The characterization of Aspergillus oryzae β-galactosidade for lactose conversion: response surface methodology approach

The enzyme β-galactosidase (β-gal) is not only used for the production of hydrolyzed milk, but has also been studied in various industrial processes such as the production of galacto-oligosaccharides. In this study, considering the possible use of the β-gal extracted from Aspergillus oryzae in the lactose bioconversion process, the optimum value of the temperature and pH for the activity of this enzyme in aqueous solution has been determined. We also determined the optimum value for its thermal stability in aqueous solutions with different temperatures, for processes either being with or without the reuse of the enzyme. The experiments were carried out as a Central Composite Design and evaluated using Response Surface Methodology. According to the polynomial model adjusted, higher values of the specific activity of β-gal (287,594–310,672 U g-1) were predicted for temperatures between 50 and 60°C and pH 4.5. After the experimental validation of this result, the stability of the enzyme was evaluated at different temperatures (phosphate buffer pH 4.5). It was observed that there was a complete loss of the enzymatic activity at 60 °C and only a 30% loss of the residual activity after 3 hours of incubation at 55 °C. On the other hand, at temperature of 50°C, the residual activity was about 64% and 44% after 9 and 24 hours of incubation, respectively. At storage temperatures equal to or lower than 30 °C, a residual activity of 65% after 216 hours of incubation was observed. It indicates the use of these temperatures for long-term processes of lactose conversion and/or for processes with reuse of the enzyme. In order to maximize the lactose bioconversion into galacto-oligosaccharides using the A. oryzae β-galactosidase, and with the conditions analyzed and presented in this article, a temperature of 50 °C and pH of 4.5 were suggested.