Food Microbiology and Spoilage
What the exam tests
This domain tests microbial groups, growth requirements, spoilage signs, foodborne pathogens and the difference between spoilage and safety. Expect questions on intrinsic and extrinsic factors, growth curves, canned-food spoilage, and pathogens such as Salmonella, Listeria, E. coli O157 and Clostridium botulinum.
Core rules and concepts
- Bacteria, yeasts and moulds differ in size, growth rate and tolerance; bacterial spores survive ordinary cooking and need a scheduled thermal process.
- Intrinsic factors include pH, water activity, nutrients and antimicrobial compounds; extrinsic factors include temperature, atmosphere and humidity.
- Microbial growth has lag, log, stationary and death phases. Refrigeration extends lag; freezing stops growth but preserves cells.
- Spoilage organisms may not be pathogens. A food can smell normal and still carry pathogens; a spoiled food is not automatically unsafe, but it is unacceptable.
- Low-acid canned foods (pH above 4.6) support C. botulinum if anaerobic and aw is above 0.85; bulging cans may indicate gas production.
Worked example
A food has 100 bacterial cells and a generation time of 30 min at room temperature. After 4 h there are 8 generations, because 240 divided by 30 = 8. Population = 100 x 2^8 = 100 x 256 = 25,600 cells. If the same food is held at 5 degrees Celsius and growth stops, the count stays near 100. The reasoning shows why time-temperature control matters more than the initial count alone.
Traps
Assuming refrigeration kills bacteria; judging safety by smell; ignoring anaerobic pathogens in canned food; confusing spoilage with foodborne illness; forgetting that moulds can grow at lower aw than most bacteria.