The structural and hydration properties of heat-treated rice studied at multiple length scales

The impact of heat-treatment on structure and hydration properties of rice was studied at different length scales (μm–nm). Heat-treatment introduced micro- and macro-pores within rice kernels (μCT) and, within intact cell walls, disintegrated starch granules were observed (SEM, CSLM). In native kernels starch predominantly occurred as crystalline A-type starch and, upon heat treatment, amorphous and V-type starch appeared (XRD, 13C CP MAS NMR). Plasticization of amorphous starch by water was more pronounced for heat-treated than for native kernels (13C SPE MAS NMR). Within native kernels, more effective spin diffusion between water and starch chains was present (WISE-Exchange), confirming the inter-helical nanoscale order of amylose helices. Upon heat-treatment, this inter-helical nanoscale order was lost, as well as microscale granular compartmentalisation (TD NMR). These findings explain why, upon heat-treatment, vapour sorption is lower and starch is more prone to gelatinization (DSC).