Increasing Performance of Spiral-Wound Modules (SWMs) by Improving Stability against Axial Pressure Drop and Utilising Pulsed Flow.

Spacer-induced flow shadows and limited mechanical stability due to module construction and geometry are the main obstacles to improving the filtration performance and cleanability of microfiltration spiral-wound membranes (SWMs), applied to milk protein fractionation in this study. The goal of this study was first to improve filtration performance and cleanability by utilising pulsed flow in a modified pilot-scale filtration plant. The second goal was to enhance membrane stability against module deformation by flow-induced friction in the axial direction ("membrane telescoping"). This was accomplished by stabilising membrane layers, including spacers, at the membrane inlet by glue connections. Pulsed flow characteristics similar to those reported in previous lab-scale studies could be achieved by establishing an on/off bypass around the membrane module, thus enabling a high-frequency flow variation. Pulsed flow significantly increased filtration performance (target protein mass flow into the permeate increased by 26%) and cleaning success (protein removal increased by 28%). Furthermore, adding feed-side glue connections increased the mechanical membrane stability in terms of allowed volume throughput by ≥100% compared to unmodified modules, thus allowing operation with higher axial pressure drops, flow velocities and pulsation amplitudes.

Concentration, purification and quantification of milk protein residues following cleaning processes using a combination of SPE and RP-HPLC.

Detection and quantification of milk protein residues can be of utmost importance for validation of cleaning process efficiency in removing even traces of residues as well as quality assurance and product safety. However, currently available assays cannot provide a combination of high sensitivity and a simultaneous quantification of the individual milk proteins. Furthermore, a low protein-to-protein-variability and high compatibility with other reagents such as residual cleaning agents (e.g. surfactants) cannot be ensured. Therefore, a new method was developed comprised of a pre-concentration of proteins by solid-phase extraction and optimisation of the sensitivity of an existing reversed-phase high performance liquid chromatography method for the separate quantification of bovine milk proteins κ-Casein, αS2-Casein, αS1-Casein, ß-Casein, α-Lactalbumin, and ß-Lactoglobulin. Hereby, solid-phase extraction enables robust and reproducible purification and concentration of protein residues with a high protein recovery rate and flexible adjustment of concentration factors. The increased sensitivity of the reversed-phase high performance liquid chromatography method was achieved by changes in the measurement wavelength and guanidine buffer concentration. This new method enables reproducible concentration, purification and quantification of protein concentrations below 7 ng mL-1 and thus can be used to detect milk protein residues in highly diluted aqueous systems.•Concentration, purification and quantification of milk protein residues with a high recovery rate of proteins (> 94%) and high reproducibility (coefficient of variation (CV) < 3.0%)•Flexible adjustment of sample volumes allows the utilisation of high concentration factors (≤ 500) without compromising the recovery rate of proteins (recovery rate of proteins decreases by 2.74% per 100 CF).

Effect of Pre-Heating Prior to Low Temperature 0.1 µm-Microfiltration of Milk on Casein-Whey Protein Fractionation.

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 ß-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 ß-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 ß-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.

Effect of Temperature, Added Calcium and pH on the Equilibrium of Caseins between Micellar State and Milk Serum.

Micellar casein and casein monomers in milk serum are in a dynamic equilibrium. At temperature below 15-20 °C a considerable amount of casein monomers, ß-casein in particular, is released from the casein micelle into the aqueous serum phase. This study investigates the effects of added calcium and related variations of pH on this peculiar equilibrium in order to minimize the amount of caseins in the serum and to better understand the casein permeation during microfiltration. The pH was varied in the range of 6.3 to 7.3 and the content of calcium was increased up to 7.5 mM by adding CaCl2. Upon equilibration, the milk was separated by ultracentrifugation and the amounts of protein in the supernatant were analyzed. It was shown that the addition of low amounts of calcium shifts the equilibrium towards the micellar casein phase and can, thus, lower the serum casein content induced at low temperatures. Relative to that, the adjustment of pH separately from the CaCl2 addition had a minor effect on casein concentration and composition in the serum.

Effect of Temperature-Dependent Bacterial Growth during Milk Protein Fractionation by Means of 0.1 µM Microfiltration on the Length of Possible Production Cycle Times.

This study determined the maximum possible filtration time per filtration cycle and the cumulated number of operational hours per year as a function of the processing temperature during milk protein fractionation by 0.1 µm microfiltration (MF) of pasteurized skim milk. The main stopping criteria were the microbial count (max. 105 cfu/mL) and the slope of the pH change as a function of filtration time. A membrane system in a feed and bleed configuration with partial recirculation of the retentate was installed, resembling an industrial plants' operational mode. Filtration temperatures of 10, 14, 16, 20, and 55 °C were investigated to determine the flux, pH, and bacterial count. While the processing time was limited to 420 min at a 55 °C filtration temperature, it could exceed 1440 min at 10 °C. These data can help to minimize the use of cleaning agents or mixing phase losses by reducing the frequency of cleaning cycles, thus maximizing the active production time and reducing the environmental impact.

Technical Concepts for the Investigation of Spatial Effects in Spiral-Wound Microfiltration Membranes.

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.

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

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.