrolyzed almost to completion (~100%) after 8 and 24 hours of digestion, respectively, whereas high-amylose maize and normal potato starch granules reached 82% and 71% degrees of hydrolysis, respectively, after 48 hours of digestion. The SEC weight distributions of whole molecules show that amylose and amylopectin molecules in all starch samples were hydrolyzed to much smaller dextrins (<i>R<sub></i>h</sub> ~2.5 nm). The changes in the individual branches after digestion were less apparent. The XRD results show that the crystalline pattern did not change after digestion because of the substantial amount of remaining native crystalline structure. However, the DSC results show that the gelatinization temperature of waxy and normal maize starches increased with digestion times and the presence of retrograded starch with melting temperature > 125ºC became apparent after digestion, whereas these phenomena were less noticeable for high-amylose maize and normal potato starches. The results suggest that the small dextrins can retrograde into highly stable crystalline structure during digestion and the amount of retrograded starch is greater in the rapidly digested starch granules.

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