Effects of alpha-glucanotransferase treatment on the thermo-reversibility and freeze-thaw stability of a rice starch gel

Abstract

This study was designed to investigate the effect of a thermostable a-glucanotransferase (TS alpha GT) originated from Thermus scotoductus on the rheological properties, molecular weight distribution, thermo-reversibility, and freeze-thaw stability of rice starch paste. Rice starch paste samples (5%) were incubated at 70 degrees C with TSaGT and a commercial alpha-amylase, respectively, until reduction of moduli leveled off. The TS alpha GT-modified rice starch paste showed a yield stress. After the enzymatic treatment, both enzyme-treated pastes showed considerably low moduli with higher G" than G', indicating a significant liquefaction. However, when stored at 4 degrees C, both moduli of a TS alpha GT-treated sample dramatically increased with significantly higher G' than G", confirming that a solid gel structure formed. This transition was found to be reversible, when temperature fluctuated between 70 and 4 degrees C. The control starch gel did not show thermo-reversibility, and alpha-amylase- treated starch paste was incapable of forming a solid gel. The size distribution of the starch polymers, as measured by MALLS, revealed that average M-w reduced to about 5 X 105 after the TS alpha GT treatment, which was a significant decrease, but possibly high enough to form a solid gel matrix at low temperatures. The Ts alpha GT-treated gel demonstrated highly improved freeze-thaw stability.