Q. In 1861, Louis Pasteur observed that while yeasts grow in a sugar and protein medium, completely free of air, they ferment vigorously; and for every gram of yeast that forms, 60 to 80 grams of sugar disappeared. If experiment is carried out in the presence of air, for one gram of yeast that forms, only 4 to 10 grams of sugar are removed. Yeasts again ferment if transferred to a sugar-containing medium absent air.

When an experiment is repeated with a protein medium, yeasts grow only in the presence of oxygen. Pasteur concluded that the yeasts can take oxygen from air, and in the absence of air, the yeasts take oxygen from sugar.

Pasteur applied quantitative methods to his studies of fermentation and was first to report on organisms that could live and reproduce in the absence of oxygen. His conclusions were, however, incorrect. These different behaviours of yeasts are called today as the Pasteur Effect.

1. Describe the three yeast behaviours based on modern concepts of microbial metabolism.

2. What was wrong about Pasteur’s conclusion?