Fermentation of sugars to ethanol is carried out in the absence of air. What happens if air gets into the fermentation mixture?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → The oxygen of air oxidises ethanol to ethanoic acid, which destroys the taste of the drink.
Fermentation by the enzymes of yeast takes place under anaerobic conditions. If air gets in, its oxygen oxidises the ethanol formed to ethanoic acid, and the sour acid spoils the taste of the alcoholic drink:
\( \displaystyle \ce{CH3CH2OH + O2 -> CH3COOH + H2O} \)
Why the other options are incorrect
Option (1) The action of zymase is inhibited, so the yeast stops forming ethanol. — Incorrect. The action of zymase is inhibited when the percentage of alcohol formed goes above 14 percent. That limit is set by the alcohol, not by air.
Option (2) The oxygen of air oxidises ethanol to ethanal, which gives the drink a foul smell. — Incorrect. A primary alcohol is oxidised to the aldehyde, which in turn is oxidised to the carboxylic acid. The product that spoils the drink is the acid, ethanoic acid.
Option (3) The oxygen of air oxidises ethanol to methanol, which makes the drink poisonous. — Incorrect. Oxidation of ethanol keeps both carbon atoms; it gives ethanal and then ethanoic acid. Methanol has one carbon and is not formed from ethanol in this way.
NCERT (2026-27 edition):
7.5 Some Commercially Important Alcohols – “…Fermentation takes place in anaerobic conditions i.e. in absence of air. Carbon dioxide is released during fermentation.”
“…If air gets into fermentation mixture, the oxygen of air oxidises ethanol to ethanoic acid which in turn destroys the taste of alcoholic drinks.”