Cell growth and ethanol production characteristics of immobilized S. cerevisiae in a batch system with nutrient recirculation

Abstract

Recently, there has been world-wide interest in the production of ethanol by fermentation as an alternative to chemical feed stock and as a gasoline extender. Savings in utilities as well as reduction in fixed costs can be achieved if the sugar syrups can be fermented developing high ethanol concentrations. However, conventional ethanol production uses free cells and comparatively low substrate concentrations in order to ensure a speedy fermentation without inhibitory reactions by high sugar or high ethanol concentrations. Over the past few years, a new approach of great potential has been the use of immobilized cell systems to produce ethanol. Due to higher cell mass per unit fermenter volume and high rates of ethanol production in an immobilized cell system, it may be possible to achieve high ethanol concentration by fermenting solutions of high sugar concentrations. In this research, a flocculating strain of Saccharomyces cerevisiae was entrapped In polymeric support particles having a complex void structure and a range of experiments were conducted in order to obtain kinetic data on the growth and the ethanol production characteristics in an immobilized cell reactor under batch operation with nutrient recirculation. The pH conditions, ethanol production rates, residual glucose concentrations and cell densities in the system were measured at different operation times and substrate concentrations. Ethanol productivities, production rates, percent conversions and yields for four different substrate concentrations namely 6, 10, 12 and 14% glucose were compared to determine the optimum substrate concentration under these conditions and was found to be 10%. The growth patterns in the immobilized cell reactor showed that the yeast cells enter the stationary phase in about 15 hours of operation after inoculation for the 6 and 10% glucose concentrations while longer periods are required to reach the stationary phase at 12 and 14 % glucose concentrations. The specific growth rates of the yeast cells in the exponential phase were found to be 0.200, 0.124, 0.079, and 0.088 lIh for 6, 10, 12 and 14% glucose concentrations respectively.