Cell physiology of baker's yeast in chemostats subjected to lactic acid perturbations

Knowledge of microbial nutrition has become more relevant in recent years with the increased use of microorganisms and their products as food additives and supplements. In this paper, we describe the effect of lactic acid on the steady state growth of baker's yeast (Saccharomyces cerevisiae). This organism is frequently used by food producers as a flavour enhancer in various yeast extract preparations. Sacch. cerevisiae is propagated indu¬strially from sugar cane and/or beet molasses which is often of low-quality, being a zvasle product from the sugar refining process. The presence of lactic acid in batches of molasses from various sources may lead to subsequent loss of yeast biomass yield following industrial propagations. This study zvas aimed at defining the physiological effects occurring in the yeast cells subjected to perturbations, or "spikes", of lactic acid when grown in glucose-limited chemostat culture. The results of analysis of yeast biomass, cellular protein, glycogen, trehalose and RNA obtained shozved a differential response of cells to elevation of lactic acid levels zulucli zvas dependent on yeast grozuth rate. At sub-optimal growth rates the effect zvas devastating to the cells, resulting in loss in biomass, and significant changes in intermediary metabolism, possibly due to a cytotoxic effect of lactic acid diffusing into the cells. However, the yeast cells at supra-optimal grozuth rates responded with much less deviation compared to steady-state control cultures in the chemostat.