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

Graphical abstract Highlights • A. oryzae β-gal immobilized on glyoxyl agarose was tested in CPBR for the first time. • Highest Y Lu obtained was 0.6 g·g−1 for continuous operation in a packed bed reactor. • Flow, substrate molar ratio and enzyme/inert support affected CPBR performance. • Lactulose synthesis in CPBR is a sound alternative to conventional batch synthesis. 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. [ABSTRACT FROM AUTHOR]