Increasing slowly digestible starch content of normal and waxy maize starches and properties of starch products.

摘要

Changes in the digestibility and properties of starch isolated from normal and waxy maize (corn) kernels after heat-moisture treatment (HMT) followed by different temp. cycling (TC) or isothermal holding (IH) conditions were investigated. Moist maize kernels were heated at 80 degrees C for 2 h. HMT maize kernels were subjected to various conditions designed to accelerate retrogradation of starch within endosperm cells. 2 methods were used to accelerate crystallization: TC with a low temp. of -24 degrees C for 1 h and a high temp. of 20, 30 or 50 degrees C for 2, 4 or 24 h for 1, 2 or 4 cycles; and IH at 4, 20, 30 or 50 degrees C for 24 h. Starch granules were then isolated from treated kernels. Starch isolated from HMT normal maize kernels treated by TC using -24 degrees C for 1 h and 30 degrees C for 2 h for 2 cycles gave the greatest SDS content (24%) and starch yield (54%). Starch isolated from HMT waxy maize kernels treated by TC using -24 degrees C for 1 h and 30 degrees C for 24 h for 1 cycle had an SDS content of 19% and starch yield of 43%. 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 temp. than 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 temp. were higher. Starch isolated from treated normal and waxy maize kernels with the highest SDS contents was further examined using DSC, X-ray diffraction and polarized light microscopy. Onset and peak temp. 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 starch from treated normal maize kernels was higher than that of the control, while the relative crystallinity of starch from the treated waxy maize kernels was not markedly 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.