Understanding resistant-starch formation during drying high-amylose maize kernels

Interests in using high-amylose maize (HAM) flour and starch for low glycemic index foods continue to grow. The objective of this work was to understand resistant-starch formation during drying the HAM kernels. Freshly harvested HAM kernels with 28.2 % initial moisture were subjected to sun drying (30 degrees C) or hot-air drying at 50 degrees C, 70 degrees C, 90 degrees C, or 110 degrees C. The enzymatic digestibility of HAM flour decreased from 63.6 % to 41.1 % as the drying temperature increased from 30 degrees C to 110 degrees C. The swelling power, solubility, and overall viscosity of HAM flours milled from kernels dried at 110 degrees C decreased, whereas the peak and conclusion gelatinization temperatures, enthalpy change, and relative crystallinity increased compared to those of flours from kernels dried at 30 degrees C, 50 degrees C, 70 degrees C, and 90 degrees C. Light microscopic and scanning electron microscopic images showed that starch granule aggregation in HAM flour increased with increasing drying-temperatures. The aggregates remained after 16 h enzymatic hydrolysis of cooked HAM flours. These results suggested that the increase of enzymatic resistance of HAM flour resulted from the formation of high temperature-resistant ordered structures in starch granules and the starch aggregates less accessible to enzymatic hydrolysis.