From molasses to syrup: Engineering ultrafiltration membrane surface to improve invertase reusability

A by-product from membrane-mediated molasses refining process mainly contains sucrose, glucose, fructose and KCl, which is an ideal raw material to produce high fructose syrup (HFS) by invertase. Ultrafiltration (UF) membrane can achieve invertase reuse while protein adsorption on the membrane not only decreases the membrane permeability, but also induces enzyme activity loss. In this work, the effect of glucose/fructose and KCl on the catalytic behavior of invertase was investigated. Then the reusability of invertase for sucrose hydrolysis was evaluated by different UF membranes, and mussel-inspired surface modification was employed to further improve the invertase reusability. The results showed that the presence of KCl in the raw materials greatly decreased the reaction rate, and thus desalination and concentration of the raw materials should be conducted before enzymatic hydrolysis to improve the enzymatic hydrolysis efficiency, save energy and reduce reactor volume. Moreover, it was found that the sucrose conversion decline with enzyme reuse by UF was caused by enzyme adsorption in/on the membrane and/or its penetration through the membrane leading to overall enzyme loss in the retentate. A polyether sulfone (PES) membrane with molecular weight cut-off (MWCO) of 10 kDa (UP010) was selected to recycle invertase because it enabled complete invertase retention and relatively high permeate flux. When l-cysteine (Cys) was grafted on the polydopamine-coated UP010 membrane, it could reduce invertase adsorption on the membrane, and thus keep high sucrose conversion (93–95%) in four enzyme reuse cycles because the grafted Cys greatly increased the negative charge and hydrophilicity of the membrane surface. This work not only offered an alternative for HFS production by membrane technology but also developed a novel strategy to construct antifouling surface to retard enzyme adsorption.