Food Microbiology and Food Safety
This domain tests whether you can name the organisms that spoil food or make people ill, explain the conditions that let them grow, and design controls - hygienic practice, temperature control, pH, aw, preservatives and HACCP. Local exam contexts are common: groundnut paste, smoked fish, street-vended food, cooked rice left at ambient temperature, borehole water used to dilute drinks.
The main groups
- Bacteria - rod-shaped or cocci, divide by binary fission (about 20 min under ideal conditions), prefer aw > 0.90 and pH near neutral. Spoilage and pathogens: Salmonella, Staphylococcus aureus, Clostridium botulinum, Escherichia coli O157:H7, Listeria monocytogenes, Bacillus cereus.
- Yeasts - tolerate acid and sugar; cause fermentation and off-flavours in drinks and jams.
- Moulds - grow at lower aw, tolerate acid, produce mycotoxins. Aspergillus flavus on poorly dried groundnuts and maize produces aflatoxin, a liver carcinogen that survives ordinary cooking.
Intrinsic and extrinsic factors
Intrinsic: pH, aw, nutrient content, antimicrobial compounds, biological structure. Extrinsic: temperature, relative humidity, gas atmosphere, competing microflora. Preservation works by removing or disrupting at least one factor - hurdle technology stacks several mild hurdles so no single severe treatment is needed.
Foodborne illness: infection vs intoxication
- Infection: live cells must be eaten and multiply in the gut (e.g. Salmonella); incubation is typically 12–72 h.
- Intoxication: pre-formed toxin is eaten (e.g. S. aureus toxin, botulinum toxin); onset is fast, often 1–6 h for staphylococcal toxin.
- Toxi-infection: cells multiply in the gut and release toxin there (e.g. B. cereus diarrhoeal type).
HACCP in seven principles
- Conduct a hazard analysis. 2. Determine CCPs. 3. Set critical limits. 4. Monitor. 5. Establish corrective actions. 6. Verify. 7. Keep records. HACCP sits on foundations of GMP, personal hygiene and sanitation.
Worked example
A small firm bottles hibiscus (bissap) drink at pH ~2.8. Hazard analysis finds two hazards: microbial survival from unboiled water, and recontamination from bottles and hands. Because the product is acid, C. botulinum is not a credible hazard, so a full 12D retort is unnecessary; a hot-fill pasteurisation is adequate. CCP 1: fill at ≥ 85 °C with ≥ 15 min holding, temperature and time logged per batch. CCP 2: bottle rinse at 100 ppm available chlorine, verified by test strips. The reasoning - acidity removes the spore-former hazard and determines which control is proportionate - earns the marks.
Traps
Freezing stops growth but does not sterilise; drying does not destroy aflatoxin; boiling destroys staphylococcal toxin only poorly; and hazard analysis is not the same as risk assessment.