Modeling on-line selective protein recovery from a semi-solid substrate in an air-fluidized bed bioreactor

Abstract

Baker's yeast was grown on a semi-solid substrate (homogenized whole potatoes) in an air-fluidized bed bioreactor. A high velocity stream of humidified air was used to supply oxygen as well as to mix the viscous substrate. α-Amylase was used as an enzyme to convert the starch to degradable sugars. During batch growth, certain extracellular yeast proteins were trapped by sparging the effluent air into a water chamber. Surprisingly, the proteins carried out were not the most abundant ones available in the process mixture. About half of the recovered proteins had a molecular weight of 33 kDa. The Monod and the Leudeking-Piret equations were used to describe the yeast cell growth and the extracellular protein formation, respectively, in the starch-cell media. For modeling purposes, the combined protein production and separation process is visualized as being an equilibrium process between the extracellular protein in the fluidized bed and the protein carried over into micro water droplets in the air. This model, therefore, is similar to the Rayleigh batch distillation equilibrium model except that the notion of “volatility” is replaced with the idea of partitioning or extraction between two liquid phases. Another analogy (to the more traditional distillation separation process) employed here was the use of a correction term to deviations from equilibrium. In distillation, this is referred to as the Murphree vapor efficiency term. In this work, the efficiency term is correlated to the air-flow rate in the fluidized bed.