Foam fractionation of globular proteins

Abstract

Foam fractionation of bovine serum albumin (BSA) was studied as a model system for potato wastewater. The effects of feed concentration, superficial gas velocity, feed flow rate, bubble size, pH, and ionic strength on the enrichment and recovery of BSA were investigated in a single-stage continuous foam fractionation column. Enrichments ranged from 1.5 to 6.0 and recoveries from 5 to 85%. The feed concentrations were varied from 0.01 to 0.2 wt %, and enrichments were found to increase with lower feed concentrations. Enrichments also increased with lower superficial gas velocities and larger bubble sizes. At sufficiently low feed flow rates, enrichment was found to increase with an increase in the flow rate, eventually becoming insensitive to the feed flow rate at higher values. The pH was varied from 3.5 to 7.0 and ionic strength from 0.001M to 0.2M. The effects of pH and ionic strength were found to be coupled with bubble size. A minimum bubble size was found at pH 4.8, the isoelectric point of BSA, resulting in a minimum in the enrichment. Bubble size, and thus enrichment, was found to increase as the ionic strength decreased from 0.2M to 0.01M. Previous models(1,2) for the hydrodynamics of foam column were extended for a singlestage continuous foam fractionation column for the prediction of enrichment and recovery. The model assumed adsorption equilibrium, infinite surface viscosity, and bubbles of the same size. Though coalescence was formally accounted for in the model by considering bubble size as a function of foam height, calculations for the experimental runs were performed only for the case of no coalescence. Quantitative predictions of enrichment and recovery could not be made with a single representative bubble size because of the broad inlet bubble size distribution as well as broadening of the distribution as a result of coalescence. The experimental enrichments were higher and recoveries were lower than the model predictions, the discrepancy being more pronounced at lower feed concentrations because of increased coalescence. The higher enrichments are due to the predominant effect of internal reflux as a result of coalescence whereas the lower recoveries are a result of detrimental effects of broadening bubble size distributions.