Investigation of mechanisms governing membrane fouling and protein rejection in the sterile microfiltration of beer with an organic membrane

Since the early 1980s, cold sterile crossflow microfiltration has been evaluated as a potential alternative to conventional thermal processes in the brewing industry (flash- or tunnel pasteurization). Up to now, industrial membrane development for beer microfiltration was limited both by severe membrane fouling and by protein and aroma compound retention. In the present work, fouling mechanisms and protein rejection have been investigated experimentally at laboratory scale for the microfiltration of a clarified (kieselguhr filtered) beer through a 0.2 mu m polycarbonate membrane. Fouling mechanisms were analysed by using the constant pressure blocking filtration laws for which interactions arise between high molecular weight beer components and the membrane matrix resulting in pore constrictions and progressive blockage. Protein content in beer was measured using two dye-binding methods (Bradford and Lowry). It was found that permeate flux decay was governed by two successive fouling mechanisms: an internal pore fouling at the initial stages of filtration that conforms to the standard blocking model, followed by an external surface fouling conforming to the cake filtration model. The variation with time of membrane selectivity was characterized by a high initial protein transmission rate followed by a sharp decrease that occurs in a range of filtered volumes which is independent of the applied transmembrane pressure. For all experiments, protein retention level throughout the filtration was found to be closely related to the nature of fouling. It remains constant and low (<20%) in the presence of a pore diameter decrease induced by lateral adsorption of macrosolutes, then increases abruptly (up to 60%) and stabilizes when a fouling layer (gel layer) forms over the membrane surface. The significant role played in the composition of this limiting surface :fouling layer by proteins (associated with polyphenols and P-glucans to form proteinaceous haze material) has been highlighted. The understanding of membrane fouling phenomena (such as the nature and location of fouling materials) is liable to lead to the design of efficient cleaning procedures for the sterile microfiltration of beer.