The adenylate energy charge and specific fermentation rate of brewer's yeasts fermenting high- and very high-gravity worts

Abstract

Intracellular and extracellular ATP, ADP and AMP (i.e. 5'-AMP) were measured during fermentations of high- (15 degrees P) and very high-gravity (VHG, 25 degrees P) worts by two lager yeasts. Little extracellular ATP and ADP but substantial amounts of extracellular AMP were found. Extracellular AMP increased during fermentation and reached higher values (3 microM) in 25 degrees P than 15 degrees P worts (1 microM). More AMP (13 microM at 25 degrees P) was released during fermentation with industrially cropped yeast than with the same strain grown in the laboratory. ATP was the dominant intracellular adenine nucleotide and the adenylate energy charge (EC = ([ATP] + 0.5*[ADP])/([ATP] + [ADP] + [AMP])) remained high (>0.8) until residual sugar concentrations were low and specific rates of ethanol production were < 5% of the maximum values in early fermentation. The high ethanol concentrations (>85 g/l) reached in VHG fermentations did not decrease the EC below values that permit synthesis of new proteins. The results suggest that, during wort fermentations, the ethanol tolerance of brewer's strains is high so long as fermentation continues. Under these conditions, maintenance of the EC seems to depend upon active transport of alpha-glucosides, which in turn depends upon maintenance of the EC. Therefore, the collapse of the EC and cell viability when residual alpha-glucoside concentrations no longer support adequate rates of fermentation can be very abrupt. This emphasizes the importance of early cropping of yeast for recycling.