Impact of Protein Interactions and Transmembrane Pressure on Physical Properties of Filter Cakes Formed during Filtrations of Skim Milk

Abstract

Microfiltration (0,1μm) was used to fractionate casein (d50,3 = 180nm) micelles from whey proteins (2 - 6nm). The casein fraction forms a deposited layer on the membrane surface. Little is known about the structure of these layers and how transmembrane pressure affects their structure. In order to assess the properties, casein micelle deposits were characterized by dead-end microfiltrations. Casein micelle deposits of constant composition were formed following a standardized layer build up (permeate mass, pressure, pH). Then protein free milk serum was filtered through the deposit. Thus, the impact of compressive forces and protein-protein interactions on physical cake properties during constant solid height filtrations could be determined by stepwise variations of transmembrane pressure and pH without further deposition of proteins. It was found that the casein micelle deposits became more compactable, when their surface charge was lower. Specific cake properties were related to hydrophilic repulsion between casein micelles, whereas the influence of electrostatic interactions between micelles was negligible. The observed cake material properties obtained from dead-end filtrations provide valuable insights into deposit layer build-up and structure.Results obtained for dead-end filtrations were used to describe cross-flow filtrations. For this purpose a new method was developed to assess the specific cake resistances by the evaluation of the kinetic of flux decrease due to deposit layer build-up at the start of filtrations. For the first time it could be shown that deposits consisting of casein micelles during cross-flow filtration were thin layers (1-3μm), resulting in high specific resistances (up to 20•10-15 m/kg). The specific cake resistance was pressure dependent and increased when the hydrophilic repulsion between casein micelles was reduced at a pH beyond the isoelectric point.