A study of molecular structure and functional properties of rice starches

The objective of this research was to measure molecular structure and functional properties of isolated rice starch from 8 different rice varieties belonging to four different grain types which were grown in California (CA) and Texas (TX) in consecutive crop years (2002 and 2003).; Weight-averaged molecular weights (Mw) of starch were 0.52∼1.96 x 108 g/mol, Mw of amylopectin were 0.38∼2.50 x 108 g/mol and Mw of amylose were 2.20∼8.31 x 105 g/mol, respectively, in both crop years. This study showed that higher molecular weights of starch and amylopectin strongly correlated to lower amylose content. Branch chain length distribution of debranched amylopectin showed that F1 (long chain) fraction was proportional to amylose content. F2 (short chain) showed similar values for every sample's range of degree of polymerization (DPw), 19-22. No significant differences were observed in protein, Mw of starch, amylose, and distribution of chain-length amylopectin between 2002 and 2003 crops (p < 0.05) and between CA and TX.; The gelatinization and retrogradation properties were measured by differential scanning calorimetry (DSC), and the pasting properties were studied with controlled stress rheometer (CSR). The TX rices had a higher gelatinization onset temperature (Go) and peak temperatures (Gp) than the corresponding varieties grown in CA, but there was no difference in completion temperature (Gc) in the gelatinization study with DSC study. These results suggested that TX rice samples had a higher crystallinity than those of CA counterparts. Waxy starch showed a lower pasting onset temperature and a lower final viscosity than normal rice starch. The molecular structure and amylose content showed a relationship with the pasting properties and the retrogradation study. Pasting properties, gelation and retrogradation showed no significant differences between the two crop years (p < 0.05).; The combined information of the gelatinization study and, the pasting properties provided more detailed structural information during the starch-water interaction. The pasting onset temperature was located in between Gp and Gc in all the rice samples. The pasting peak temperature of normal rice starch, which occurs at maximum granule swelling, resulted after a complete loss of crystalline structure (Gc), but waxy rice starch was investigated before Gc.