Idli Batter, Temperature and Anaerobic Respiration
A School Science Club prepares idli batter using parboiled rice and black gram, adding a little yeast to one batch and relying on natural fermentation in another. Batch A (with yeast) is kept in a closed steel container in a warm kitchen at 32°C. Batch B is kept in the refrigerator at 8°C. Batch C is kept at 32°C but with the lid loosely covered. After 10 hours, Batch A has doubled in volume, smells pleasantly sour, and a glowing splint held above the batter goes out. Batch B has not risen. Batch C has risen only slightly and has a less sour smell. A student says the gas must be oxygen because the batter 'breathes in air'. Another says the sour smell proves the batter is rotting and should be thrown away. The teacher asks the club to explain the results using respiration, not just the smell.
1. Identify the gas produced in Batch A and explain the splint test. Why is the gas not oxygen?
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- The gas is carbon dioxide, produced by anaerobic respiration of yeast.
- In yeast, glucose is broken down to ethanol and carbon dioxide, releasing energy: glucose → ethanol + carbon dioxide + energy.
- Carbon dioxide does not support burning, so a glowing splint placed above the gas goes out. Oxygen would relight the glowing splint, so the result contradicts the 'breathes in air' claim.
- The closed container limits oxygen, so yeast shifts to anaerobic respiration. The gas accumulates and the batter rises as bubbles are trapped.
2. Compare Batches A, B and C and explain the differences using temperature, oxygen and gas escape. Include what would happen if the yeast were boiled before use.
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- Batch A is at a favourable warm temperature, is sealed, and has yeast. Fermentation is fast, carbon dioxide is trapped, and the batter rises well.
- Batch B is cold. Low temperature slows down enzyme activity in yeast, so little fermentation occurs and the batter does not rise.
- Batch C is warm but loosely covered. Some carbon dioxide escapes, and oxygen can enter, so the rise is smaller and anaerobic fermentation is less favoured. The sour smell is also weaker because fewer fermentation products accumulate.
- The sour smell is not proof of rotting. It comes from acids and other fermentation products that are normal in idli batter. Boiling yeast would kill it and denature its enzymes, so Batch A would not rise unless other microbes were present; any rise would be slower and less predictable.