Abstract
Starch and flour from the same rice grain source (with 20, 25 and 30% moisture content) were exposed to heat–moisture treatment (HMT) at 100 °C for 16 h in order to investigate whether there were differences in their susceptibility to modification by HMT and, if any, to determine the main causes of the differences. HMT had a far greater effect on paste viscosity of flour than of starch. A significant increase in paste viscosity after removal of proteins from HMT flour – as well as images of fast green-stained HMT flour gels – indicated that an important role was played by proteins in affecting properties of the modified samples. Greater effects of HMT on thermal parameters of gelatinization and gel hardness values of flours were observed – more so than those for starches. Following this observation, it was ascertained that components in rice flour other than rice starch granules also underwent alterations during HMT.
These studies indicated that structural rearrangement and starch-chain associations contributed to these changes. In addition, phase separation between amylose and amylopectin, compaction of granular matter by vapor pressure force, and chemical bonding/interactions that occur during HMT might be other factors influencing the strength of HMT starch (Watcharatewinkul et al.,