Yeast and Alcohol Fermentation
Location: Outside U.S.
Date: Spring 2013
If alcohol is poisonous to yeast, why does it produce it?
It is a waste product. Our waste products are poisonous to us too.
Richard E. Barrans Jr., Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
Thanks for the question. Yes, alcohol is poisonous to yeast, when the alcohol is present in large amounts, say greater than 8%. Alcohol is produced by yeast as a by-product of their metabolism (digesting) of sugars when there is no oxygen present (so-called anaerobic conditions). What happens is that yeast produce alcohol (their waste product) while they are eating sugars. Eventually, their waste product (alcohol) builds up enough that it kills them. For this reason, the alcohol in beer and wine is usually not more than 8% (or less) since the yeast simply die when too much alcohol is present.
I hope this helps. Please let me know if you have more questions.
The large scale answer "why" is because it has enabled yeast to survive as a species.
It's somewhat similar to the "why" do humans produce carbon dioxide if it's poisonous to us?
The details of the why?
Most of the time, the amount we produce is not a problem, we survive, reproduce, and the species survives.
However, occasionally, members of the species get trapped in scenarios where they can't escape from their own waste products (humans in collapsed mines, yeast in fermentation tanks), and die. But so long as they produced (before they died) or others have produced some offspring that survive outside of that environment, this is not a problem for the species.
With all toxins, two parameters define their toxicity. The first is a measure of its inherent toxicity to the various living species in the culture. The second is the concentration of the toxic substance in the medium. It may be different for different media, but the concentration nonetheless. When a the concentration of alcohol reaches a certain upper limit, the yeast (or other microbe) decreases in its ability to produce ethanol, the product of fermentation, and eventually dies. This is why certain types of wine, such as sherry or port, have to have ethanol added after fermentation, and why those wines are called “fortified”.
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