Flux and transmission of β-casein during cold microfiltration of skim milk subjected to different heat treatments.

Raw skim milk was subjected to different heat treatments: thermization (65°C, 20 s), pasteurization (72°C, 15 s), and no heat treatment (milk was microfiltered using 1.4-µm membranes at 50°C for bacteria removal; 1.4 MF). The milk (thermized, pasteurized, and 1.4 MF) was cooled and stored at 2°C until processing (at least 24 h) with cold (∼6°C) microfiltration using a benchtop crossflow pilot unit (Pall Membralox XLAB 5, Pall Corp., Port Washington, NY) equipped with 0.1-µm nominal pore diameter ceramic Membralox membrane (ET1-070, α-alumina, Pall Corp.). The flux was monitored during the process, and β-casein transmission and removal were calculated. The study aimed to indicate the conditions that should be applied to maximize β-casein passage through the membrane during cold microfiltration (5.6 ± 0.4°C) of skim milk. The proper selection of heat treatment parameters (temperature, time) of the feed before the cold microfiltration process will increase β-casein removal. It is not clear whether the difference in β-casein transmission between 1.4 MF, thermized, and pasteurized milk results from the effect of heat treatment conditions on β-casein dissociation from the casein micelles or on passage of β-casein through the membrane. The values of the major parameters (permeation flux and tangential flow velocity, through the wall shear stress) responsible for a proper membrane separation process were considerably lower than the critical values. It seems that the viscosity of the retentate has a great effect on the performance of the microfiltration membranes for protein separation at refrigerated temperatures.