Question
Name the microorganisms used in the production of curd, bread, alcohol, and antibiotics. What is fermentation and how does it differ from aerobic respiration?
(NEET 2022, similar pattern)
Solution — Step by Step
Lactobacillus (lactic acid bacteria, LAB) converts lactose in milk to lactic acid through fermentation.
The acid causes milk proteins (casein) to coagulate, turning milk into curd. This is why we add a small amount of old curd to warm milk — it serves as an inoculum containing live Lactobacillus.
LAB also produce vitamin B12 and check the growth of disease-causing microbes in the gut.
Saccharomyces cerevisiae ferments sugar in dough and produces :
The bubbles get trapped in the dough, causing it to rise (leavening). During baking, the alcohol evaporates and the CO₂ expands further, giving bread its soft, porous texture.
The same species of yeast is used, but the process is optimised for ethanol production rather than CO₂.
- Beer: Yeast ferments malted barley sugars
- Wine: Yeast ferments grape juice sugars
- Toddy: Yeast naturally present ferments palm sap
The ethanol concentration is limited to about 12-14% because higher alcohol levels kill the yeast. For spirits (whisky, rum), the fermented liquid is distilled to concentrate the alcohol.
| Antibiotic | Microbe | Significance |
|---|---|---|
| Penicillin | Penicillium notatum/chrysogenum | First antibiotic discovered (Alexander Fleming, 1928) |
| Streptomycin | Streptomyces griseus | Used against TB |
| Tetracycline | Streptomyces aureofaciens | Broad-spectrum antibiotic |
| Chloramphenicol | Streptomyces venezuelae | Used against typhoid |
Antibiotics are secondary metabolites — produced during the stationary phase of microbial growth, not during active growth.
Why This Works
Fermentation is anaerobic respiration — it extracts energy from organic molecules without using oxygen. Unlike aerobic respiration (which fully oxidises glucose to CO₂ and H₂O, yielding 30-32 ATP), fermentation only partially breaks down glucose, yielding just 2 ATP per glucose.
Humans have exploited microbial fermentation for thousands of years. The microbes do the chemistry for us — converting sugars into useful products like lactic acid (curd, cheese), ethanol (beverages), CO₂ (bread), or organic acids (vinegar).
Alternative Method — Microbe-Product Quick Map
For NEET, this is the must-memorise table:
| Product | Microbe |
|---|---|
| Curd | Lactobacillus |
| Bread | Saccharomyces cerevisiae |
| Beer/Wine | Saccharomyces cerevisiae |
| Toddy | Natural yeast in palm sap |
| Vinegar | Acetobacter aceti |
| Idli/dosa batter | Leuconostoc + yeast |
| Swiss cheese (holes) | Propionibacterium shermanii (produces CO₂) |
| Roquefort cheese | Penicillium roqueforti |
NEET loves the cheese question — the holes in Swiss cheese come from CO₂ produced by Propionibacterium.
Common Mistake
Students often say “fermentation produces only alcohol.” There are different types of fermentation: alcoholic fermentation (by yeast — produces ethanol + CO₂) and lactic acid fermentation (by LAB — produces lactic acid). Both are anaerobic, but they produce different end products. When NEET asks about curd formation, mention lactic acid fermentation — not alcoholic fermentation.