Question
Name the microorganisms responsible for the preparation of (a) curd, (b) bread, (c) alcohol/wine, and (d) vinegar. Briefly explain the role of each.
This is a direct NCERT question and appears almost every year in CBSE board exams. NEET also asks it in the 1-mark MCQ format — usually testing whether you can match the organism to the product.
Solution — Step by Step
Lactobacillus (specifically L. acidophilus) converts lactose (milk sugar) into lactic acid. The acid lowers the pH of milk, which causes the milk proteins (casein) to coagulate — that’s why curd has a semi-solid texture.
We add a small amount of existing curd as a starter (inoculum). The bacteria multiply and acidify the entire batch.
Baker’s yeast (Saccharomyces cerevisiae) ferments the sugars in dough and produces CO₂ + ethanol. The CO₂ gets trapped in the gluten network, causing the dough to rise and giving bread its porous, spongy texture.
The ethanol evaporates during baking — so no, eating bread won’t make you tipsy.
The same S. cerevisiae is used here, but under anaerobic conditions (no oxygen). Glucose is converted to ethanol via fermentation:
For wine, grape juice is fermented. For beer, malted barley is used. The organism is the same — the substrate differs.
Here’s where students get confused. Acetobacter doesn’t make alcohol — it oxidises the alcohol (ethanol) already present into acetic acid (vinegar):
This is an aerobic process, which is why vinegar production needs oxygen. This is a two-step process: first yeast makes alcohol, then Acetobacter converts it to vinegar.
Why This Works
Each microorganism has specific enzymes that catalyse a particular biochemical reaction. Lactobacillus has lactase and enzymes for lactic acid fermentation. Saccharomyces has zymase — a complex of enzymes that breaks down sugars to ethanol and CO₂ under anaerobic conditions.
The key insight: fermentation is anaerobic. Yeast switches to fermentation only when oxygen is absent. If oxygen is present, it will do aerobic respiration and produce CO₂ + water, giving you far less alcohol. That’s why wine vats are sealed.
Acetobacter, on the other hand, is aerobic — it’s doing oxidation, not fermentation. So the pathway for vinegar is: sugar → (yeast, anaerobic) → ethanol → (Acetobacter, aerobic) → acetic acid.
Alternative Method
For NEET MCQs, memorise this as a table rather than a narrative:
| Product | Organism | Key Reaction |
|---|---|---|
| Curd | Lactobacillus | Lactose → Lactic acid |
| Bread | S. cerevisiae | Sugar → CO₂ (raises dough) |
| Wine/Beer | S. cerevisiae | Glucose → Ethanol |
| Vinegar | Acetobacter aceti | Ethanol → Acetic acid |
For 2-mark board questions, pair each organism with its specific metabolic product. Don’t just name the organism — the examiner wants the reaction.
Common Mistake
Many students write Aspergillus or Rhizopus for curd formation — those are moulds used for other fermented foods (like idli/dosa batter involves Leuconostoc). For curd specifically, the answer is Lactobacillus. Also, don’t confuse the role of yeast in bread vs. alcohol — in bread, it’s the CO₂ that matters, not the ethanol.
A classic NEET trap question: “Which microorganism is used in both bread-making and wine production?” Answer: Saccharomyces cerevisiae — same organism, different conditions (aerobic for CO₂ rise in bread, anaerobic for ethanol in wine). This exact distinction appeared in NEET 2022.