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

This study was designed to investigate the effect of a thermostable α-glucanotransferase (TSαGT) 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 °C with TSαGT and a commercial α-amylase, respectively, until reduction of moduli leveled off. The TSαGT-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 °C, both moduli of a TSαGT-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 °C. The control starch gel did not show thermo-reversibility, and α-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 Mw reduced to about 5×105 after the TSαGT treatment, which was a significant decrease, but possibly high enough to form a solid gel matrix at low temperatures. The TsαGT-treated gel demonstrated highly improved freeze-thaw stability.