Effects of added yeast on protein transmission and flux in cross-flow membrane microfiltration

Microfiltration membranes may be used to separate valuable proteins from suspensions containing cells or cell debris. Although a clean microfiltration membrane allows for complete protein transmission and high flux, both of these quantities decline in time due to membrane fouling. Using bovine serum albumin (BSA) as a model protein, flux and protein transmission during cross-flow microfiltration were studied with and without added yeast cells. Cross-flow microfiltration of BSA-only solutions results in a BSA fouling layer with low permeability forming on the membrane surface. Due to this layer, the long-term BSA transmission is typically only 25-40%. In contrast, during microfiltration of yeast-BSA mixtures, the yeast forms a cake layer on the membrane surface. The yeast cake acts as a dynamic or secondary membrane, allowing BSA monomers to pass through but preventing protein aggregates from fouling the membrane. The result is slower flux decline and higher long-term BSA transmission of typically 60-90%. For filtration of yeast-BSA mixtures at low yeast concentrations (<1 g/L), 50-100% higher BSA recovery is obtained than for BSA-only solutions with the same BSA concentration. At high yeast concentrations (>5 g/L), the protein transmission remains high, but the recovery may be lower due to reduced flux.