Synthesis and separation of galacto-oligosaccharides using membrane bioreactor

In the present investigation, a dead-end membrane module, equipped with β-galactosidase immobilized flat-sheet ultrafiltration (UF) membrane, has been considered as a membrane bioreactor (MBR) for galacto-oligosaccharide (GOS) synthesis and separation. β-galactosidase was immobilized on the membrane surface by the combination of a series of mechanisms, such as adsorption of polyethyleneimine (PEI) on membrane matrix, formation of PEI and β-galactosidase aggregates, and finally cross-linking of PEI and β-galactosidase complex on the membrane surface by glutaraldehyde. The optimum pH for PEI-enzyme aggregation was found to be 8.2 and the optimum pH for crosslinking of PEI-enzyme aggregates on the membrane surface was found as 7.0 when the ratio of PEI to β-galactosidase was 1:25. Stable immobilization could be achieved over broad temperature range (4 °C to 65 °C). It was found that at optimum condition β-galactosidase loading on the membrane surface was 85% at multilayer form. GOS was synthesized by MBR with retentate recirculation, using simulated lactose solution, as well as de-proteinated whey as a substrate. Optimum operating conditions for the GOS synthesis were found at 0.045 M initial substrate concentration, 25 mL·h− 1 feed flow rate, 45 °C temperature, 3.0 kgf cm− 2 trans-membrane pressure (TMP), 80 r.p.m. stirrer speed.

► Highest β-galactosidase loading on membrane surface was 85% at pH 8.2.
► Optimum ratio of PEI to β-galactosidase was 1:25.
► Optimum substrate concentration was 0.045 M.
► Optimum feed flow rate was 25 mL·hr− 1.
► Optimum conditions: 45 °C temperature, 4.0 kgf cm− 2 TMP, 80 r.p.m. stirrer speed.