HYDROLYSIS OF LACTOSE IN ACID WHEY BY LACTASE BOUND TO POROUS GLASS PARTICLES IN TUBULAR REACTORS

Partially purified lactases (β-galactosidase, EC3.2.1.23) from Aspergillus niger were covalently bound to acetone-silanized, diazotized porous glass particles (mean pore diam, 86.5 nm; particle diam, 75–125 μm). The temperature (～55°C) and pH (3.5–4.0) optima were established in acid whey containing 5% total whey solids. Lactose hydrolyzing activity was stable during 43 days of semicontinuous operation at 55°C with reconstituted acid whey (pH = 4.5) at total solids (TS) concentrations varied between 4 and 25% and to which 5 ml/liter toluene had been added to retard microbiological contamination. Kinetic experiments with acid wheys gave results reproducible when assayed by both thin layer chromatography (TLC) and glucose oxidase (GS) procedures, although the TLC method gave systematically higher Values at intermediate conversions and high TS concentrations. The kinetics of lactose hydrolysis by columns of lactase bound to porous glass (LBG) of 1.6 cm diam and lengths of 1, 5 and 10.5 cm showed some evidence for reduction of the rate of lactose hydrolysis by film diffusion resistances. Calculations using correlations for packed beds also suggest the presence of diffusional effects. Lactose was hydrolyzed slightly more rapidly in whey than in deproteinized whey. Lactose hydrolysis rates in both types of reconstituted whey increased as the TS concentrations increased from 4 to 25%. The data did not obey any of a number of integrated reaction rate equations, including a rate equation which accounted for competitive product inhibition of Michaelis enzyme kinetics. Failure of simple models is due in part to diffusional resistances and in part to the large range of concentrations studied. The LBG preparation retained appreciable activity after more than 8 months of frequent use at a wide variety of conditions.