135 immobilized β-galactosidase on nanofiltration (0.4 kDA) and ultrafiltration (5 to 50 kDA) membranes via covalent grafting, and compared their catalytic performance towards the continuous membraneassisted GOS production. A higher GOS yield was obtained when using the enzyme-loaded nanofiltration membrane, which aligns with the enhanced permeation of monosaccharides and improved retention of GOS observed with the nanofiltration membrane (compared to enzyme-immobilized ultrafiltration membrane). Furthermore, the nanofiltration process resulted in substantial retention of the lactose substrate, providing extended residence time and greater interaction between the substrate and immobilized enzyme, which contributed to the increased GOS production. Similarly, leveraged on the use of membrane-immobilized dextranase in order to selectively permeate the produced oligodextran featuring the desired molecular weight (5-8 kDa), and avoid over-degradation of products.