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

•The d-Tagatose 3-Epimerase was efficiently expressed in food-grade Bacillus subtilis.•Using engineered B. subtilis to produce d-psicose by fermentation, firstly.•The substrate conversion rate could attain to 56.26 % through optimization.•Wasted apple hydrolysate could be fermented at a conversion rate of 25.86 % (w/w).