D-tagatose biotransformation from lactose by lactase and recombinant Bacillus subtilis expressing l-arabinose isomerase, and the product separation, purification and crystallization.

d-Tagatose is a low-calorie rare sugar with notable physiological benefits. The recombinant Bacillus subtilis expressing L-arabinose isomerase (LAI) was constructed and used together with lactase for biotransformation of d-tagatose from cheap substrate of lactose. Under optimum conditions, 45.6 g/L d-tagatose was produced from 200 g/L lactose, achieving a conversion yield of 0.228 g d-tagatose/g lactose. d-Glucose produced by lactose hydrolysis was removed by anaerobic fermentation with Saccharomyces cerevisiae. Subsequently, the sugar solution containing d-galactose and d-tagatose was decolorized by using powdered activated carbon, desalinated by anion and cation exchange resin beds, and separated by chromatographic column, resulting in decolorization rate of 95.5 %, desalinization rate of 93.8 % and d-tagatose solution purity of 97.9 %. Finally, the separated d-tagatose solution was concentrated and crystallized, resulting in d-tagatose crystals with 99.9 % purity. In summary, this paper establishes a complete bioprocess for d-tagatose from lactose catalyzed by using recombinant B. subtilis and lactase. The methods developed in this study show promise for mass production of food-grade d-tagatose from the inexpensive substrate lactose.