Optimisation of synthesis of oligosaccharides derived from lactulose (fructosyl-galacto-oligosaccharides) with β-galactosidases of different origin

Batch synthesis of fructosyl-galacto-oligosaccharides from lactulose was performed with commercial β-galactosidase preparations from Aspergillus oryzae, Kluyveromyces lactis and Bacillus circulans. The enzyme from A. oryzae produced the highest yield and specific productivity of synthesis, being selected for further studies. Optimization of fructosyl-galacto-oligosaccharides synthesis was carried out using response surface methodology, considering temperature and initial sugar concentration as variables and yield and specific productivity as response parameters. Maximum yield of 0.41 g g−1 fructosyl-galacto-oligosaccharides was obtained at 70 °C and 60% w/w lactulose concentration, while maximum specific productivity of 1.2 g h−1 mg−1 was obtained at 70 °C and 40% w/w lactulose concentration.

► Different microbial β-galactosidases were evaluated as catalysts for the synthesis of fGOS.
► Highest yield was 0.41 g fGOS/g lactulose, obtained with the β-galactosidase from A. oryzae.
► Fructose was a total competitive inhibitor of the transgalactosylation reaction.
► Synthesis of fGOS with A. oryzae β-galactosidase was optimised by surface of response methodology.