Reasons for the apparent difference in the effects of produced and added ethanol on culture viability during rapid fermentation by Saccharomyces cerevisiae

Abstract

By feeding ethanol at various high rates to low cell density cultures of Saccharomyces cerevisiae it was shown that the sharp fall in viability when ethanol is produced during rapid fermentations is in part a direct consequence of the high rate of change of extracellular ethanol concentration. Nevertheless, the fall in viability in high cell density rapid fermentations which produced 98 g L(-1) ethanol in 3 h considerably exceeded that of control low cell density cultures to which ethanol was added at the same rate. This difference was shown to be not due to intracellular ethanol accumulation or to differences in glucose concentration between the cultures. The concentrations of a range of potentially toxic fatty acids, higher alcohols, and esters were measured during rapid fermentations, but when added at these concentrations to control cultures in the presence of ethanol they had no significant toxic effect. However, when rapid fermentations were conducted in rich medium containing 80 g L(-1) yeast extract, the apparent difference in toxicity of produced and added ethanol virtually disappeared. Magnesium was shown to be the component of yeast extract primarily responsible for this effect. The high rate of fall of viability when ethanol is rapidly produced is suggested to be partly due to the inability of the cells to adapt quickly enough to the rising ethanol concentration and partly to an increased demand for magnesium at higher ethanol concentrations which cannot be met in Mg-unsupplemented high cell density fermentations.

Dynamic microbial response under ethanol stress to monitor Saccharomyces cerevisiae activity in different initial physiological states

“…As already reported [22,33,34] when the initial ethanol concentration was near the critical value of 100 g l -1 , no growth nor ethanol production were observed. Moreover, stress induced by initial ethanol concentrations led to an increase in the global yield of glycerol production on glucose.…”

Microbial Cellulose Utilization: Fundamentals and Biotechnology

“…A difference between the inhibitory effect of added and produced ethanol, in conjunction with different intracellular and extracellular ethanol concentrations, was proposed for yeast in studies prior to 1983 (34,479,497). However, subsequent unrebutted work has discounted this hypothesis for yeast (133,134,135,229,395), Z. mobilis (313), and E. coli (168). In light of these data and in particular biophysical analyses indicating that substantial differences between intracellular and extracellular ethanol concentrations across plasma membranes can exist for at most short periods (313,395), it would appear unlikely that this discrepancy is due to a difference in the inhibitory effect of added and produced ethanol.…”

Physiology of yeasts in relation to biomass yields

“…For instance it has been shown that addition of biotin (Winter et al 1989) or magnesium (Dasari et al 1990) can relieve the effect of apparent ethanol inhibition. Furthermore, the observation that added ethanol is less toxic than endogenously produced ethanol (Novak et al 1981) has led to suggestions that many of the effects orginally ascribed to ethanol are due to other metabolites, such as longer alcohols (Oko-10 et al 1987) or weak acids (Viegas et al 1985).…”