Structure‐Viscosity Relationships for Thai Rice Starches

Abstract

Abstract The viscosities of five Thai rice starches with different amylose contents were measured by rotational rheometry and using a Rapid Visco Analyser. The viscosity response was interpreted in terms of the swelling volume and the structural properties of the granule determined by calorimetry, infrared spectroscopy and X‐ray diffraction. It was found that the waxy rice starches had a higher swelling volume resulting in a higher viscosity than the other rice classes. Critical concentrations for close packing, identified from the concentration where the flow behaviour departs from Newtonian, were 2.5%, 1.5% and 1.10% for the high amylose, medium amylose and waxy rice starch, respectively. An exception to this was a commercial high amylose starch that had a critical concentration similar to the medium amylose category. This was ex‐plained in terms of a relatively higher loss of short‐range molecular order on gelatini‐sation as shown by FTIR for this starch. Medium amylose Jasmine rice starch, extracted from rice that had been aged for 5 months, showed differences in granule size, crystallinity, pasting profile and enthalpy of gelatinisation when compared with the results for starch extracted from freshly harvested rice. This was provisionally interpreted in terms of granule damage during the extraction process, though more work would be required to confirm this interpretation.