Structural organization and behaviour of casein micelles fouling layer during crossflow filtration of milk at low temperature : A Small-Angle X-Ray Scattering (SAXS), osmotic stress and rheology study

International audienceDuring filtration of skimmed milk, the casein micelles accumulate at the membrane surface and their presence has a crucial impact on the performances of the operation and on the products properties. Information is still scarce, especially at ‘low’ temperature (≈8-12°C), about how the casein micelles behave at the membrane surface and interact in such concentrated regimes. The aim of this work was to understand the structural organization and behavior of concentrated casein micelles accumulated at the membrane during crossflow filtration (concentration, sol-gel transition, cohesiveness and reversibility) with a focus on impact of temperature. The strategy consisted in combining in-situ measurements by Small-Angle X-Ray Scattering coupled to crossflow filtration, with ex-situ characterization of concentrated dispersions of casein micelles using osmotic stress technique and rheology. Swelling-re-dispersion experiments were carried out to characterize the cohesiveness of micelles concentrates. During in-situ filtration performed at constant pressure, increasing temperature (12, 25, 42°C) increased permeate flux (which was mainly attributed to the decrease of viscosity of permeate) and led to higher accumulation of casein micelles and a thicker deposit. The relaxation of pressure showed that casein micelles deposit was more cohesive at low temperature. With osmotic stress and rheology experiments, it was observed that compressibility (concentration reached at given osmotic pressure) and sol-gel transition concentration of casein micelles increased with the temperature increasing from 7 to 20°C. Moreover, swelling of casein micelles gels at 7°C resulted in lower redispersion rate as compared to the swelling at 20°C regardless of the gel formation temperature. It was also observed that casein micelles gels obtained at 7°C are more cohesive than those prepared at 20°C. This work shows that carrying out filtration at low temperature led to a lower accumulation of casein micelle (lower compressibility of casein micelles), but to a higher cohesiveness of the accumulated micelles (lower sol/gel transition and swelling properties). These results were attributed to physico-chemical modifications of casein micelles (voluminosity, hydration) under temperature changes