Milk Protein Fractionation by Means of Spiral-Wound Microfiltration Membranes: Effect of the Pressure Adjustment Mode and Temperature on Flux and Protein Permeation

Abstract

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.

Structural Characterisation of Deposit Layer during Milk Protein Microfiltration by Means of In-Situ MRI and Compositional Analysis

“…Since in cross-flow membrane filtration the transmembrane pressure decreases along the flow path from module inlet to outlet, the extent of deposit formation varies accordingly, while the shear stress almost stays the same as at the inlet. As found by Hartinger et al [5], Piry et al [19], and Kulozik & Kersten [4] using custom-made module systems able to measure flux and permeate composition in segregated modules separately, the targeted maximum of whey protein permeation is achieved only at or towards the end of the module. At this point, the ratio of transmembrane pressure and shear stress shifts to the benefit of shear forces imposed on already deposited or depositing material.…”

“…During MF of food systems, material accumulations at the membrane surface mainly consist of biopolymers such as proteins or polysaccharides [1][2][3][4]. This applies even under crossflow conditions, where the wall shear stress only reduces the amount of deposited material, but a complete prevention of material accumulation cannot be achieved [5][6][7]. This is referred to deposit formation [8].…”

Influence of Spacer Design and Module Geometry on the Filtration Performance during Skim Milk Microfiltration with Flat Sheet and Spiral-Wound Membranes

“…Experiments on SWM were conducted on a pilot-scale filtration plant, which is described in detail elsewhere [20]. The piping and instrumentation (P&I) diagram of the plant is shown in Figure 2.…”

“…After 60 min, permeate and retentate samples were taken and the filtration was terminated. This time was sufficient to reach steady state filtration as we showed in an earlier work [20]. The membrane was removed from the test cell and immersed in deionized water (20 • C) for 30 s to remove residual milk loosely adhered to the deposited protein before further analyses.…”

“…P&I diagram of the pilot scale plant for SWM (LIR: Level indicator recorder; TRC: Temperature recorder controller; PIR: Pressure indicator recorder; FRC: Flow recorder controller; FIR: Flow indicator recorder)[20].…”