Continuous enzymatic synthesis of lactulose in packed-bed reactor with immobilized Aspergillus oryzae β-galactosidase

Abstract

Lactulose synthesis from fructose and lactose in continuous packed-bed reactor operation with glyoxyl-agarose immobilized Aspergillus oryzae β-galactosidase is reported for the first time. Alternative strategies to conventional batch synthesis have been scarcely explored for lactulose synthesis. The effect of flow rate, substrates ratio and biocatalyst-inert packing material mass ratio (M_B/M_IM) were studied on reactor performance. Increase in any of these variables produced an increase in lactulose yield (Y_Lu) being higher than obtained in batch synthesis at comparable conditions. Maximum Y_Lu of 0.6 g·g^-1 was obtained at 50 °C, pH 4.5, 50% w/w total sugars, 15 mL·min^-1, fructose/lactose molar ratio of 12 and M_B/M_IM of 1/8 g·g^-1; at such conditions yield of transgalactosylated oligosaccharides (Y_TOS) was 0.16 g·g^-1, selectivity (lactulose/TOS molar ratio) was 5.4 and lactose conversion (X_Lactose) was 28%. Reactor operation with recycle had no significant effect on yield, producing only some decrease in productivity.

Keywords: Continuous operation; Glyoxyl agarose; Lactulose; Packed-bed reactor; β-Galactosidase.