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Microbes That Help Make Food
- Yeast is a microorganism, a type of fungus.
- Yeast respires and releases carbon dioxide gas.
- The gas forms bubbles that make dough soft and fluffy.
- Some bacteria, like Lactobacillus, ferment milk into curd.
- These microbes grow best in warm conditions.
Example 1 — Yeast in Dough
- Yeast is mixed into dough with sugar and warm water.
- It makes carbon dioxide bubbles.
- So the dough rises and becomes fluffy.
Example 2 — Curd from Milk
- Lactobacillus from curd is added to warm milk.
- It ferments the milk and makes lactic acid.
- So the milk sets into sour curd.
Important Points
- Yeast also makes a little alcohol, giving dough its smell.
- Lactobacillus makes lactic acid, which makes curd sour.
- Bacteria like Rhizobium in legume roots fix nitrogen for plants.
Definition — Yeast
Yeast is a single-celled fungus that releases carbon dioxide and is used to make breads, cakes, idlis, and dosas rise.
Definition — Fermentation
Fermentation is the process in which microbes break down sugars, as when Lactobacillus turns milk into curd.
Materials needed
Two bowls and flour (atta or maida).
Sugar, yeast powder, and warm water.
Sugar, yeast powder, and warm water.
Procedure
Put flour and a pinch of sugar in each bowl.
Add yeast to bowl A only, none to bowl B.
Knead both with warm water into soft dough.
Cover with a damp cloth and keep warm for 4 to 5 hours.
Add yeast to bowl A only, none to bowl B.
Knead both with warm water into soft dough.
Cover with a damp cloth and keep warm for 4 to 5 hours.
Observation
The dough in bowl A rises, turns fluffy, and smells different. Bowl B stays flat.
Explanation
Yeast releases carbon dioxide, which makes bubbles in the dough. So bowl A rises but bowl B does not.
◆ Summary
- Yeast in bowl A only.
- Bowl A rises and puffs.
- Bowl B stays flat.
- Yeast gives off gas.
Materials needed
Two glass bowls.
Lukewarm milk, cold milk, and a spoon of curd.
Lukewarm milk, cold milk, and a spoon of curd.
Procedure
Pour lukewarm milk in bowl A and cold milk in bowl B.
Mix a spoon of curd into each.
Keep bowl A warm and bowl B cold (in a fridge).
Observe after a few hours or overnight.
Mix a spoon of curd into each.
Keep bowl A warm and bowl B cold (in a fridge).
Observe after a few hours or overnight.
Observation
Bowl A sets into sour curd. Bowl B stays as milk.
Explanation
Lactobacillus in the curd ferments warm milk and makes lactic acid. It grows well in warm conditions, so bowl A sets but bowl B does not.
◆ Summary
- Curd added to both bowls.
- Warm bowl A sets into curd.
- Cold bowl B stays milk.
- Bacteria need warmth.
⭐ A Step Further — Rhizobium and Nitrogen
Rhizobium bacteria live in swollen nodules on the roots of legumes like beans, peas, and lentils.
They trap nitrogen from the air and make it useful for the plants. So farmers grow legumes in rotation to keep the soil rich in nitrogen, without chemical fertilisers.
