Food Microbiology and Food-Borne Pathogens
What this domain tests
Questions cover microbial growth factors, recognised pathogen-food pairs, indicator organisms, thermal death kinetics and standard detection methods. Expect one applied calculation and several "which organism/which food" matches.
Growth factors to memorise
- Water activity: most bacteria stop near aw 0.90, most yeasts near 0.88, most moulds near 0.80; Staphylococcus aureus tolerates down to about 0.86.
- pH: Clostridium botulinum will not grow and toxigenesis is inhibited below pH 4.6 - the reason acidified canned foods need less severe heat treatment.
- Temperature: the danger zone is 5–60 degrees C; psychrotrophs such as Listeria monocytogenes and Yersinia enterocolitica grow at refrigeration temperature.
Pathogen-food associations
- Clostridium botulinum - anaerobic, low-acid canned foods, vacuum-packed fish; preformed neurotoxin is heat labile.
- Staphylococcus aureus - enterotoxin is heat stable, so reheating cooked rice or dairy products does not make them safe.
- Bacillus cereus - emetic type in cooked rice held warm; diarrhoeal type in starchy foods.
- Salmonella - poultry, eggs; Campylobacter jejuni - raw milk, poultry; E. coli O157:H7 - undercooked beef, unpasteurised juice; Vibrio parahaemolyticus - seafood.
- Moulds: Aspergillus flavus produces aflatoxin in groundnut and maize; Fusarium produces trichothecenes in cereals.
Worked example
A canning problem gives D-value at 121 degrees C for C. botulinum spores as 0.21 minutes. The 12D concept requires F0 = 12 x 0.21 = 2.52 minutes at 121 degrees C, meaning the coldest point must accumulate at least that lethality. This is why low-acid canned foods are given a scheduled process far harsher than pasteurisation.
Traps
- Treating coliforms or total plate count as proof of pathogen presence; they indicate hygiene failure only.
- Forgetting that staphylococcal toxin survives boiling while the organism does not.
- Applying the botulinum pH 4.6 rule to acid foods such as citrus juice.