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 an d temperature on the productivity of the alcohol fermentation of starch hyd rolysate 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 degrees C) or concentration of carboxymeth yl cellulose (0.4<C-CMC<2.0g/L), chosen as viscosity-raising additive. The results of tests carried out at even viscosity demonstrate that the volumet ric productivity increases with temperature up to an optimal value (32-36 d egrees C). At higher temperatures a productivity drop occurs. In addition, a viscosity increase up to about 12g m(-1) s(-1) (value determined at 30 de grees C) improves the fermentation kinetics, while the process is strongly negatively affected at viscosity values higher than this threshold. Both th e Arrhenius and the so-called "thermodynamic" models have then been used to estimate the related thermodynamic quantities referred to both fermentatio n and thermal deactivation. A comparison of the values of these quantities suggests that both cell growth repression provoked by mass transfer limitat ions due to viscosity rise and the reduction of product inhibition are poss ible simultaneous causes of the observed productivity enhancement at low CM C levels.