Useful Microbes in Food: Bacteria, Yeast and Mushrooms (ICSE Class 8 Biology)
Updated 17 October 2026 · Fact-checked
Fermentation is the change of sugars into acids, alcohol or gas by microbes. Lactobacillus bacteria change lactose into lactic acid to set milk into curd; Acetobacter makes vinegar. Yeast, a unicellular fungus, ferments sugar in dough to give carbon dioxide that makes bread rise. Edible mushrooms are protein-rich and low in fat, but wild ones can be poisonous.
Understand Bacteria, Yeast and Mushrooms in the Food Industry
Fermentation. The breakdown of sugars by microbes into acids, alcohol or gas. It changes taste and texture and helps preserve food because the acid stops harmful germs.
Bacteria in food.
- Curd (dahi) and yoghurt: *Lactobacillus* turns milk sugar (lactose) into lactic acid; the acid thickens the milk proteins and gives a sour taste. A spoon of old curd added to warm milk supplies the bacteria.
- Buttermilk: made by churning curd; contains the same bacteria.
- Cheese: bacteria (sometimes moulds) curdle milk and ripen the cheese.
- Idli and dosa batter: bacteria with yeast ferment the soaked rice and urad dal overnight and make it fluffy.
- Vinegar: *Acetobacter* changes alcohol into acetic acid.
Fungi in food.
- Yeast: a unicellular fungus that reproduces by budding. In dough, yeast ferments sugar:
glucose → alcohol + carbon dioxide
The CO₂ forms bubbles, the dough rises and the bread is soft and spongy; baking drives off the alcohol. Yeast is also used in cakes, bhatura, alcoholic drinks and vinegar, and is rich in vitamin B and protein.
- Mushrooms: fleshy fruiting bodies of fungi.
- Edible mushrooms (button mushroom) are rich in protein, vitamins and minerals and low in fat.
- Grown on compost in dark, damp rooms, giving farmers extra income from farm waste.
- Caution: many wild mushrooms are poisonous; never eat one that you cannot identify.
| Microbe | Kind | Product | Action |
|---|---|---|---|
| Lactobacillus | bacterium | curd | lactose to lactic acid |
| Acetobacter | bacterium | vinegar | alcohol to acetic acid |
| Yeast | fungus | bread, idli batter | sugar to alcohol and CO₂ |
| Button mushroom | fungus | food | protein-rich vegetable |
Flow chart: curd. Warm milk + a spoon of curd → Lactobacillus multiplies → lactose to lactic acid → proteins thicken → curd.
Bacteria, Yeast and Mushrooms in the Food Industry: key facts
- Bacteria
- Lactobacillus: lactose to lactic acid, curd
- Acetobacter: alcohol to acetic acid, vinegar
- Also cheese, buttermilk
- Bacteria are prokaryotes.
- Yeast
- Unicellular fungus
- Budding
- Glucose to alcohol and carbon dioxide
- Dough rises
- CO₂ makes the bubbles.
- Mushrooms
- Fungus fruiting body
- Rich in protein, low in fat
- Grown on compost
- Wild ones may be poisonous
- Eat only edible kinds.
How to answer Bacteria, Yeast and Mushrooms in the Food Industry questions
Typical tasks: name the microbe behind a food, state the chemical change, explain why dough rises or milk sets, or give the importance and precautions on mushrooms.
- 1Identify whether the microbe is a bacterium (curd, vinegar) or a fungus (yeast, mushroom).
- 2State the starting substance and the product: lactose to lactic acid; glucose to alcohol and CO₂.
- 3Link the product to the observation: acid sets milk; CO₂ raises dough.
- 4For mushrooms, give a nutrition point and a safety point.
- 5Mention preservation when the question asks the benefit of fermentation.
Bacteria, Yeast and Mushrooms in the Food Industry in real life
The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.
Dough overnight
Suppose a baker mixes flour, sugar and a pinch of yeast in warm water, covers the dough, and finds it almost double in size after two hours.
Explain the rise and say why the dough would not rise if the water were boiling hot.
Try it first, then show the solutionSolution
- Yeast ferments the sugar and gives off carbon dioxide gas.
- The gas is trapped as bubbles, so the dough swells.
- Boiling water would kill the yeast, so no gas would form.
Answer: CO₂ from yeast fermentation raises the dough; boiling water kills the yeast.
Memory hook: Warm helps; hot kills.
Milk in winter
Suppose a girl sets curd with a spoon of old curd, and it sets in 6 hours in summer but takes 12 hours on a cold winter night.
Why does it take longer in winter, and what is the ratio of the two times?
Try it first, then show the solutionSolution
- Lactobacillus multiplies and works faster in warmth.
- In the cold the bacteria are slower, so it takes longer to produce lactic acid.
- Ratio of times = 12 : 6 = 2 : 1.
Answer: Bacteria work slowly in cold; winter time is twice summer time.
Memory hook: Warm milk, quick curd.
Tricks to remember Bacteria, Yeast and Mushrooms in the Food Industry
Short memory aids. Say each one aloud once, then use it on a question the same day.
L for lactose, L for lactic
Lactobacillus turns lactose into lactic acid.
Use it when: Curd.
Yeast breathes bubbles
Yeast gives CO₂ bubbles that raise dough.
Use it when: Bread.
Mushroom caution
Edible kinds feed; unknown kinds may kill.
Use it when: Safety.
Bacteria, Yeast and Mushrooms in the Food Industry: solved examples
Ordered from board level to HOTS. Try each one before you open the solution.
Example 1: Which microbe · 1 mark · MCQBoard level
The microbe that converts milk into curd is:
(a) yeast (b) Lactobacillus (c) Rhizopus (d) Plasmodium
Try it first, then show the solutionSolution
- Lactobacillus is the lactic acid bacterium.
Answer: (b).
Example 2: Type of microbe · 2 marks · Short answerBoard level
State whether each is a bacterium or a fungus: Lactobacillus, yeast, button mushroom, Acetobacter.
Try it first, then show the solutionSolution
- Bacteria: Lactobacillus and Acetobacter (1 mark).
- Fungi: yeast and button mushroom (1 mark).
Answer: Bacteria: Lactobacillus, Acetobacter; fungi: yeast, mushroom.
Example 3: Curd formation · 3 marks · Short answerHarder board level
Describe how curd is formed from milk, naming the bacterium, the change in the milk sugar and the effect on the proteins.
Try it first, then show the solutionSolution
- A spoon of curd in warm milk adds Lactobacillus (1 mark).
- The bacteria turn lactose into lactic acid (1 mark).
- The acid thickens the milk proteins so the milk sets and turns sour (1 mark).
Answer: Lactobacillus changes lactose to lactic acid, which sets the proteins.
Example 4: Mass balance · 4 marks · Long answerHarder board level
In the fermentation of one molecule of glucose, two molecules of alcohol (C₂H₅OH) and two of carbon dioxide are produced. Taking C = 12, H = 1, O = 16, calculate the mass of glucose (C₆H₁₂O₆) per molecule, and show that mass is conserved.
Try it first, then show the solutionSolution
- Glucose: 6 × 12 + 12 × 1 + 6 × 16 = 72 + 12 + 96 = 180 (1 mark).
- Alcohol: 2 × 12 + 6 × 1 + 16 = 46; two molecules = 92 (1 mark).
- Carbon dioxide: 12 + 2 × 16 = 44; two molecules = 88 (1 mark).
- Products: 92 + 88 = 180, equal to glucose, so mass is conserved (1 mark).
Answer: 180 = 92 + 88.
Example 5: Mushroom farm · 4 marks · Case-basedHOTS / competency
A farmer wants to start growing button mushrooms in a spare room. (a) Give two reasons why it is a good idea. (b) Suggest a medium and two conditions needed. (c) Why should she never sell or eat mushrooms picked from a forest unless an expert checks them?
Try it first, then show the solutionSolution
- (a) Mushrooms are rich in protein and sell for good income; they use farm waste (1 mark).
- (b) Compost of straw and manure; darkness and damp (1 mark).
- (c) Many wild mushrooms are poisonous and look like edible ones (1 mark).
- Eating a poisonous kind can cause serious illness or death (1 mark).
Answer: Profit and protein; compost, dark and damp; wild mushrooms can poison.
Where students lose marks in Bacteria, Yeast and Mushrooms in the Food Industry
The slips examiners see most often, and what to write instead.
Mistake: Calling yeast a bacterium
Both are microbes used in food.
Do this: Yeast is a unicellular fungus.
Usually costs: 1 mark.
Mistake: Writing oxygen as the gas in bread-making
The gas is confused with respiration.
Do this: The gas is carbon dioxide.
Usually costs: 1 mark.
Mistake: Saying all mushrooms are edible
Market mushrooms are generalised to wild ones.
Do this: Write edible mushrooms such as button mushroom; wild ones may be poisonous.
Usually costs: 1 mark.
Mistake: Not naming the sugar and acid in curd
The answer says milk turns into curd.
Do this: Lactose to lactic acid by Lactobacillus.
Usually costs: 1 mark.
Exam tips
- Curd: Lactobacillus, lactose, lactic acid.
- Bread: yeast, glucose to alcohol and CO₂.
- Mushrooms: protein, low fat, caution about wild kinds.
- Fermentation also preserves food.
Practice questions on Bacteria, Yeast and Mushrooms in the Food Industry
Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.
Q1 · 1 mark · Very short answerBoard level
What is fermentation?
Hint
Sugar to acid or alcohol.
Answer
The breakdown of sugars by microbes into acids, alcohol or gas.
Q2 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Yeast cannot be used to make curd.
Reason (R): Curd is formed when Lactobacillus changes lactose into lactic acid.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).Hint
Which microbe?
Answer
(c).
Q3 · 3 marks · Short answerHarder board level
Compare Lactobacillus and Acetobacter by the substance each changes, the product formed and a food in which it is used.
Hint
Lactic or acetic.
Answer
Lactobacillus changes lactose of milk into lactic acid and is used in making curd. Acetobacter changes alcohol into acetic acid and is used in making vinegar.
Q4 · 4 marks · Long answerHarder board level
Dough with yeast is kept in a refrigerator overnight and hardly rises, while the same dough kept in a warm kitchen doubles. Explain with reasons, and say why the bread is soft after baking.
Hint
Activity of yeast and its gas.
Answer
In the cold the yeast ferments sugar very slowly and releases little carbon dioxide, so the dough hardly rises; in warmth it ferments quickly and the gas bubbles swell the dough. After baking the trapped bubbles leave holes, making the bread soft and spongy, and the alcohol is driven off by heat.
Q5 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a shopkeeper stores curd at room temperature for three days in summer and notices it becomes very sour.
(a) Name the acid. (1)
(b) Why does the sourness increase? (1)
(c) How can he slow this down? (1)Hint
Bacteria keep working.
Answer
(a) Lactic acid. (b) Lactobacillus continues to work and the acid keeps building up. (c) By storing the curd in a refrigerator, as the cold slows down the bacteria.
Test your recall
Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.
1.Which bacterium makes vinegar?Show answer
Acetobacter.
2.What type of organism is yeast?Show answer
A unicellular fungus.
3.What sugar is in milk?Show answer
Lactose.
4.What is the vitamin found in yeast?Show answer
Vitamin B.
5.Name an edible mushroom.Show answer
Button mushroom.
Bacteria, Yeast and Mushrooms in the Food Industry: frequently asked questions
Is curd with bacteria safe to eat?
Yes; the lactic acid bacteria in curd are useful.
Does baking kill the yeast?
Yes, heat kills it and drives off the alcohol.
Are moulds part of this topic?
The syllabus names bacteria, mushrooms and yeast.
Where are crops and animals covered?
In the next topic.