Medicneet Logo

Dough Fermentation by Baker's Yeast (Idli, Dosa, Bread)

Biology · Microbes in Human Welfare · NEET

Bread dough is fermented by the yeast Saccharomyces cerevisiae (baker's yeast), while idli and dosa dough is fermented by bacteria. In both cases the dough becomes soft and puffed-up because the microbes make carbon dioxide (CO2) gas that gets trapped inside. Memory hook: "Yeast makes gas, dough gets a raise" - the bubbles you see are CO2.
Dough Fermentation: CO2 Makes It RiseRaw dough+ microbeflat & denseferment(anaerobic)puffed doughsoft & spongyCO2 bubbles trappedWhich microbe?Bread: yeastS. cerevisiae (fungus)Idli/Dosa:bacteria
Fermenting microbes release CO2 that gets trapped in the dough, making it puffed-up and soft. Bread dough uses yeast (Saccharomyces cerevisiae, a fungus); idli/dosa dough uses bacteria.

Your doubts, answered

Which microbe ferments bread dough, and which one ferments idli/dosa dough?

This is the most common mix-up. As per NCERT, dough used to make BREAD is fermented by the yeast Saccharomyces cerevisiae (baker's yeast, a fungus). But the dough used to make idli and dosa is fermented by BACTERIA, not yeast. So bread = yeast, idli/dosa = bacteria. NEET often frames a statement saying idli/dosa dough is fermented by yeast, which is wrong.

Why does the dough become puffed-up and soft?

During fermentation the microbes carry out anaerobic respiration and release carbon dioxide (CO2) gas. This CO2 gets trapped inside the sticky dough as tiny bubbles, so the dough rises and looks puffed-up. NCERT states clearly: 'The puffed-up appearance of dough is due to the production of CO2 gas.' The soft, spongy texture of bread, idli and dosa comes from these gas pockets.

Is Saccharomyces cerevisiae a bacterium or a fungus?

Saccharomyces cerevisiae is a yeast, which is a type of FUNGUS - it is not a bacterium. It is called baker's yeast when used for bread and brewer's yeast when used for making alcohol. NEET decoys sometimes write 'Streptococcus cerevisiae' or call it a bacterium - both are wrong. Remember: -myces in the name points to a fungus.

Which metabolic pathway makes the CO2 in dough?

NCERT asks this as a thought question. The CO2 is produced by fermentation, the anaerobic breakdown of sugars (glycolysis followed by fermentation). Yeast breaks down sugar into ethanol and CO2. The CO2 raises the dough; in bread-making the small amount of ethanol evaporates during baking.

Same yeast for bread and beer - how is that possible?

Yes. The same species, Saccharomyces cerevisiae, is used both to leaven bread (as baker's yeast) and to ferment cereals and fruit juices into alcohol (as brewer's yeast). The difference is only in how it is used, not in the organism. This overlap is a favourite NEET fact.

⚠️ The NEET trap
✗ Idli and dosa dough is fermented by yeast (Saccharomyces cerevisiae), just like bread.
✓ Only BREAD dough is fermented by the yeast Saccharomyces cerevisiae. Idli and dosa dough is fermented by BACTERIA. The common point is only the CO2 that puffs up the dough.
🧠 Bread = yeast, Idli/Dosa = bacteria. Same CO2, different microbe.

Real NEET questions

NEET 2019

Match the following organisms with the products they produce. Column-I: (a) Lactobacillus (b) Saccharomyces cerevisiae (c) Aspergillus niger (d) Acetobacter aceti; Column-II: (i) Cheese (ii) Curd (iii) Citric Acid (iv) Bread (v) Acetic Acid

A · (a)-(ii), (b)-(iv), (c)-(v), (d)-(iii)
B · (a)-(ii), (b)-(iv), (c)-(iii), (d)-(v) ✓
C · (a)-(iii), (b)-(iv), (c)-(v), (d)-(i)
D · (a)-(ii), (b)-(i), (c)-(iii), (d)-(v)
Solution: Saccharomyces cerevisiae (baker's yeast) ferments dough for BREAD, so (b)-(iv). Lactobacillus makes curd (ii), Aspergillus niger makes citric acid (iii), and Acetobacter aceti makes acetic acid (v). This gives option (b).
NEET 2025

Which of the following microbes is NOT involved in the preparation of household products? A. Aspergillus niger B. Lactobacillus C. Trichoderma polysporum D. Saccharomyces cerevisiae E. Propionibacterium sharmanii

A · C and D only
B · C and E only
C · A and B only
D · A and C only ✓
Solution: Household-product microbes include Lactobacillus (curd), Saccharomyces cerevisiae (bread/dough) and Propionibacterium sharmanii (Swiss cheese). Aspergillus niger (industrial citric acid) and Trichoderma polysporum (cyclosporin/biocontrol) are NOT household-product microbes, so the answer is A and C only, option (d).

Solved Microbes in Human Welfare NEET PYQs

Try the real previous-year questions from this chapter — each with the answer and a full solution.

See all 26 Microbes in Human Welfare NEET PYQs ›
Next concept: Saccharomyces cerevisiae →Keep learning — 2 minFeeling ready? Solve the Microbes in Human Welfare NEET PYQs ›Or practice on your phone — get the free MedicNEET app ›

Frequently asked

What is baker's yeast in NEET?

Baker's yeast is the yeast Saccharomyces cerevisiae, a fungus used to ferment bread dough. It releases CO2 that makes the dough rise and become soft and spongy.

Does bread fermentation produce alcohol?

Yes, yeast produces a small amount of ethanol along with CO2, but the ethanol evaporates during baking. The CO2 is what leavens (raises) the dough.

Are idli and dosa fermented by the same microbe as bread?

No. Bread dough is fermented by the yeast Saccharomyces cerevisiae, while idli and dosa dough is fermented by bacteria. Both produce CO2 to puff up the dough.

Why is the dough puffed-up according to NCERT?

NCERT states the puffed-up appearance of dough is due to the production of CO2 gas by the fermenting microbes, which gets trapped inside the dough.

Related concepts:
Saccharomyces cerevisiae: Baker's and Brewer's YeastHow Curd Is Made from Milk (Lactobacillus)Microbes in Household Products: An OverviewLactic Acid Bacteria (LAB): Meaning and ExamplesFermented Beverages: Wine, Beer, Whisky, Brandy, Rum
Practise Biology for NEET:
Biology MCQs (free, chapter-wise)Microbes in Human Welfare PYQs