Upward Dead-End Ultrafiltration of Binary Protein Mixtures

Characteristics of filter cake exfoliation in upward ultrafiltration of nanoparticle suspensions

Downward and upward ultrafiltration (UF) was performed using the suspensions of nanosized colloidal silica with different particle diameters and their filtration rates were compared. In downward UF, the filtration rate decreases as the particle diameter decreases because the specific filtration resistance of the filter cake becomes significantly higher. In contrast, the filtration rate in upward UF increases with the decrease in the particle diameter because the filter cake consisting of small particles is exfoliated much more easily under the influence of gravity than that of large ones. In order to evaluate the characteristics of the filter cake exfoliation, the steady filtration rate in the upward mode was measured. The steady filtration rate has a tendency to decrease with particle concentration as well as mean particle diameter. Therefore, when the small particles are added into a given concentration of large particle suspension, the mean particle diameter decreases and the total particle concentration increases due to the dosage of small particles. This results in a maximum of the steady filtration rate at a certain dosage of small particles. Moreover, an estimation equation was proposed for predicting the steady filtration rate in upward UF of colloidal silica suspensions with various mean particle diameters and total particle concentrations.

Fractionation of proteins with two-sided electro-ultrafiltration

Downstream processing is a major challenge in bioprocess industry due to the high complexity of bio-suspensions itself, the low concentration of the product and the stress sensitivity of the valuable target molecules. A multitude of unit operations have to be joined together to achieve an acceptable purity and concentration of the product. Since each of the unit operations leads to a certain product loss, one important aim in downstream-research is the combination of different separation principles into one unit operation. In the current work a dead-end membrane process is combined with an electrophoresis operation. In the past this concept has proven successfully for the concentration of biopolymers. The present work shows that using different ultrafiltration membranes in a two-sided electro-filter apparatus with flushed electrodes brought significant enhancement of the protein fractionation process. Due to electrophoretic effects, the filtration velocity could be kept on a very high level for a long time, furthermore, the selectivity of a binary separation process carried out exemplarily for bovine serum albumin (BSA) and lysozyme (LZ) could be greatly increased; in the current case up to a value of more than 800. Thus the new two-sided electro-ultrafiltration technique achieves both high product purity and short separation times.

Separation of alpha-lactalbumin and beta-lactoglobulin using membrane ultrafiltration

There is considerable commercial interest in the preparation of individual whey proteins as high-value food additives, nutraceuticals, and therapeutics. This study examined the use of membrane filtration for the separation of alpha-lactalbumin and beta-lactoglobulin. Stirred cell filtration experiments were performed using both cellulosic and polyethersulfone membranes to determine the optimal pH, ionic strength, and filtration conditions. Selectivities of greater than 55 could be achieved at pH 5.5 and 50 mM ionic strength using a 30-kD cellulose membrane. A diafiltration process was then designed for the protein separation. A 16-diavolume filtration yielded beta-lactoglobulin as the retentate product with a purification factor of 100 and recovery of 90%. The alpha-lactalbumin was recovered in the filtrate with a purification factor of more than 10 and nearly 99% yield. Model calculations were in good agreement with the experimental data.