Food Microbiology and Spoilage
This domain tests the microorganisms that spoil food or cause illness, the factors that control their growth, and the meaning of thermal death parameters. Expect questions on pathogens, spores, toxins and spoilage signs.
Core rules and concepts
- Bacteria, yeasts and moulds cause spoilage. Pathogens of concern include Salmonella, Listeria monocytogenes, E. coli O157:H7, Clostridium botulinum, Bacillus cereus and Staphylococcus aureus.
- Intrinsic factors: pH, water activity, nutrient content, antimicrobial compounds. Extrinsic factors: temperature, oxygen, humidity.
- Microbial growth phases: lag, log, stationary, death. Refrigeration extends lag phase; freezing stops growth but not all enzyme activity.
- D-value is the time at a given temperature to reduce a population by 90%, a one-log reduction. z-value is the temperature change needed to change D tenfold.
- The 12D concept means a 12-log reduction of C. botulinum spores; it underpins low-acid canned food sterilization.
- Some toxins, such as staphylococcal enterotoxin, are heat-stable and survive cooking.
Worked example
A spore suspension has D121°C = 0.2 minutes. A 12D process needs 12 × 0.2 = 2.4 minutes at 121°C. This is a simplified calculation; real processes also consider heating lag, product consistency and cold-point location.
Traps that cost marks
- D-value is not the time for complete sterility.
- Refrigeration does not kill pathogens.
- Boiling does not destroy all spores or heat-stable toxins.
- Mould growth may be visible, but bacterial spoilage can be invisible and odourless.
- Anaerobes such as C. botulinum can grow in vacuum-packed low-acid foods.