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 (Δp_TM) 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: Δp_TM was set directly to various target values. No. 2: Starting from a low Δp_TM, we increased and subsequently decreased Δp_TM stepwise. The comparison of both protocols tested the effect of the mode of Δp_TM increase to target level. The latter protocol alone tested the effect of the deposit layer history with regard to the Δp_TM. As expected, flux and protein permeation were both found to be functions of the Δp_TM. Further, both measures were independent of the filtration protocol as long as Δp_TM was held at a constant level or, as part of protocol No. 2, Δp_TM was increased. Thus, we can state that the mode of Δp_TM increase to target level does not affect filtration performance in SWM. We found that after completion of a full cycle of stepping Δp_TM 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 Δp_TM 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.

Existing works on the influence of spatial effects on flux and permeation of proteins in microfiltration (MF) have focused on ceramic membranes. There is little information on spiral-wound membranes (SWMs). Since the inner core of a SWM is practically inaccessible by non-destructive techniques, three different prototypes were constructed in this study to optimize suitability for the investigation of spatial effects on filtration performance. To measure the pressure drop, shortened SWMs 0.25, 0.50, and 0.75 times the length of a standard industrial SWM (0.96 m) were designed. Second, a sectioned membrane (0.96 m) with separated compartments on the permeate side was constructed to analyze spatial effects on flux and protein permeation along the flow path of a SWM. Three different features characterized this sectioned module: sectioned permeate pockets, a sectioned permeate collection tube, and sectioned permeate drain and measurement systems. Crossflow filtration experiments showed that these modifications did not alter the filtration performance compared to an unmodified control SWM. Thus, it can be applied to assess spatially-resolved filtration performance in SWMs. The third prototype designed was a test cell with accessible flat sheet membranes and spacer material, as in SWMs. The flow path in this test cell was designed to match the characteristics of the channels between the membrane sheets in a standard SWM as closely as possible. The flow path length and the combination of membrane material and spacer architecture were the same as in the control SWM. This test cell was designed to assess the effects of length and processing conditions on the formation of a deposit layer. The combined results of these test modules can yield new insights into the spatial distribution of flux, permeation of target components, and deposit formation.

Spacer design in spiral-wound membranes (SWMs) significantly affects the axial pressure drop in the flow channel but also the deposit layer removal. However, the effects of the spacer design and feed flow distribution in the module on the filtration performance have not yet been investigated during the highly fouling-susceptible fractionation of proteins from skim milk by SWMs. Therefore, a parallel spacer with no turbulence promotion and a less homogeneous feed flow distribution in the SWM was compared to a diamond spacer with regard to its impact on deposit formation and filtration performance. The experiments were conducted in a flat sheet test cell and in SWMs. The parallel spacer induced a more homogeneous deposit layer formation. However, no difference in filtration performance could be observed in the experiments with the test cell. Even though deposit layer formation dominates the microfiltration, its amount and spatial distribution could not be directly linked to the filtration performance. Furthermore, both spacers were assessed in SWM. Despite the higher crossflow velocity applicable in the more open channels of the parallel spacer, the performance of the parallel spacer was inferior to the diamond spacer. This was independent of the viscosity of the feed. Due to the high curvature of the membrane sheets close to the permeate collection tube, the cross-section of the flow channels in the SWM equipped with the parallel spacer was reduced. This resulted in a distinctly lower deposit layer control and performance, which could not be compensated by the resulting higher crossflow velocity far from the permeate collection tube.

During skim milk microfiltration (nominal pore size of 0.1 µm) at 10 °C, the whey protein purity in the permeate is reduced by an enhanced serum casein permeation, primarily of β-casein. To decrease casein permeation, the possibility of a pre-heating step under pasteurization conditions before the filtration step was investigated, so as to shift the equilibrium from soluble serum casein monomers to impermeable micellar casein. Immediately after the pre-heating step, low temperature microfiltration at 10 °C was conducted before the casein monomers could diffuse into the serum. The hypothesis was that the dissociation of β-casein into the serum as a result of a decreasing temperature takes more time than the duration of the microfiltration process. It was found that pre-heating reduced the β-casein permeation during microfiltration without significantly affecting the flux and whey protein permeation, compared with a microfiltration at 10 °C without the pre-heating step. Furthermore, the addition of calcium (5 and 10 mM) not only reduced the casein permeation and thus increased the permeate purity, defined as a high whey protein-to-casein (g L⁻¹/g L⁻¹) ratio, but also decreased the filtration performance, possibly due to the structural alteration of the deposited casein micelle layer, rendering the deposit more compact and more retentive. Therefore, the possible combination of the addition of calcium and pre-heating prior to microfiltration was also investigated in order to evidence the potential increase of whey protein (WP) purity in the permeate in the case of Ca²⁺ addition prior to microfiltration. This study shows that pre-heating very close to low temperature microfiltration results in an increased purity of the whey protein fraction obtained in the permeate.

Fouling distinctly reduces the filtration performance of membranes. A characterization of the fouling in membranes, however, is difficult due to its spatial distribution. Currently applied methods for deposit layer analysis are rather complex or do not offer a spatial resolution. Knowledge of the spatial distribution, however, could be used to improve the design of membranes, modules, and spacers. Staining with Coomassie Brilliant Blue, related to the staining of PAGE gels, is a simple method to visualize and analyze the deposited proteins semi-quantitatively. We improved an existing staining technique for protein deposits on membranes by adding a calibration for the semi-quantitative analysis and optimizing the sample handling. The method provides a spatially resolved analysis of deposited proteins up to a concentration of 10 g m^-2. Apart from staining, data processing is described in order to generate false colors or topographic images of deposits. Thus, the paper describes a simple method to assess and visualize the influence of module characteristics such as spacer design on the spatially resolved protein fouling of polymeric and ceramic membranes. Therefore, the method can contribute to the improvement of the module design and processing conditions with regard to the filtration performance. •Visualization of proteinaceous deposits on membranes•Spatially resolved quantification of proteinaceous deposits.