Characterization of a lactic acid bacterium-derived β-glucosidase for the production of rubusoside from stevioside

Rubusoside, which is used as a natural sweetener or a solubilizing agent for water-insoluble functional materials, is currently expensive to produce owing to the high cost of the membrane-based technologies needed for its extraction and purification from the sweet tea plant (Rubus suavissimus S. Lee). Therefore, this study was carried out to screen for lactic acid bacteria that possess enzymes capable of bio-transforming stevioside into rubusoside. Subsequently, one such rubusoside-producing enzyme was isolated from Lactobacillus plantarum GS100. Located on the bacterial cell surface, this enzyme was stable at pH 4.5-6.5 and 30-40 °C, and it produced rubusoside as a major product through its stevioside-hydrolyzing activity. Importantly, the enzyme showed higher β-glucosidase activity toward the β-linked glucosidic bond of stevioside than toward other β-linked glucobioses. Under optimal conditions, 70 U/L of the rubusoside-producing enzyme could produce 69.03 mM rubusoside from 190 mM stevioside. The β-glucosidase activity on the cell surface was high at 35 h of culture. This is the first report detailing the production of rubusoside from stevioside by an enzyme derived from a food-grade lactic acid bacterium. The application of this β-glucosidase could greatly reduce the cost of rubusoside production, hence benefiting all industries that use this natural product.