Dead-end filtration of sponge-like colloids: The case of casein micelle

In colloidal filtration, understanding the mechanisms responsible for deposit formation and determining the relations between filtration conditions and deposit characteristics are still major issues. This is especially the case for soft colloids which can deform or deswell when they accumulate at the membrane surface. In this work, we focus on the behavior of one particular soft and sponge-like natural colloid, the milk casein micelle. For that purpose, specific dead-end filtration experiments were designed and performed, involving series of filtrations interspersed with rinse periods at zero transmembrane pressure. From the results obtained, we first demonstrate that deposit formation is intimately linked to the colloidal nature of the micelle, as no irreversible deposit is formed when caseins are not organized into micelles. We then discuss the critical conditions for deposit formation and we estimate the corresponding critical osmotic pressure at which colloidal phase transition occurs. Interestingly, the obtained value is a bit higher than the one measured in “static” osmotic stress experiments; which could be a direct effect of liquid flow between the particles. We finally demonstrate that our results are consistent with a deposit made of closely-packed and connected casein micelles that are compressed during filtration, and that fully relax to their “native” initial dimensions when pressure is released.

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► In filtration, casein micelles form an irreversible deposit at the membrane surface.
► No deposit is formed when caseins are not organized into micelles.
► Critical osmotic pressure for deposit formation is Πcrit=31–35 kPa.
► Micelles being soft and elastic, the deposit is both compressible and relaxable.
► Deposit resistance is fully governed by the internal structure of the casein micelle.