Aminopterin Derangement of Yeast Folic Acid Metabolism and Theoretical-Practical Consequences Thereof in Fermentation and Packaging

Abstract

Stationary and aerated fermentations conducted using synthetic or complex (wort and beer) fermentation media, brewing strains of Saccharomyces cerevisiae and a beer spoilage yeast S. diastaticus, with increasing aminopterin (4-amino folic acid) concentrations are described. The package beer biological preservative studies include synergism examples (EDTA+aminopterin and EDTA+WS-7). Fermentation assays include total yeast cell population/ml of beer, yeast crop dry weight, average cell size, per cent budding and viable cells, cell protein, flocculation, and fermentation liquor pH (indirect measure of fermentation rate). Additional assays include the effect of aminopterin on yeast cell ultrastructure and aerobic respiration. In fermentations conducted with increasing aminopterin concentrations, progressive changes were observed: decreased yeast cell population, dry weight, viability, fermentation rate, and respiration; increased cell size, budding, flocculation, floc formation, and beer pH. Cellular protein remained essentially the same. In electron microscope studies, aminopterin-treated cells showed better fixative penetration and increased formation of lipid inclusions and folds in the cytoplasmic membrane. Aminopterin retarded S. diastaticus growth in package beer. Synergism was also shown. The relationship of results to a change in cell membrane permeability and diacetyl production in beer is discussed.