Question

Pasteur’s effect is :
(a) Stoppage of fermentation in presence of oxygen
(b) Increase of fermentation in presence of oxygen
(c) No effect on fermentation
(d) None of the above

Answer 1

Hint: Berzelius and Liebig proposed that fermentation was caused by decomposition. Louis Pasteur, a French scientist demonstrated that their theory was incorrect. During this demonstration, he observed a phenomenon which was known as Pasteur's effect.

Complete Solution:
Pasteur’s effect is an inhibitory effect of oxygen on the fermentation process.
Explanation:
Yeast is a facultative anaerobe, it can produce energy by using two different metabolic pathways.
(i) When the oxygen concentration is low, pyruvate which is the product of glycolysis is turned into ethanol and $CO_{2}$. The efficiency of energy produced is low (2 moles of ATP per one mole of glucose.
(ii) If the oxygen content grows, pyruvate gets converted to acetyl CoA which can be used in the citric acid cycle. This increases the efficiency to 36 moles of ATP per one mole of glucose.
Negative pasteur’s effect, which is the inhibition of alcoholic fermentation under anaerobic conditions, is stimulated in the presence of elementary oxygen molecules. This effect was demonstrated in Brettanomyces strains and Torulopsis cylindrica.
Louis Pasteur was one of the important fathers of the germ theory and was famous for developing the pasteurization process by stating that microbes were responsible for souring alcohol. He developed vaccines for rabies and saved the silk industry.

So, the correct answer is ‘Stoppage of fermentation in presence of oxygen’.

Note:
Pasteur’s effect should not be confused with Pasteurization which is a process in which the liquids such as milk are subjected to high temperatures between $60-100 ^{o} C$ for short periods and then the liquid is cooled. This eliminates most of the microorganisms present in the liquid.