Efficient Production of d-Tagatose Using a Food-Grade Surface Display System

d-Tagatose, a functional sweetener, is commonly transformed from d-galactose by l-arabinose isomerase (l-AI). In this study, a novel type of biocatalyst, l-AI from Lactobacillus fermentum CGMCC2921 displayed on the spore surface of Bacillus subtilis 168, was developed for producing d-tagatose. The anchored l-AI, exhibiting the relatively high bioactivity, suggested that the surface display system using CotX as the anchoring protein was successfully constructed. The stability of the anchored l-AI was significantly improved. Specifically, the consolidation of thermal stability representing 87% of relative activity was retained even at 80 °C for 30 min, which remarkably favored the production of d-tagatose. Under the optimal conditions, the robust spores can convert 75% d-galactose (100 g/L) into d-tagatose after 24 h, and the conversion rate remained at 56% at the third cycle. Therefore, this biocatalysis system, which could express the target enzyme on the food-grade vector, was an alternative method for the value-added production of d-tagatose.