Tailoring the enzymatic synthesis and nanofiltration fractionation of galacto-oligosaccharides

Continuous galacto-oligosaccharides (GOS) synthesis by bead-immobilized β-galactosidase and nanofiltration fractionation of derived sugar mixtures by cellulose acetate membranes are investigated. Upon enzyme adsorption assays (8 mL/g of solution/resin ratio, 150 mg/mL of initial enzyme concentration and 14 ± 1.4 mg protein/g resin), a Langmuir-type isotherm is observed. Enzyme concentration of 10 U/mL, selected in batch reactor experiments, is used for continuous GOS production carried out at lactose concentrations of 150 g/L and 300 g/L, and at 40 °C. Total dissolved solids concentration influences both reaction and saccharide mixtures fractionation. All observed sugar retention coefficients, mainly for monosaccharides, increase with transmembrane pressure and a gap between the retention values of synthesized disaccharides and lactose (87% versus 77%, respectively, for 2 bar/40 °C) is displayed as a function of the effective transmembrane pressure and total sugars concentration. At 150 g/L total sugars in feed, trisaccharides are totally retained. Doubling sugars concentration, trisaccharides retentions decreased for all applied pressures, reaching 90%.