A fundamental study of concentration polarisation and fouling in skim milk ultrafiltration

Abstract

Skim milk ultrafiltration (UF) is an important dairy process operation in which milk proteins are preferentially concentrated for downstream manufacturing of cheese and milk protein concentrates (MPC), and also for protein standardisation. However its operational efficiency is impacted by the accumulation of retained particles at the membrane surface (concentration polarisation, CP) and the irreversible deposition of particles onto the membrane surface and in membrane pores (fouling). CP and fouling ultimately result in lower throughput (flux decline), alterations in product quality, significant operational downtime and vast consumption of water and cleaning chemicals. Development of effective optimisation strategies and technologies for the mitigation or prevention of CP and fouling requires the mastery of these flux decline phenomena. However CP and fouling are still not completely understood despite over 30 years of industrial application, owing partly to the complex physicochemical nature of skim milk. This thesis aims to extend our current understanding of CP and fouling in skim milk UF. Specific investigations include the influence of processing temperature and diafiltration (DF) on CP and fouling in skim milk UF, characterisation of the osmotic compressibility of skim milk, and a visual assessment of gel formation during skim milk UF.

A comprehensive examination of skim milk UF behaviour as a function of processing temperature (between 10-50 °C) showed, for the first time, that fouling at 10 °C is shown to be primarily proteinaceous, consistent with fouling at 50 °C (mainly alpha-lactalbumin, a-LA, and peptides). This provides a validation of existing fouling observations at 50 °C for UF at 10 °C, for which there are very few fouling investigations despite its widespread use. Despite higher fluxes, higher processing temperatures led to a greater magnitude and rate of fouling. This was found to be caused by increased pore blocking by a-LA and deposition/adsorption of beta-lactoglobulin (b-LG) onto the membrane surface, attributed respectively to thermal pore expansion and reversible conformational changes.

Diafiltration involves dilution of the concentrated feed stream with water, followed by filtration to facilitate further removal of salts, lactose and water in order to achieve a higher protein purity. This primarily caused a substantial decrease in ionic strength due to mineral removal, resulting in an increase in electrostatic repulsive interactions between casein micelles and thus a decrease in CP resistance. No effects of DF on irreversible fouling were observed. Restoration of lactose concentrations only resulted in a minor decrease in ionic strength, but had no significant effect on diafiltration behaviour. CP resistance showed a strong correlation with ionic strength (in the range of 32-85 mM), with analyses of existing literature suggesting possible extrapolation (at least qualitatively) to lower ionic strengths (i.e. further extents of DF).

Osmotic stressing experiments show that skim milk is more resistant to osmotic compression/concentration than casein micelle dispersions. Comparisons of osmotic pressure profiles between skim milk, heat-treated skim milk and casein micelle dispersions suggest that whey proteins contribute resistance to osmotic concentration via two mechanisms: (a) below the gel point, whey proteins contribute a counteracting osmotic pressure, and (b) above the gel point, denaturation and attachment of whey proteins onto the casein micelle surface by thiol-disulphide reactions, resulting in an increase of mechanical strength of the casein micelles (i.e. more rigid, less susceptible to deformation or compression). The inclusion of whey proteins provides a more accurate representation of the behaviour of casein micelles under concentration in the CP layer during skim milk UF.

Visual inspection of gel formation resulting from CP during dead-end and cross-flow skim milk UF reveals that at pressures above the gel point, less concentrated gels (i.e. formed in shorter durations or lower pressures) had a turbid white appearance and were brittle to the touch, while more concentrated gels appeared more translucent and were more mechanically robust. Whereas solid gel sheets were formed in dead-end filtration, gel formation in cross-flow skim milk UF were fragmented in nature, and primarily located in dead-zones or areas of low-flow. This is exacerbated when operating at higher feed concentrations, where gel formation was also observed near the feed inlet (particularly due to the flow geometry of the filtration cell used). Description of gel formation allows distinguishing between the different gels that can potentially form during skim milk UF – information which has not been available despite gel formation being mentioned frequently in literature. Gel formation in low-flow zones is detrimental to cleaning and sanitary operation, and is particularly relevant to spiral wound and plate-and-frame modules which have some degree of flow tortuosity.