Question
Describe the role of microorganisms in the production of curd, bread, and cheese. Name the specific microorganism responsible for each.
Solution — Step by Step
Curd (Yogurt) is produced by bacterial fermentation of milk.
Microorganism: Lactobacillus species (primarily L. acidophilus, L. lactis)
Process:
- A small quantity of curd (starter/inoculum) containing live Lactobacillus is added to warm milk (~40°C)
- Lactobacillus carries out lactic acid fermentation:
- Lactic acid lowers the pH of milk, causing milk proteins (mainly casein) to precipitate and coagulate
- This coagulation gives curd its semi-solid texture
- The process takes about 6–8 hours at room temperature or faster in warm conditions
Additional benefit: Lactobacillus produces vitamins B12 and improves the nutritional value of curd. The bacteria remain alive in curd — that’s why curd is a probiotic.
Bread is made using yeast fermentation to produce CO₂ that makes dough rise.
Microorganism: Saccharomyces cerevisiae (baker’s yeast)
Process:
- Yeast is mixed into dough (flour + water + sugar)
- Yeast carries out anaerobic fermentation of sugars:
- The CO₂ gas gets trapped in the elastic gluten network of the dough, causing it to rise and become porous
- During baking, the CO₂ expands further and the alcohol evaporates → light, fluffy texture
- The heat also kills the yeast, stopping fermentation
Why dough rises: The gluten proteins in wheat flour form elastic networks that trap gas bubbles. Without gluten (e.g., in rice flour), the dough doesn’t rise well — which is why rice bread is denser.
Saccharomyces cerevisiae is also used for fermenting wine and beer.
Cheese production uses bacteria and sometimes fungi, depending on cheese type.
Bacteria involved: Lactococcus lactis, Lactobacillus species, Propionibacterium shermanii (for Swiss cheese)
Fungi involved: Penicillium roqueforti (for Roquefort/blue cheese), Penicillium camemberti (for Brie/Camembert — white rind)
Process:
- Milk is pasteurised and a bacterial starter culture is added
- Bacteria ferment lactose to lactic acid → milk curdles
- Rennet (enzyme — rennin/chymosin) is added to further coagulate casein protein
- The solid curd is separated from liquid whey, cut, and pressed into the desired shape
- The cheese is then ripened/aged — bacteria or fungi continue to act, developing flavour and texture
Swiss cheese holes: Propionibacterium shermanii produces CO₂ during ripening → forms the characteristic holes (eyes) in Swiss cheese.
Blue cheese flavour: Penicillium roqueforti produces blue-green veins and a sharp, tangy flavour through its enzyme activity.
For completeness (relevant for board exam higher-order questions):
| Food | Microorganism | Process |
|---|---|---|
| Idli/Dosa | Leuconostoc mesenteroides + Lactobacillus | Lactic acid + CO₂ fermentation |
| Toddy | Saccharomyces | Fermentation of palm sap |
| Vinegar | Acetobacter aceti | Ethanol → acetic acid (aerobic) |
| Soya sauce | Aspergillus oryzae | Fermentation of soya + wheat |
Why This Works
All these food production methods rely on microbial metabolism — the microorganism’s own biochemical processes produce the compounds we need:
- Lactobacillus needs to ferment lactose to lactic acid for its own energy → curd forms as a side effect of the bacteria’s metabolism
- Yeast ferments sugar for energy → CO₂ and ethanol are byproducts we exploit
- Cheese-making bacteria and fungi produce enzymes that break down proteins and fats, releasing flavour compounds while also preserving the food (acidic pH prevents pathogen growth)
Humans have used these processes for thousands of years — curd-making in India and bread-making in ancient Egypt — long before we understood the microbiology.
Alternative Method — Focus on the Chemical Change
To classify these as fermentation processes:
| Food | Substrate | Product we use | Type of fermentation |
|---|---|---|---|
| Curd | Lactose | Lactic acid (coagulates milk) | Lactic acid fermentation |
| Bread | Sucrose/glucose | CO₂ (leavens dough) | Alcoholic fermentation (anaerobic) |
| Cheese | Lactose/casein | Coagulation + flavour compounds | Lactic acid + proteolysis |
NEET and CBSE Class 12 (Chapter 10 — Microbes in Human Welfare) test this topic regularly. Key facts to memorise: (1) Curd: Lactobacillus, lactic acid, pH drops. (2) Bread: Saccharomyces cerevisiae, CO₂ makes dough rise. (3) Large holes in Swiss cheese: Propionibacterium. (4) Roquefort (blue cheese): Penicillium roqueforti. NEET 2022 had a direct question on Swiss cheese holes.
Common Mistake
Students often say “yeast makes bread rise by producing oxygen.” This is incorrect. Yeast produces CO₂ (carbon dioxide) through anaerobic fermentation, and it is the CO₂ that gets trapped in the dough, making it rise. Yeast does not produce oxygen — it is the reverse: yeast uses oxygen when available (aerobic) but switches to fermentation (producing CO₂ and ethanol) in the absence of oxygen (anaerobic conditions inside thick dough).