Studies On hydrolysis of skimmed milk using immobilized β-galactosidase in a membrane reactor

Abstract

In the present investigation, an attempt has been made to investigate the hydrolysis of skimmed milk in a batch membrane bioreactor using β-galactosidase immobilized on a 30 kDa PES membrane. The skimmed milk solutions of different concentrations(30 kg m-3 – 80 kg m-3)were deproteinated through 30 kDa and 5 kDa cross-flow ultrafiltration membrane modules successively and about 95%-97% lactose present in the feed was recovered in the permeate stream. The deproteinated milk permeates thus obtained were subjected to hydrolysis in a batch membrane bioreactor fitted with the enzyme-immobilized PES membrane. Immobilization of enzyme on the ultrafiltration membrane was performed by cross-linking using two different concentrations of glutaraldehyde (3%v/v and 4%v/v). It was observed that high percent retention ofenzyme activity(94.2%) and percent enzyme loading(98%) were attained when the immobilization was performed using 4%v/v glutaraldehyde. For an initial feed concentration of 70 kg m-3,the percentage lactose conversion achieved were 45.2% and 21.4% using β-galactosidase immobilized with 4%v/v and 3%v/v glutaraldehyde respectively. However, a significant percentage reduction in pure water fluxwas observed in case of enzyme-immobilized membranes (27.5% and 67.5% for immobilization carried out using 3% and 4%v/v glutaraldehyde respectively)from that obtained using the fresh membrane. The enzyme-immobilized membrane(cross-linked with 4%v/v glutaraldehyde) could be used for six successive runs with more than 90% retention in activity and less than 5% loss due to leaching. The unsteady state mass balance equation was developed in order to predict the concentration profile above the membrane surface.The model equation could also predict the permeate concentration well within 95% confidence interval