Enzymatic modification of raw corn starch using thermostable glucoamylase from Thermothelomyces thermophilus

Abstract

Enzymatic modification is an important approach to changing the structure and expanding industrial application of natural starch. In the process of starch modification, thermostable enzymes are favored owing to their compatibility with higher reaction temperatures and lower required dosage. In this study, thermostable glucoamylase from Thermothelomyces thermophilus (TtGA) was heterologously expressed in Pichia pastoris, and its effects on the structure and physicochemical properties of raw corn starch were determined. The purified TtGA had a molecular weight of approximately 66 kDa, and its optimum reaction temperature and pH were 50 °C and 5.0, respectively. TtGA retained ＞ 60% of its activity following treatment at 60 °C for 2 h and remained stable within a pH range of 4.0–7.0 for 6 h. The enzymatic modification of raw corn starch with TtGA led to 3% hydrolysis at 50 °C for 24 h. In comparison with natural raw corn starch, TtGA-modified starch had a smaller particle size with an unchanged crystalline structure, increased relative crystallinity, and amylose content. Scanning electronic observation showed that larger pores were formed on the surface of starch particles, and Fourier-transform infrared spectroscopy indicated that TtGA increased the degree of order in the raw corn starch. TtGA modification caused enhanced viscosity of the raw corn starch and altered the rheological properties with decreases in storage and loss moduli, as well as shear viscosity. Moreover, TtGA treatment enhanced the thermal characteristics of the raw corn starch, and decreased gelatinization enthalpy. This study provides detailed evidence for TtGA modification of raw corn starch, which would be helpful for its practical utilization in starch modification.