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The effects of acetic acid and extracellular pH (pHex) on the intracellular pH (pHi) of nonfermenting, individualSaccharomyces cerevisiae cells were studied by using a new experimental setup comprising a fluorescence microscope and a perfusion system. S. cerevisiae cells grown in brewer’s wort to the stationary phase were stained with fluorescein diacetate and transferred to a perfusion chamber. The extracellular concentration of undissociated acetic acid at various pHex values was controlled by perfusion with 2 g of total acetic acid per liter at pHex 3.5, 4.5, 5.6, and 6.5 through the chamber by using a high-precision pump. The pHi of individual S. cerevisiae cells during perfusion was measured by fluorescence microscopy and ratio imaging. Potential artifacts, such as fading and efflux of fluorescein, could be neglected within the experimental time used. At pHex 6.5, the pHi of individualS. cerevisiae cells decreased as the extracellular concentration of undissociated acetic acid increased from 0 to 0.035 g/liter, whereas at pHex 3.5, 4.5, and 5.6, the pHi of individual S. cerevisiae cells decreased as the extracellular concentration of undissociated acetic acid increased from 0 to 0.10 g/liter. At concentrations of undissociated acetic acid of more than 0.10 g/liter, the pHi remained constant. The decreases in pHi were dependent on the pHex; i.e., the decreases in pHi at pHex 5.6 and 6.5 were significantly smaller than the decreases in pHi at pHex 3.5 and 4.5.

No abstract

The effect of yeast trehalose content at pitching on the fermentation performance during brewing fermentations was studied using a commercial strain of lager yeast, Saccharomyces cerevisiae (AJL 2155). Pitching yeasts with different trehalose contents were obtained by collecting cells in suspension after 96 h and 144 h of fermentation in EBC tubes in 10.8°P brewers wort at 14°C. The trehalose content of the pitching yeast had no effect on growth, specific gravity and ethanol pro duction during the subsequent fermentation. A high trehalose content of the pitching yeast, however, sustained cell viability during the initial stage of fermentation, increased the carbohydrate utilisation rate and increased the production of isoamyl alcohol and isobutanol. For these aspects of fermentation performance, the trehalose content of the pitching yeast may prove useful in evaluating the vitality of pitching yeasts within the brewery.

Flow cytometry was used for the determination of the intraceilular esterase activity of unstressed and stressed Saccharomyces cerevisiae cells using fluorescein diacetate as substrate during brewing fermentations in EBC tubes. The determination of intraceilular esterase activity by flow cytometry was compared with In vitro assays for determination of yeast esterase activity. The method was regarded valid with a high degree of reproducibiiity. Intraceliuiar esterase activity during brewing fermentations was dependent on the yeast strain applied but independent of the wort compositions applied within this study. Further, the intraceliuiar esterase activity during fermentation was correlated with cell proliferation determined by DNA staining and flow cytometry and by calculating the percentage of G1-phase cells. Yeast esterase activity in both unstressed and ethanol stressed cells followed a similar pattern during brewing fermentations. Furthermore, this pattern could be correlated with the percentage of G1-phase cells during fermentation indicating that the esterase activity was in some way related to cell cycle progression.