Continuous synthesis of lactulose in an enzymatic membrane reactor reduces lactulose secondary hydrolysis

•Continuous operation was carried out at precisely controlled flux (i.e., HRT).•Secondary hydrolysis decreased by removal of lactulose during reaction.•Enhanced productivity of lactulose synthesis was achieved in continuous operation.•An efficient reduction of enzyme consumption in continuous synthesis was realised.•Continuous lactulose production for ∼100h is amenable. Newly developed parallel small-scale enzymatic membrane reactors (EMRs) were used to enhance the synthesis of lactulose using β-galactosidase. Under batch operation, the productivity of lactulose decreased abruptly from 2.72 down to 0.04 mglactulose/(Uenzymeh) over 35h of reaction. This was presumably caused by the action of β-galactosidase which performed secondary hydrolysis upon the produced lactulose. The continuous operations of an EMR system led to continuous removal of lactulose in the reactors restricting lactulose degradation caused by secondary hydrolysis. Therefore, continuous lactulose syntheses in the EMRs yielded significantly higher specific productivities under “steady state” conditions. Approximately 0.70 and 0.50 mglactulose/(Uenzymeh) for hydraulic residence times of 5 and 7h were reached, respectively. Continuous lactulose synthesis performed in an EMR system conclusively can circumvent the drawbacks (e.g., secondary hydrolysis) of lactulose synthesis encountered in batch operation. It is, therefore, beneficial in terms of enhanced lactulose productivity and reduced enzyme consumption.