Ionic strength dependence of skimmed milk microfiltration: Relations between filtration performance, deposit layer characteristics and colloidal properties of casein micelles

Abstract

Crossflow microfiltration (0.1 μm) of skimmed milk is largely used in the dairy industry. The overall performance of the process is governed by the accumulation of casein micelles at the membrane surface, which leads to the formation of an irreversible deposit (a gel) in critical hydrodynamic conditions (CHC). To control the filtration performance, it is then necessary to understand how the micelles accumulate at the membrane and how they interact with each other during accumulation. In that aim, microfiltration of skimmed milks is performed at different ionic strengths (NaCl addition 0–0.3 M) and the properties of the casein micelles deposit are determined in each case in terms of structuration, transmission properties and fouling resistances. Upon addition of salt, our results indicate that the casein micelles turn into a gel at a lower “critical” casein concentration than with native milk. With the increase in ionic strength, the deposit is also more cohesive and harder to remove from the membrane. Finally, the addition of NaCl results in a change in the selectivity of the process, with a decrease in the overall protein transmission through the deposit layer. All these results are explained through a shift in the balance of interactions between and within the micelles as ionic strength is changed. They are also very consistent with the generic colloidal behavior of casein micelles as determined through osmotic stress experiments.