Influences of operating conditions on continuous lactulose synthesis in an enzymatic membrane reactor system: A basis prior to long-term operation

Lactulose synthesis was performed in a continuous stirred enzymatic membrane reactor. Each investigated operating condition (agitation, pH, feed molar ratio of lactose to fructose (mL/mF ratio), hydraulic residence time (HRT)) had an influence on reaction performances, in terms of lactulose concentration, productivity and selectivity. Lactulose concentration was maximum at an mL/mF ratio of 1/2. Higher than this ratio, synthesis of galactooligosaccharides was promoted rather than lactulose. At mL/mF ratios lower than 1/2, enzyme inhibition was pronounced to the detriment of lactulose production. At 7 or 9 h HRT, higher lactulose concentrations were obtained than at shorter HRTs. Applying an mL/mF ratio of 1/2 and an HRT of 9 h in a long-term operation, nearly constant lactulose concentration was reached after 23 h and lasted up to 32 h with a mean concentration of 14.51 ± 0.07 g/L and a reaction selectivity of 0.075-0.080 mollactulose/molcons.lactose. After 7 d, lactulose concentration reduced by 31%. A continuous synthesis of lactulose at lab-scale was shown to be amenable using a membrane reactor process. Moreover, for process evaluation, this study can bridge the gap between batch laboratory scale and continuous full-scale operation regarding lactulose synthesis.