Food Microbiology and Food Safety
This domain tests microorganisms in food, the factors that control their growth, spoilage and foodborne illness, and preventive systems such as HACCP and good hygiene practices.
Core concepts
- Microbial groups: bacteria, yeasts and moulds. Yeasts and moulds tolerate lower pH and lower water activity than most bacteria; bacterial spores survive ordinary cooking.
- Growth factors: nutrients, moisture (water activity, aw), pH, temperature, oxygen and time. Lower aw, acid pH, refrigeration and competitive flora slow growth.
- Important hazards: Salmonella, Staphylococcus aureus, Clostridium botulinum, Escherichia coli and Listeria monocytogenes. Spore-forming bacteria are a major concern in low-acid canned foods.
- HACCP: seven principles - hazard analysis, critical control points, critical limits, monitoring, corrective actions, verification, and record keeping.
Worked example
A canned low-acid food has an initial spore load of 10^6 per container. If the D-value at 121 °C is 0.2 min, a process of 1.2 min gives 1.2 ÷ 0.2 = 6 decimal reductions. The expected survivors are 10^6 ÷ 10^6 = 1 per container, which is still a risk. The process must be increased to achieve a safe margin; 12D would require 2.4 min. This shows why sterilisation processes are designed with a safety margin, not just to reach one survivor.
Traps
Saying refrigeration kills bacteria - it only slows growth; assuming pasteurisation sterilises; ignoring spore survival; treating pH 4.6 as safe for all low-acid canned foods without controlling botulinum risk.