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There is little information concerning the fractionation by microfiltration (MF) of casein micelles and immunoglobulins plus other minor whey proteins with ceramic gradient membranes. The order of transmission was α-lactalbumin (α-La, 2.3 nm)> β-lactoglobulin (β-Lg; 4.2 nm)>IgG (10.7 nm)> lactoperoxidase (8.2 nm)> IgA (18.1 nm)> IgM (23.8 nm)> lactoferrine > blood serum albumin (7.8 nm), irrespective of the applied transmembrane pressure (0.6–3 bar) and equal to 55% > 50% > 47% > 41%>39% > 32% > 22% > 19%. Including preconcentration, it was possible to obtain 90% of the initial IgG, IgA and IgM within 85, 119, and 160 min, based on 1 m2 of membrane area and 50 L of skim milk volume. The long-term process exposure at 50 °C did not affect α-La and IgG but β-Lg (3–5% denaturation), which, however, was selectively retained by the MF. In conclusion, MF is not only suitable for fractionation of the major whey proteins and caseins, but also for the minor and far bigger immunoglobulins.

Compared to tubular membranes, spiral-wound membranes (SWM) cannot be operated at very high crossflow velocities due to mechanical instabilities of the module. We therefore applied a novel approach using custom-made microfiltration SWM modules to study the effects of crossflow velocities beyond the level currently applicable for commercial SWM. First, we used a modified industrial SWM (0.96 m) to measure flux and protein permeation spatially resolved within the limits of crossflow conditions applied in routine operation (maximum ΔpL of 1.3 bar). Secondly, we applied a short SWM (0.24 m) to assess the effect of extreme crossflow velocities with a corresponding ΔpL of up to 2.8 bar m−1. The goal was to investigate the effects of deposit formation reduced to the minimum, and thus, to measure the theoretically achievable improvement of flux and protein permeation even though this is currently not practicable. As key result, we found a critical crossflow velocity up to which a significant increase of protein mass flow could be achieved; beyond which, however, no further reduction in filtration resistance caused by deposit formation was observed. The filtration became independent of the transmembrane pressure. Based on these findings, more rigid modules could potentially improve the performance of SWM.

During microfiltration, the length-dependent transmembrane pressure (ΔpTM) causes an inhomogeneous filtration performance along the module. Elimination of this length effect was found to increase the overall filtration efficiency of ceramic membranes significantly. Up to the authors’ knowledge, a systematic investigation on the spatial behavior of polymeric spiral-wound membranes (SWM) during microfiltration and the potential of a homogeneous ΔpTM has not been carried out yet. A modified SWM with permeate pockets divided in four hermetically separated sections was used to assess the length-resolved filtration performance during skim milk fractionation. Along the membrane, flux and fouling resistance decreased, whereas protein permeation increased. By adjusting the permeate pressure comparable to the uniform transmembrane pressure mode in ceramic membranes, the length dependency of ΔpTM along the module was reduced. In turn, the filtration performance was found to be more balanced. The overall whey protein mass flow increased as all sections were operating at the same ΔpTM under optimal condition. For the first time, we have proved with experimental data that SWM behave similar to ceramic membranes with regard to the length dependency of the filtration performance. Thus, the concept of a length-independent ΔpTM seems promising to enhance filtration performance of SWM at industrial scale.

Author Summary: A dead-end filtration process of skim milk with ceramic hollow fiber membranes was investigated using in situ Magnetic Resonance Imaging. Special emphasis was put on contrast optimization exploiting water relaxation times being sensitive to casein concentration in the formed deposit layer.

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.