Production of 1-lactulose and lactulose using commercial β-galactosidase from Kluyveromyces lactis in the presence of fructose

Production of 1-lactulose and lactulose using commercial β-galactosidase DSM Maxilact® 5000 in the presence of fructose was investigated. Experiments were performed at 40 °C and pH 7.5. Lactose starting concentration was constantly 250 g/l. A novel transgalactosylation product 1-lactulose was detected besides lactulose. Effects of fructose concentration, reaction time and enzyme concentration on transgalactosylation reactions were discussed. In all reactions, the yield ratio 1-lactulose:lactulose was close to 3:1 due to the regioselectivity of β-galactosidase. The maximum production of 1-lactulose and lactulose was approximately 22 and 8 g/l, respectively, when fructose concentration was increased to 100 g/l. Lactose hydrolysis was significantly retarded since fructose strongly attracted water molecules. Higher enzyme concentration can accelerate transgalactosylation reactions without affecting the maximum production of transgalactosylation products. Fructose was a more preferred galactosyl acceptor than lactose, since the synthesis of galactooligosacchairdes was found to be absolutely inhibited in the presence of fructose.