Food Microbiology and Food Spoilage
Domain 3 tests food microbiology and spoilage: which organisms grow, why food spoils, and how processing and storage select for or suppress them.
Core rules
- Bacteria, yeasts and moulds differ in size, cell type, pH and aw tolerance. Bacteria generally need higher aw; moulds tolerate lower aw.
- Intrinsic factors include pH, aw, nutrient content, redox potential and natural antimicrobials. Extrinsic factors include temperature, relative humidity and gas atmosphere.
- Microbial growth passes through lag, log, stationary and death phases; refrigeration lengthens lag and slows log growth but does not kill all cells.
- Spoilage signs include off-odour, slime, gas, discolouration and mould; pathogens such as Salmonella, Listeria, Staphylococcus aureus and Clostridium botulinum may cause illness without obvious spoilage.
- Control uses temperature, pH, aw, preservatives, competitive flora, packaging and hygiene.
Worked example
A canned low-acid vegetable swells at the ends. Reason: Clostridium botulinum is an anaerobic spore-former that survives inadequate retorting. In the anaerobic can it germinates, ferments and produces gas and neurotoxin. The can must not be tasted; it is discarded and the retort process is verified. The swelling is a process failure signal, not proof that only a harmless gas-producer is present.
Traps
- Assuming a spoiled food is the only unsafe food; pathogen-contaminated food can look normal.
- Saying freezing sterilises; it preserves by stopping growth, and survivors grow on thawing.
- Treating all moulds as harmless; some produce mycotoxins.
- Forgetting that yeasts can spoil high-sugar, high-acid products where bacteria fail.