You are here
๐ฌ Think about it
Add a spoon of curd to warm milk and leave it overnight — the whole bowl becomes curd. A pinch of yeast makes dough puff up and rise. Tiny living things are doing the work. How? Let's find out.
What does yeast do to dough?
What does yeast make inside dough?
Yeast in dough
It respires and breaks food
Carbon dioxide
Forms bubbles in dough
Makes it soft and fluffy
Example:
Bread
Cakes
Alcohol
Made in small amount
Gives a different smell
Example:
The smell of risen dough
- Yeast is a microorganism (a fungus).
- It breaks down sugar and releases carbon dioxide gas.
- The gas bubbles make the dough soft and fluffy.
How is curd formed from milk?
How does warm milk turn into curd?
Warm milk + a spoon of curd
Curd carries Lactobacillus bacteria
↓
Lactobacillus multiplies
It feeds on lactose, the sugar in milk
↓
Lactic acid is produced
Not alcohol, as yeast would make
↓
The milk sets into curd and tastes sour
- Curd contains bacteria like Lactobacillus .
- These bacteria feed on milk sugar and multiply.
- They make lactic acid, which turns milk into sour curd.
What are legumes?
| Legume | Where the nodules are |
|---|---|
| Bean | On the roots, holding Rhizobium |
| Pea | On the roots, holding Rhizobium |
| Lentil | On the roots, holding Rhizobium |
- Legumes are plants whose seeds grow in pods.
- Their roots have tiny swellings called root nodules.
- These nodules are home to helpful bacteria.
Example:
Peas, beans, gram, and lentils are all legumes.
How do bacteria in legumes help plants grow?
What do plant and bacteria give each other?
Rhizobium in root nodules
A partnership in soil
Plant gives shelter
Swollen root nodules
Example:
Beans
Peas
Lentils
Bacteria give nitrogen
Trap nitrogen from air
No chemical fertiliser
Example:
Legumes grown in rotation
- Rhizobium bacteria live in the root nodules of legumes.
- They trap nitrogen from the air and give it to the plant.
- So legumes grow well and make the soil fertile without chemical fertilisers.
๐ฅ
Activity 2.8 — Let us perform
How do we test what yeast does to dough?
Take two bowls, A and B
200 g of flour and a pinch of sugar in each
↓
Add yeast to bowl A only
Bowl B has no yeast — it is the comparison
↓
Knead both with warm water
Make a soft dough in each bowl
↓
Cover and keep them warm
A damp cloth, in a warm place, for 4–5 hours
↓
Bowl A has risen, turned fluffy and smells different; bowl B has not
In this Activity, we will knead two bowls of dough, one with yeast and one without, and compare them.
Materials needed
Flour, water, sugar, yeast, and two bowls (A and B).
Procedure
1. In bowl A, mix flour, water, a little sugar, and yeast into a dough.
2. In bowl B, mix flour, water, and sugar into a dough, but add no yeast.
3. Cover both bowls and keep them in a warm place.
4. Leave them for a few hours.
5. Compare the size and smell of the dough in both bowls.
2. In bowl B, mix flour, water, and sugar into a dough, but add no yeast.
3. Cover both bowls and keep them in a warm place.
4. Leave them for a few hours.
5. Compare the size and smell of the dough in both bowls.
Observation
Bowl A rises and smells different; bowl B stays the same.
Explanation
The yeast respires, releasing carbon dioxide (fluffiness) and a little alcohol (smell). This is used in breads, cakes, idli, dosa, and bhatura.
Yeast releases carbon dioxide, which makes the dough in bowl A rise and turn fluffy.
โ Summary
- Yeast dough rises and smells
- Caused by carbon dioxide gas
- Used in breads and cakes
๐ฅ
Activity 2.9 — Let us prepare
How do we test what makes milk set?
Take two glass bowls, A and B
Label them clearly
↓
Lukewarm milk in A, cold milk in B
The only difference is the temperature
↓
Add a spoonful of curd to each
Mix it in well
↓
Cover both and wait
Bowl A in a warm place, bowl B in a cool place, overnight
↓
Bowl A has set into sour curd; bowl B has not curdled — Lactobacillus needs warmth
In this Activity, we will set warm milk and cold milk with a spoon of curd, and find out which one sets.
Materials needed
Milk, a spoon of curd, and two bowls (A and B).
Procedure
1. Warm some milk slightly and pour it into bowl A.
2. Pour cold milk into bowl B.
3. Add a spoon of curd to each bowl and stir.
4. Cover both bowls.
5. Leave them undisturbed overnight.
6. Check both bowls the next morning.
2. Pour cold milk into bowl B.
3. Add a spoon of curd to each bowl and stir.
4. Cover both bowls.
5. Leave them undisturbed overnight.
6. Check both bowls the next morning.
Observation
Bowl A sets into curd; bowl B does not set well.
Explanation
Lactobacillus
bacteria ferment warm milk into curd, producing lactic acid. Warmth helps them grow.
Lactobacillus ferments warm milk into curd; the cold milk in bowl B does not set.
โ Summary
- Warm milk sets into curd
- Cold milk does not
- Bacteria need warmth to work
Important Points
- Yeast makes carbon dioxide (and a little alcohol), used in baking.
- Lactobacillus makes lactic acid, used to set curd and ferment food.
- Rhizobium in legume roots fixes nitrogen and enriches the soil.
โ
Test Yourself
-
Which microbe makes dough rise?
View Answer
Yeast, a fungus. -
Which gas does yeast release?
View Answer
Carbon dioxide. -
Which bacteria turn milk into curd?
View Answer
Lactobacillus bacteria, which make lactic acid. -
What are legumes? Give one example.
View Answer
Plants whose seeds grow in pods, like peas or beans. -
How does
Rhizobium
help legumes?
View Answer
It traps nitrogen from the air and enriches the soil.
Important Definitions
- Yeast — A unicellular fungus that releases carbon dioxide, used to make dough rise.
- Fermentation — The breakdown of sugar by microbes, used in making curd, idli, dosa, etc.
- Legumes — Plants whose seeds grow in pods, such as peas, beans, gram, and lentils.
- Rhizobium — Bacteria living in the root nodules of legumes that trap nitrogen from the air.
๐ NCERT Question 2 — Aanandi took two test tubes
Aanandi took two test tubes and marked them A and B. She put two spoonfuls of sugar solution in each of the test tubes. In test tube B, she added a spoonful of yeast. Then she attached two incompletely inflated balloons to the mouth of each test tube. She kept the set-up in a warm place, away from sunlight.
View Answer →
๐ NCERT Question 3 — A farmer was planting wheat
A farmer was planting wheat crops in his field. He added nitrogen-rich fertiliser to the soil to get a good yield of crops. In the neighbouring field, another farmer was growing bean crops, but she preferred not to add nitrogen fertiliser to get healthy crops. Can you think of the reasons?
View Answer →
๐ NCERT Question 8 — A student observes that curd
A student observes that when curd is left out for a day, it becomes more sour. What can be two possible explanations for this observation?
View Answer →
๐ NCERT Question 9 — Observe the set-up given
Observe the set-up given in Fig. 2.15 and answer the following questions.
View Answer →
