Food Microbiology and Food Safety
Microbiology questions are usually applied: which organism, which food, which control, which limit.
Organisms and their signatures
- Clostridium botulinum: anaerobic spore-former, pre-formed neurotoxin, low-acid canned foods - hence the 12D botulinum cook.
- Staphylococcus aureus: enterotoxin formed in food by handlers, heat-stable, so reheating does not make it safe.
- Salmonella: poultry and eggs, cross-contamination; infection, not intoxication.
- Listeria monocytogenes: multiplies at refrigeration temperature, a danger in ready-to-eat and dairy foods.
- E. coli O157:H7: Shiga toxin, undercooked meat, haemolytic uraemic syndrome.
- Bacillus cereus: emetic type in cooked rice held warm; diarrhoeal type in protein foods.
- Aspergillus flavus → aflatoxins in groundnut and maize; aflatoxin M1 appears in the milk of exposed animals.
Standards and indicators
- Total plate count reflects overall hygiene and shelf life; coliforms and E. coli indicate hygiene or faecal contamination, not necessarily pathogens.
- Ready-to-eat pathogen limits are typically "absent in 25 g" for Salmonella and Listeria monocytogenes under FSS Regulations.
- The danger zone is 5–60 °C; growth runs through lag, log, stationary and death phases.
Worked example
A low-acid canned vegetable must be commercially sterile. If D121 for C. botulinum is 0.21 min, a 12D process needs 12 x 0.21 = 2.52 min at the cold point at 121.1 °C. If the retort delivers only 2 min there, the process is short and the lot must be held. Note the basis: time at the cold point, not the retort thermometer reading.
Traps
- Equating a high plate count with the presence of pathogens.
- Calling staphylococcal food poisoning an infection.
- Forgetting that "commercially sterile" permits no growth in normal storage, not the absence of every organism.
- Using 100 °C as the reference temperature instead of 121.1 °C.