Food Microbiology and Fermentation
Food Microbiology and Fermentation
This domain tests microbial growth, spoilage, foodborne pathogens and controlled fermentation. Questions often ask why a process fails or how a starter culture changes a food.
Core rules
- Growth factors: temperature, pH, aw, oxygen, nutrients and competition. Psychrotrophs grow at chill temperatures; thermophiles at high temperatures; anaerobes grow without oxygen.
- Pathogens: Salmonella, Listeria monocytogenes, Escherichia coli O157:H7, Staphylococcus aureus and Clostridium botulinum are key hazards. C. botulinum spores survive boiling and grow in low-acid anaerobic packs.
- Spoilage: off-odours, gas, slime and mould indicate microbial or enzymatic activity; spoilage organisms are not always pathogens.
- Fermentation: lactic acid bacteria lower pH in curd, idli and pickle; yeasts produce ethanol and CO2 in bread and wine; acetic acid bacteria make vinegar.
- Starters: use active, uncontaminated cultures at the correct temperature; back-slopping carries contamination risk.
Worked example
To set 10 L of curd, pasteurized milk is cooled to about 40–45 °C, inoculated with 2% active curd (about 200 g), mixed and held for 6–8 h. Lactic acid bacteria ferment lactose to lactic acid; pH falls toward 4.6 and casein coagulates. If the milk is too hot, the starter is killed; if too cold, setting is slow; prolonged holding causes whey separation and sourness.
Traps
Sterilization is not the same as sanitization; refrigeration does not kill bacteria; a sterile product can still spoil if the seal leaks; never judge safety by smell alone.