Food Microbiology and Spoilage
3. Food Microbiology and Spoilage
This domain tests the types of microorganisms in food, factors that control their growth, and how spoilage and foodborne illness arise. Expect numerical questions on thermal death time and conceptual questions on intrinsic and extrinsic factors.
Core rules
- Bacteria, yeasts and moulds cause spoilage; pathogens such as Salmonella, Listeria monocytogenes, Escherichia coli O157:H7 and Clostridium botulinum cause illness without necessarily spoiling food.
- Intrinsic factors: pH, water activity, nutrient content, antimicrobial compounds. Extrinsic factors: temperature, atmosphere, humidity.
- Most bacteria grow best at aw above 0.90 and pH near neutral; moulds tolerate lower aw; C. botulinum is an anaerobic spore-former of concern in low-acid canned foods.
- Growth curve: lag, log, stationary, death. Logarithmic reduction is described by D-value (time for 90% reduction) and z-value (temperature change for 10-fold D change).
- Spoilage signs: off-odour, slime, gas, colour change, mould; refrigeration slows but does not stop psychrotrophic growth.
Worked example
A canned food has a target 12-log reduction of C. botulinum. If D at 121°C is 0.2 min, the required F0 = 12 × 0.2 = 2.4 min at 121°C. If the initial load is 10^5 spores and a process gives 3 log reductions, surviving load = 10^(5−3) = 10^2 spores. Reasoning: each D-value reduces the population by one log; multiply D by the required log reduction.
Traps
Sterility is not the same as commercial sterility. Refrigeration does not kill pathogens. Vacuum packaging suppresses aerobes but can favour anaerobes. Do not add log reductions; subtract them from the initial log count.