Microfiltration of soy sauce: Efficiency, resistance and fouling mechanism at different operating stages

Microfiltration is a promising green alternative to the current thermal-based soy sauce refinement process. In this work, an integral investigation into the fouling mechanism of soy sauce microfiltration was conducted. The models of resistance-in-series, fouling propensity and pore blocking were adopted to identify the corresponding mechanisms. Foulant/membrane interaction was evaluated by XDLVO theory. Filtration efficiency and permeate quality were also evaluated. The results suggested that PES-foulant formed a hydrophilic repulsive interaction while PVDF-foulant interaction was mainly controlled by attractive force. Compared to PES membranes, PVDF membranes showed lower fouling propensity, tightly related to their high surface roughness; compared to 0.22 mu m as nominal membrane pore size, 0.45 mu m showed lower irreversible fouling resistance. The flux decay in the early stage (2 min) accounted for 66% to 93% of initial permeate flux, which related to an internal pore blocking mechanism. After a quick transition of intermediate pore blocking, cake layer formation, mainly composed of protein and polysaccharide, then dominated the steady-state filtration. This work provides not only guidance for soy sauce refinement but also reference for microfiltration of protein-polysaccharide complex colloid.