A complete specific cleavage of glucosyl and ester linkages of stevioside for preparing steviol with a β-galactosidase from Sulfolobus solfataricus

β-Galactosidases from Sulfolobus solfataricus have been used to synthesize galactooligosaccharide and lactulose. In this work, a β-galactosidase from S. solfataricus with weak β-glucosidase activity but high lipase activity was employed as catalyst to assist hydrolysis of stevioside to obtain steviol, an important starting reagent of synthetic bioactive materials and the main metablite of stevioside in human digistion. The β-galactosidase presented a strict substrate specifity on converting stevioside to steviol in a stoichiometric yield. The β-galactosidase favors the cleavage of glycoside linkages prior to cleavage of glycosyl ester linkage. The hydrolysis is external diffusion controlled and hence has to bear low substrate concentration in regular process, but this can be solved with product removal or enzyme immobilization. The immobilization of the β-galactosidase onto cross-linked chitosan microspheres did not enhance the enzymeʹs thermal or pH stability but eliminated the external diffusion, and therefore speeded the hydrolysis in 3 folds. The relative reaction activity dropped only 1.75% after 6 runs of using the immobilized β-galactosidase.