Effect of enzymatic and heat-moisture treatments on the physical and structural properties of granular starches

Abstract

The glucosyl-transferase changes the amylose contents, amylose-amylopectin ratio, and the size of branched-chain of amylopectin in starch, and these factors influence the physical and structural properties of starch. Corn, sorghum, and wheat starches were treated with α- 1,4-α- 1,4-glucosyl transferase from Thermocus aquaticus YT1 (TAαGT) to change the physical and structural properties of starch.. The amount of amylose was measured using colorimetric method by amylose-iodine complex, and the change of wave-scanning pattern was measured in the range of 400 - 800 nm. Enzymatically treated starches showed lower levels of amylose content than raw starches, and the patterns of wave-scanning were turned to more amylopectin-like patterns. Thermal properties of enzymatically treated starches were measured by differential scanning calorimetery (DSC). DSC profiles showed that enzymatically treated starches showed lower gelatinization enthalpy, higher peak temperature, and broader melting temperature area than raw starches. The size distribution of enzymatically treated starch was measured by high-performance size exclusion chromatography (HPSEC). The results showed that the size of amylopectin was shorter and more variable than raw starches, and the intermediate molecular weight (IMW) and low molecular weight (LMW) fractions were increased. The rheological behavior was measured by using an oscillatory rheometer. The results of frequency sweep measurement showed that the gel of enzymatically treated starch had higher storage modulus (G) and loss modulus (G) than raw starches, and the structure of the enzymatically treated starch gels was collapsed during temperature sweep measurements. The results of dynamic viscoelasticity during heating and cooling cycle showed that the solid-like structure of enzymatically treated starch at room temperature turned to liquid-like structure at 70oC and the liquid-like structure turned to solid-like structure relatively reversibly at room temperature. Thus enzymatically treated starches could be used in the starch-containing food system as the anti-staling materials and the fluidity-enhancing materials. For the more precise study of structural properties, the fine structure of amylopectin shoud be investigated.