The effect of the immobilization time on the activity of immobilized beta-galactosidase from K. lactis was investigated. Six biocatalytic membranes, different only for the time of the enzyme immobilization, were obtained by using nylon membranes grafted with glycidyl methacrylate (GNU) and activated by hexamethylenediamine (HMDA) and glutaraldehyde (Glu), used as spacer and coupling agent, respectively. Comparison between the isothermal and nonisothermal yield of these biocatalytic membranes was carried out in the process of lactose hydrolysis in milk. All of the results, reported as a function of the immobilization time, have evidenced the influence of our variable parameter on the activity of the catalytic membranes. The membrane giving highest yield under isothermal and nonisothermal conditions was that obtained with 2 h of immobilization time. The industrial application of these membranes has been discussed in terms of percentage reduction of the production times.