Improvement of Alcohol Fermentation of a Corn Starch Hydrolysate by Viscosity-Raising Additives

Aim of this work is to investigate the simultaneous effects of viscosity and temperature on the productivity of the alcohol fermentation of starch hydrolysate by Saccharomyces cerevisiae. Batch fermentations have been carried out at given pH, broth composition, inoculum, and agitation intensity, but at varying temperature (24<T<46°C) or concentration of carboxymethyl cellulose (0.4<CCMC<2.0g/L), chosen as viscosity-raising additive. The results of tests carried out at giver viscosity demonstrate that the volumetric productivity increases with temperature up to an optimal value (32-36°C). At higher temperatures a productivity drop occurs. In addition, a viscosity increase up to about 12 g m-1 s-1 (value determined at 30°C) improves the fermentation kinetics, while the process is strongly negatively affected at viscosity values higher than this threshold. Both the Arrhenius and the so-called "thermodynamic" models have then been used to estimate the related thermodynamic quantities referred to both fermentation and thermal deactivation. A comparison of the values of these quantities suggests that both cell growth repression provoked by mass transfer limitations due to viscosity rise and the reduction of product inhibition are possible simultaneous causes of the observed productivity enhancement at low CMC levels.