Product Inhibition Influence on Immobilized Cell Biocatalyst Performance

Abstract

AbstractThe potential steady‐state influence of product inhibition on the performance of immobilized cell cultures is thoroughly explored with the aid of a simple unstructured reaction‐diffusion model. Production inhibition was assumed to diminish the specific growth rate directly. The range of product sensitivity explored corresponds directly with the observed range of ethanol sensitivity of Clostridium thermocellum, Clostridium ace‐tobutylicum, Saccharomyces uvarum, Zymomonas mobilis, and Saccharomyces cerevisiae. The consequent influence on the immobilized cell growth rate, thickness of steady‐state viable cell layer, formation of growth‐associated product, and global performance of the immobilized cell particle is explored by simulated variation of both the external bulk solution product level and the cell intrinsic sensitivity to the inhibitory product. Thus, both the thickness of the steady‐state viable cell layer and the consequent biomass loading of the bead are predicted variables of the model; these are variables due to the adaptive growth and biomass redistribution which occur in porous supports. The calculated influences are appreciable and even dramatic under some circumstances; the implications for packed‐bed reactor operations for ethanol and solvent production are discussed.These model calculations indicate that product inhibition can very significantly influence the performance of immobilized cell particles. Immobilization alone necessarily increases the impact of product inhibition on the culture since intraparticle product levels must be higher that external solution conditions. The presence of product inhibition lowers the specific growth rate of the culture pointwise in the carrier, thus diminishing the particle conversion of substrate to product and the total productivity and specific activity of the biocatalyst. Counterintuitively, perhaps, increased sensitivity to product inhibition results at first in an increased viable biomass loading of each carrier particle; then at still higher sensitivities a decreased loading and eventual extinction of viable cells is predicted. The application of these calculations for packed‐bed reactor design is suggested and constitutes a portion of our ongoing studies.