Synthesis of Lactulose in Continuous Stirred Tank Reactor With β-Galactosidase of Immobilized in Monofunctional Glyoxyl Agarose Support.

Lactulose synthesis from fructose and lactose in continuous stirred tank (CSTR) reactor operation with glyoxyl-agarose immobilized β-galactosidase is reported for the first time. The effect of operational variables: inlet concentrations of sugar substrates, temperature, feed substrate molar ratio, enzyme loading and feed flow rate was studied on reactor performance. Even though the variation of each one affected to a certain extent lactulose yield (Y ), specific productivity (π ) and selectivity of the reaction (lactulose/transgalactosylated oligosaccharides molar ratio) (S ), the most significant effects were obtained by varying the inlet concentrations of sugar substrates and the feed substrate molar ratio. Maximum Y of 0.54 g⋅g was obtained at 50°C, pH 4.5, 50% w/w inlet concentrations of sugar substrates, feed flowrate of 12 mL⋅min, fructose/lactose molar ratio of 8 and reactor enzyme load of 29.06 IU ⋅mL. At such conditions S was 3.7, lactose conversion (X ) was 0.39 and total transgalactosylation yield was 0.762 g⋅g, meaning that 76% of the reacted lactose corresponded to transgalactosylation and 24% to hydrolysis, which is a definite advantage of this mode of operation. Even though X in CSTR was lower than in other reported modes of operation for lactulose synthesis, transgalactosylation was more favored over hydrolysis which reduced the inhibitory effect of galactose on β-galactosidase.