Sterols are essential grain-derived yeast nutrients. They are types of cholesterol that form part of the yeast’s cell wall membrane. They have two principal functions. They make the cell wall permeable for the metabolic transfer of sugars into and alcohol out of the cell and they make the yeast more tolerant to increasing levels of alcohol. After completing fermentation, yeast sterol levels are low, and repitched yeast needs sterol levels to be restored to carry out the next healthy fermentation. The level of sterols within yeast cells after pitching is an indicator of the yeast’s viability during fermentation and its ability to continue to actively ferment wort through the primary fermentation and into any intended conditioning, maturation, or lagering phase. Fermentation scientists, therefore, often refer to the sterol level in yeast by the French descriptor of facteur de survie, meaning survival factor.
Yeast uses oxygen to synthesize sterols and unsaturated fatty acids (UFA), largely during the aerobic phase of the fermentation cycle; this is why ample wort aeration at pitching time is essential for successful fermentations. Incomplete or stuck fermentations, by contrast, are often the result of the yeast not having synthesized sufficient sterols because of a lack of dissolved oxygen in the wort at the start of its development. Interesting recent research has suggested the possibility that oxygen addition at the time of yeast storage, used in conjunction with an addition of olive oil to provide the UFA, may be used to replace wort aeration, allowing repitched yeast to begin fermentation with ample levels of both sterols and UFA.