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Increasing Slowly Digestible Starch Content of Normal and Waxy Maize Starches and Properties of Starch Products

¹Food Technology Program, Faculty of Science, Mahidol University, Kanchanaburi, Thailand. ²Department of Biotechnology, Faculty of Science, Mahidol University, Bangkok, Thailand. ³Whistler Center for Carbohydrate Research, Purdue University, West Lafayette, IN 47907. ⁴Corresponding author. Phone: 1-765-494-5684. Fax: 1-765-494-7953. E-mail: bemiller@purdue.edu

Changes in the digestibility and the properties of the starch isolated from normal and waxy maize kernels after heat-moisture treatment (HMT) followed by different temperature cycling (TC) or isothermal holding (IH) conditions were investigated. Moist maize kernels were heated at 80°C for 2 hr. The HMT maize kernels were subjected to various conditions designed to accelerate retrogradation of the starch within endosperm cells. Two methods were used to accelerate crystallization: TC with a low temperature of –24°C for 1 hr and a high temperature of 20, 30, or 50°C for 2, 4, or 24 hr for 1, 2, or 4 cycles, and IH at 4, 20, 30, or 50°C for 24 hr. The starch granules were then isolated from the treated kernels. The starch isolated from HMT normal maize kernels treated by TC using –24°C for 1 hr and 30°C for 2 hr for 2 cycles gave the greatest SDS content (24%) and starch yield (54%). The starch isolated from HMT waxy maize kernels treated by TC using –24°C for 1 hr and 30°C for 24 hr for 1 cycle had an SDS content of 19% and starch yield of 43%. The results suggest that TC after HMT changes the internal structure of maize starch granules in a way that results in the formation of SDS (and RS). They also suggest that thermal treatment of maize kernels is more effective in producing SDS than is the same treatment of isolated starch. All starch samples isolated from treated normal maize kernels exhibited lower peak viscosities, breakdown, and final viscosities and higher pasting temperatures than did the control (untreated normal maize starch). Although peak viscosities and breakdown of the starch isolated from treated waxy maize kernels were similar to those of the control (untreated waxy maize starch), their pasting temperatures were higher. The starch isolated from treated normal and waxy maize kernels with the highest SDS contents (described above) were further examined by DSC, X-ray diffraction, and polarized light microscopy. Onset and peak temperatures of gelatinization of both samples were higher than those of the controls. Both retained the typical A-type diffraction pattern of the parent starches. The relative crystallinity of the starch from the treated normal maize kernels was higher than that of the control, while the relative crystallinity of the starch from the treated waxy maize kernels was not significantly different from that of the control. Both treated starches exhibited birefringence, but the granule sizes of both starches, when placed in water, were slightly larger than those of the controls.