High Expression of β-Glucosidase in Bifidobacterium bifidum BGN4 and Application in Conversion of Isoflavone Glucosides During Fermentation of Soy Milk.

In spite of the reported probiotic effects, Bifidobacterium bifidum BGN4 (BGN4) showed no β- glucosidase activity and failed to biotransform isoflavone glucosides into the more bioactive aglycones during soy milk fermentation. To develop an isoflavone-biotransforming BGN4, we constructed the recombinant B. bifidum BGN4 strain (B919G) by cloning the structural β- glucosidase gene from B. lactis AD011 (AD011) using the expression vector with the constitutively active promoter 919 from BGN4. As a result, B919G highly expressed β- glucosidase and showed higher β-glucosidase activity and heat stability than the source strain of the β-glucosidase gene, AD011. The biotransformation of daidzin and genistin compounds using the crude enzyme extract from B919G was completed within 4 h, and the bioconversion of daidzin and genistin in soy milk during fermentation with B919G also occurred within 6 h, which was much faster and higher than with AD011. The incorporation of this β-glucosidaseproducing Bifidobacterium strain in soy milk could lead to the production of fermented soy milk with an elevated amount of bioavailable forms of isoflavones as well as to the indigenous probiotic effects of the Bifidobacterium strain.