Immobilization of α-amylase to a composite temperature-sensitive membrane for starch hydrolysis

A composite membrane was made by casting hydrogel onto a nonwoven polyester support and used for enzyme immobilization. The hydrogel consists of N-isopropylacrylamide, cross-linker N,N'-methylenebis(acrylamide), 2-hydroxyethyl methacrylate, soluble starch, and N-(acryloxy)succinimide (NAS). The composite membrane is temperature-sensitive with a lower critical solution temperature (LCST) around 35 °C. It responds to temperature change by swelling below the LCST and shrinking above the LCST, corresponding to opening and closing of the membrane pores. α-Amylase was immobilized to the membrane by covalent bonds through reacting with the high reactive ester groups in NAS. The membrane-immobilized enzyme retained 32% of specific activity toward soluble starch when compared with that of free enzyme, and its properties were characterized and compared with those of the free enzyme. The immobilized enzyme was more thermally stable than the free enzyme. Kinetic constants, (K(m)) and the activation energy of the immobilized enzyme were both larger than those of the free enzyme. Starch hydrolysis with the immobilized enzyme was investigated in two-compartment permeation cells with a composite membrane between the cells. Reaction was carried out by hydrolyzing soluble starch in the donor side and collecting the hydrolyzed products in the receptor side. This reactor could be operated with temperature cycling to enhance the reaction and facilitate separation of products from the substrate. The best operating condition is cycling the temperature between 50 and 20 °C every 5 min. The membrane reactor was operated up to eight times for successive starch hydrolysis.