I've read somewhere (but cannot find it anymore) that yeast will multiply until they reach a certain density (cells/ml) or exhaust their supply of sterols, whichever comes first.

But, then I've also read that sterol-deficient yeast (presumably from under-oxygenation at pitching time, right?) will have weak cell membranes, ultimately resulting in low alcohol tolerance and high Final Gravities.

These two seem to be contradictory. Which is correct?

- Al Korzonas

RESPONSE:

Sterol, fatty acids, lipids can act as a growth factor. They keep the cell wall elastic and fluid allowing transport systems into and out of the cell to operate. When there is a deficiency during the growth phase the yeast can not bud, the cell wall is too leathery for the bud to form. When there is a deficiency later in the fermentation, the cell wall is no longer fluid and the transport systems do not allow sugar into the cell and alcohol to move out of the cell. Alcohol builds up to the point that it becomes toxic. The cell eventually dies unless it receives oxygen to produce more lipids.

Yeast will usually continue to multiply until they have exhausted their nutrient supply: sugar, minerals, vitamins, nitrogen etc. or until they stop because they have produced a high enough level of their waste product (alcohol) that it becomes toxic. The toxic effect of the alcohol begins almost as soon as the alcohol production begins. The toxic effect is only very slight in the beginning but becomes very noticeable as the it reaches 5%. There is limited growth after the fermentation reaches 5% alcohol. The yeast still produce alcohol for cell maintenance.

In the commercial production of yeast, cell density becomes a factor even though the yeast is healthy. The cell population becomes so great that no know oxygenating equipment can supply enough oxygen to keep the cells multiplying.

Dr. Clayton Cone