Removal of impurities from protein foam in foam fractionation process by wash water addition.

This work introduces a simple and efficient method to remove the impurities from a protein foam through washing the foam in a foam fractionation process (figure 1). Since the protein molecules adsorb irreversibly on the air interface, they do not desorb upon wash water addition and are transferred to the foam outlet. However, the entrained substances are directed downward by wash water to the drain outlet together with the liquid. Here, we performed experiments on bovine serum albumin (BSA), as a model protein and NaCl salt, as a model of soluble impurities. The experiments were conducted in a glass foam fractionation cell, where the liquid level was kept constant. The wash water was added on the foam top with different flow rates and BSA and NaCl concentrations were measured at the outlets for further analysis. The influence of initial bubble size and the wash water rate on the purification efficiency were investigated. The results revealed that the wash water displaces the entrained liquid in the foam and reduces the salt content at the foam outlet. The process shows higher salt removal for higher wash water rates as well as for foams with larger bubble sizes, where up to 93% of the salt was removed from the main solution in a steady state process. The washing efficiency is also influenced by air flow rate. Salt removal is enhanced in principle at lower air flow rates. However, the foam stability becomes an important issue at smaller air flow rates, since the increased foam collapse significantly reduces the foam flow to the outlet.