Biotransformation of D-galactitol to tagatose by acetic acid bacteria

Tagatose, a ketohexose C-4 fructose epimer present in nature in low concentration, is a potential low calorie bulking sweetener that can be obtained by the microbial oxidation of the corresponding polyalcohol galactitol. The screening of strains belonging to the acetic acid bacteria resulted in 100-160 mg tagatose/1, produced at 24 h in non growing conditions, while 260-340 mg tagatose/1 was obtained at 48 h with growing cells of Gluconobacter strains, with a specific activity rate of tagatose production, 1.4 × 10-3 1/h. After galactitol adaptation the Gluconobacter oxydans DSM 2343 strain gave a notable increase in tagatose yield, reaching 3160 mg/1 with a corresponding 6.6 × 10-3 1/h specific activity rate at 24 h of reaction. Preliminary enzyme characterisation experiments indicated that the dehydrogenase activity may be attributable to a sorbitol dehydrogenase (SDH).