Cross-flow microfiltration of beta-lactoglobulin solutions and the influence of silicates on the flow resistance

With the aim of studying the mechanism of fouling during cross-flow microfiltration (CF-MF) of beta-lactoglobulin (beta-lg) solutions, it was found that the presence of particles severely influenced the membrane filtration process. Capturing these particles by a dead-end prefiltration of the feed removed the particles adequately but not completely. The formation of beta-lg particles due to aggregation during processing was excluded as the cause of the initial flux decline. By infrared absorption and energy dispersive spectroscopy analysis it was found that silicates were present in the experimental system. A complex interplay of silicates, an unidentified aliphatic component, beta-lg and the membrane surface properties is responsible for the formation of particles in the system and the development of a deposit on the membrane surface, which results in flux decline. The practical implication of these findings is that small quantities of particles present in protein solutions completely determine flux behaviour during CF-MF. The presence of particles (non-protein material, protein aggregates, bacteria) in industrial feed is inevitable, which means that during industrial CF-MF they also determine the decline in flux. It was also shown that waters meeting cleaning-in-place (CIP) standards for membrane processing can still cause flux decline. By analysing the deposition mechanism occurring during CF-MF of beta-lg using a method combining two procedures recently discussed by Bowen et al. [1] and Field and Arnot [2], it was found that four successive stages occurred: (i) pore blocking in the presence of a back flux of particles, (ii) pore blocking which is irreversible, (iii) irreversible cake formation and (iv) the build-up of a reversible cake including the development of a concentration polarization layer. In addition, the method allowed the identification of the build-up of a reversible and an irreversible flow resistance.