Food Microbiology and Food Safety
Exam focus
This domain tests the microbes that spoil food and those that cause illness, the factors that control their growth, and the preventive systems - GMP, GHP and HACCP - used in a food business.
Core concepts
- Intrinsic factors: pH, water activity, nutrient content, redox potential, antimicrobial constituents.
- Extrinsic factors: temperature, relative humidity, atmosphere, competing microflora.
- Spoilage organisms change appearance, odour or texture; pathogens such as Salmonella, Listeria monocytogenes, Escherichia coli O157:H7, Clostridium botulinum, Staphylococcus aureus and Bacillus cereus may be present without obvious spoilage.
- D-value is the time at a given temperature to reduce a population by 90%, or one log. z-value is the temperature change that changes D-value tenfold.
- The 12D concept applies to low-acid canned foods: Clostridium botulinum spores must be reduced by 12 log cycles.
- HACCP has seven principles: hazard analysis; determine CCPs; establish critical limits; monitor; corrective actions; verification; record keeping.
- GMP covers facilities, equipment and personnel practices; GHP covers hygiene, cleaning and sanitation.
Worked example
If D at 121°C for C. botulinum is 0.21 min, a 12D process needs 12 × 0.21 = 2.52 min at 121°C. The reasoning is that each D-value reduces the count by one log, so twelve consecutive D-values give a 10^-12 survival fraction. In practice a safety margin and product heating lag are added, and the scheduled process is established by a competent process authority.
Traps
HACCP is preventive and does not rely on finished-product testing alone. A control point is not automatically a CCP; a CCP must control a significant hazard. Freezing stops growth but does not kill pathogens. Spore-formers survive ordinary pasteurisation. Do not confuse sterilisation with commercial sterility.