Engineered Bacillus subtilis harbouring gene of d-tagatose 3-epimerase for the bioconversion of d-fructose into d-psicose through fermentation.

d-Psicose, as important rare sugar and epimer of d-fructose on the C-3 position, displays unique health benefits and physiological functions in various fields. The production of d-psicose is currently obtained via enzymatic bioconversion, mostly with of d-tagatose 3-epimerase. In the present study, the isomerization of d-fructose into d-psicose was achieved by constructing engineered food-grade Bacillus subtilis through fermentation. After optimizing the fermentation conditions of the recombinant strain, the yield of d-psicose reached up to 4.56 g/L, representing an over 8-fold increase in the yield compared to unoptimized conditions, with a d-fructose conversion rate of 56.26 % when the strain was cultured at 35 ℃ for 24 h with 5 g/L d-fructose and 5 mM of Mn. In addition, 30 g of wasted apple hydrolysate were fermented by engineered B. subtilis and gained 7.76 g/L of d-psicose at a conversion rate of 25.86 % (w/w).