Yeast Survival Factors are also commonly referred to as oxygen substitutes(1) or anaerobic growth factors(2). These are compounds that ensure yeast viability under stress and ultimately, the survival of the yeast. The compounds include sterols, fatty acids, and peptides. You might be thinking that Yeast Survival Factors are all about how yeast protects itself to stay alive and reproduce. Ultimately, this is correct but, Survival Factors also play a big role in every fermentation. This is because of oxygen and ethanol. The later because of the ever increasing amount and the former because of the lack of it.
Generally, yeast prefer to live in an oxygen rich environment. The respiration process is the most efficient for yeast to create energy, (ATP) but that process needs oxygen. As cider makers, we usually create an anaerobic environment (no oxygen) to promote fermentation over respiration. An oxygen rich, aerobic, environment would generally be less stressful to yeast but result in an undesirable hard cider. Anaerobic environments and fermentation are not as friendly to yeast. Ethanol, for example, is toxic to yeast cells because is makes the yeast membrane more permeable. This permeability prevents the yeast cell from maintaining proper balance leaving it exposed to acidification and other harmful conditions. For example, the pH of yeast cells is usually 6-7, remember water is 7 and cider is usually 3-4. Many yeast processes release H+ ions and the cells have to excrete these to maintain their pH levels. If the cell membrane is permeable, it can’t keep the H+ ions out and the cell will shutdown.
Survival Factors define how well yeast are capable of resisting these stressful environments. The fewer Survival Factors (i.e. sterols, fatty acids, and similar compounds) that are stored in the yeast cell, the lower the probability the yeast will survive stressful conditions. Yeast need oxygen to process and absorb Survival Factors. This is why you generally want lots of oxygen available during the first phase of fermentation followed by no oxygen. The high level of oxygen allows the yeast to store Survival Factors and prepare for the coming stressful environment you will create in a sealed fermenter with no oxygen, high ethanol, and limited nutrients. This is also why it is good practice to use a yeast starter when reusing yeast that has been harvested from a batch of cider. It helps ensure that yeast can perform properly in the next cider batch.
(1) K. O’Kennedy and G. Reid, Yeast nutrient management in winemaking, The Australian & New Zealand Grapegrower & Winemaker, Issue 537, pop. 92-100, October 2008
(2) S. Lafon-Lafourcade and associates, Evidence for the Existence of “Survival Factors” as an Explanation for Some Peculiarities of Yeast Growth, Especially in Grape Must of High Sugar Concentration, Applied and Environmental Microbiology, Vol. 38, No. 6, p. 1069-1073, Dec 1979
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