Protein fractionation by means of microfiltration (MF) is significantly affected by fouling, especially when spiral-wound membranes (SWMs) are used. We investigated the influence of the mode of transmembrane pressure (ΔpTM) increase to target level and the deposit layer pressure history on the filtration performance during skim milk MF at temperatures of 10 °C and 50 °C. Two filtration protocols were established: No. 1: ΔpTM was set directly to various target values. No. 2: Starting from a low ΔpTM, we increased and subsequently decreased ΔpTM stepwise. The comparison of both protocols tested the effect of the mode of ΔpTM increase to target level. The latter protocol alone tested the effect of the deposit layer history with regard to the ΔpTM. As expected, flux and protein permeation were both found to be functions of the ΔpTM. Further, both measures were independent of the filtration protocol as long as ΔpTM was held at a constant level or, as part of protocol No. 2, ΔpTM was increased. Thus, we can state that the mode of ΔpTM increase to target level does not affect filtration performance in SWM. We found that after completion of a full cycle of stepping ΔpTM up from 0.5 bar to 3.0 bar and back down, flux and deposit layer resistance were not affected by the deposit layer history at 10 °C, but they were at 50 °C. Protein permeation, however, was lower for both 10 °C and 50 °C, when the ΔpTM cycle was completed. The processing history had a significant impact on filtration performance due to remaining structural compression effects in the deposited layer, which occur most notably at higher temperatures. Furthermore, temperatures of 50 °C lead to deposit layer aging, which is probably due to an enhanced crosslinking of particles in the deposit layer. Apart from that, we could show that fouling resistance does not directly correlate with protein permeation during skim milk MF using SWM.
Keywords: MF; SWM; deposit layer formation; deposit layer history; microfiltration; mode of ΔpTM increase; skim milk; temperature.