Differential analysis of deposition layers from micellar casein and milk fat globule suspensions onto ultrafiltration and microfiltration membranes

Particle deposition behaviour on membrane filters is of the utmost importance for the flux and selectivity. For milk filtration not only the particle behaviour as such but also the behaviour in time is a factor to take into account. Therefore we applied a differential analysis to the flux decline during dead-end filtration of latex, casein micelles and milk fat globules. Based on our analysis for latex we distinguished two regions, one best described by pore blocking and one by cake filtration. Besides these two regions, no other time dependent behaviour was observed, as was expected for latex. However, for casein micelles and milk fat globules various other time dependent effects were observed and quantified. Pore blocking of an UF membrane by micellar casein was relatively small, as concluded from the values of the pore blocking parameter of 0.1-0.2. In contrast, the cake resistance was relatively high, 35 times higher than expected from the Carman-Kozeny theory. This may be attributed to a decrease of the size of the casein micelles, leading to the formation of very compact layers. The specific cake resistance of micellar casein was lower for a MF membrane where depth filtration played a role. Milk fat globules generate layers with a very high cake resistance, both for liquid and for partly crystallised fat. For liquid fat, spreading of milk fat globules onto the membrane and deformation of milk fat globules seem to be the main cause for the high cake resistance. When a large part of the milk fat was crystallised (T = 5 °C), partial coalescence of milk fat globules in the cake layer seems to play a role. Differential cake analysis proved to be a valuable method for the evaluation of concentrated particle layers, resulting in better knowledge about the specific (time dependent) particle behaviour in these layers. In this way, parameters are identified that are essential for the design of microfiltration or ultrafiltration processes.