초록
The β-galactosidase-catalyzed conversion of lactose to galactooligosaccharides offers an attractive approach for the efficient production of galactooligosaccharides. In this study, the β-galactosidase from Bacillus circulans was overexpressed in Escherichia coli BL21(DE3), and its extracellular secretion was characterized and optimized. Using initial parameters determined during fermentation tests in shake-flasks, extracellular expression was optimized in a 3-L fermentor. Three key parameters were studied: time of induction initiation, induction temperature, and inducer (lactose) feeding rate. Optimal extracellular β-galactosidase activity was observed when: induction was initiated when the optical density at 600nm reached 40; (2) expression was induced at 37<SUP>o</SUP>C; and (3) lactose was added at a constant feeding rate of 1.0gL<SUP>-1</SUP>h<SUP>-1</SUP>. Under optimal conditions, the extracellular activity reached 220.0UmL<SUP>-1</SUP>, which represents 65.0% of the total β-galactosidase activity expressed. This high-level production of soluble β-galactosidase in a semisynthetic medium provides a model for the industrial production of β-galactosidase.