Ethanol-Tolerant Yeast Strains are important in brewing beer. When a brewer considers what yeast strain he will use to ferment his beer, there are many qualities he may look for, but the yeast’s ability to finish the fermentation is among the most important. All brewing yeasts create ethanol as a by-product of their fermentation of sugars. Once alcohol concentration in the fermenting beer reaches a certain point, however, fermentation will cease. The ability of yeast to continue fermentation in the presence of high alcohol concentrations is referred to as “ethanol tolerance” and is highly dependent on the yeast strain itself. Large breweries often produce high-gravity beers for dilution later in the process.
For centuries, brewers yeast has been passed down from fermentation to fermentation by harvesting and repitching the yeast sediment. At 3%–5% alcohol by volume (ABV), brewer’s yeast is more tolerant of ethanol than most competing microorganisms. In fact, many microbiologists believe that ethanol production evolved as a type of defense mechanism for yeast. But only certain strains will withstand ethanol concentrations above 8%, with some particularly hardy strains able to handle up to 15% in normal fermentations. In recent centuries Belgian brewers in particular have tended to produce a large number of strong beers, and many Belgian yeast strains are quite alcohol tolerant. Some yeasts can ferment past the 8% level, but need coaxing to do so. The addition of nutrients, a high concentration of pitching yeast, rousing, and warmer temperatures will tend to result in greater alcohol tolerance. Some craft brewers have produced beers with ABVs around 20% by slowly dosing additional yeast and additional sugars into fermenting beers; however, many of these beers turn out to be strong but unpalatable. When brewers select yeast strains offered for sale by commercial laboratories, the normal ethanol tolerance of every strain will usually be listed as part of the yeast’s profile.
It has been suggested that high ethanol concentrations affect the porosity of the yeast plasma membrane. The yeast cell is then unable to transport nitrogen and sugar into the yeast cell despite their presence in the wort. Without the uptake of these nutrients, the yeast cell “shuts down” fermentation. Ethanol-tolerant yeast strains may have a plasma membrane makeup that gives them a particular ability to survive high ethanol concentrations.