Question

Some of your friends are attempting to make their own wine. They start with grapes (a sugar source) and add yeast. However, when they check on their wine sometime later, they find that no ethanol is present. What might have prevented the yeast cells from producing the ethanol

Answer 1

There could be two reasons why yeast cells failed to produce alcohol. First, ethanol fermentation by yeast is a biological process where the sugars from grapes are converted into alcohol where CO2 is released as the by-product. Fermentation is an anaerobic process i.e it occurs in the absence of oxygen. If oxygen is present in the environment, the pyruvate obtained is completely oxidized into CO2 and water which is a process of cellular respiration. Yeasts ferment sugars to produce alcohol only in the anaerobic environment called as the Pasteur effect. Thus, if oxygen is present in the environment, yeast cells would not be able to produce ethanol. Secondly, if the grapes used in this process has very low sugar content in them, yeast cells would not be able to produce ethanol. Good quality grapes with high sugar concentration should be used to obtain high yield of alcohol.

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