This complicates using mathematical calculations to predict protein recovery by foaming. However some trends have been determined; high recovery rates have been linked to high concentrations of protein in the initial solution, high gas flow rates, and high feed flow rates. Enrichment is also known to increase when foaming is performed using shallow pools. Using pools with low heights allows for only a small amount of protein to adsorb from the solution to the surface of the bubbles in the foam resulting in lower surface viscosity. This leads to coalescence of the unstable foam higher up in the column causing an increase in the bubble size and an increase in the reflux of the protein in the foam. However, an increased velocity of the gas being pumped into the system has been shown to lead to a decrease in the enrichment ratio. Since these calculations are difficult to predict, bench and then pilot scale experiments are often performed in order to determine if foaming is a viable technique for extraction on an industrial scale. Bacterial cell extraction Separation of cells is typically done using centrifugation, however foam separation has also been used as a more energy efficient technique. This method has been used on many species of bacteria cells such as Hansenula polymorph, Saccharomyces carlsbergensis, Bacillus polymyxa, Escherichia coli, and Bacillus subtilis, being most effective on cells that have hydrophobic surfaces. Current and Future Directions Continuous foam extraction was initially used in regard to wastewater treatment in the 1960s. Since then there has not been a lot of research in foaming as an extraction technique. However, in recent years foaming for protein and pharmaceutical extraction has gained increased interest for researchers. Purification of products is the most expensive part of product production in biotechnology, foaming offers an alternative method that is less expensive than some current techniques. Separation equipment Foaming apparatus Continuous foam separation is one of two major modes of foam separation with the other being batch foam separation. The difference between the two modes is that in continuous mode, surfactant solution is continuously fed through a feed into the foam column and a solution, extracted of surfactant, is also continuously exiting the bottom of the apparatus. The figure to the right shows a diagram of a basic continuous foam separator. The process is stationary (or in steady state) as long as the volume of liquid is constant as