Bioconversion of d-galactitol to tagatose and dehydrogenase activity induction in Gluconobacter oxydans

Tagatose, a new probiotic sweetener, is obtainable from d-galactitol by microbial oxidation employing Gluconobacter oxydans DSM 2343 strain. Several polyols were tested as carbon source and inducer of dehydrogenase activity and galactitol was found to be the best substitute for glycerol in the base medium (3160 mg/l tagatose) although the effects of the induction did not prove to be stable throughout cell storage. The gradual addition of galactitol in high glycerol content media resulted in relevant and stable tagatose yields (4400 mg/l) highlighting that the dehydrogenase activity induced during the adaptation procedure is correlated to the presence of glycerol. This enzyme activity can be attributable to a minor activity of a sorbitol dehydrogenase (SDH), the highest levels (200×10−2 IU/g) being evidenced in galactitol adapted cells with sorbitol as substrate. The adaptation procedure determined a decrease in sorbitol affinity (KS from 37 to 57 mM) and an increase in that related to galactitol (KS from 98 to 21 mM).