In situ bubble fractionation strategies for separating individual proteins in a batch baker's yeast fermentation process.

Extracellular proteins produced by yeast have been observed to stratify in the extracellular fluid of a batch bioreactor, thus creating a vertical concentration gradient. We observed that, in the four different experiments conducted, each varied in their protein recovery characteristics. For example, sparging the system with gas accentuates the separation, though even in a nonsparged system, the in situ generation of minute carbon dioxide bubbles by yeast cells creates a protein gradient as the bubbles carry proteins upward. Based on these and other observations, we propose possible strategies for recovering the individual proteins from a system containing the four major proteins considered. A simple steady-state mathematical model, based on convective upward protein transport being balanced by downward protein diffusion, has been used to describe the behavior of each of these four extracellular proteins in the fermentation broth.