Food Science, Safety and Preservation
Here the exam tests two linked ideas: why food behaves as it does during preparation, and how biological, chemical and physical hazards are controlled so that food stays safe and keeps longer.
Functional properties
- Gelatinisation - starch granules absorb water and swell on heating; amylose leaches out and sets on cooling.
- Dextrinisation - dry heat breaks starch into shorter chains (toast browning).
- Caramelisation - sugar melts and browns; Maillard - amino groups react with reducing sugars, needing heat and low moisture.
- Coagulation of protein (egg, meat), emulsification (lecithin in egg yolk), aeration (egg white foam), gluten development (kneading, hydration).
Safety systems
- Hazards are biological, chemical, physical and allergenic. HACCP uses seven principles: hazard analysis, determine CCPs, set critical limits, monitor, corrective action, verify, document.
- Danger zone 5–63 °C; refrigerator ≤5 °C; freezer −18 °C.
- Pathogens to know: Campylobacter (poultry), Salmonella (eggs), E. coli O157 (minced beef), Listeria monocytogenes (grows at chill temperature; soft cheese, pâté - avoid in pregnancy), Clostridium botulinum (anaerobic, low-acid, spore-forming), Bacillus cereus (reheated rice).
Preservation principles
Heat (pasteurisation 72 °C/15 s; UHT 135–140 °C for seconds), cold, drying and reduced water activity (aw below about 0.6 stops growth), acidification, sugar/salt osmotic preservation, fermentation, modified-atmosphere and vacuum packing, irradiation.
Worked example
Home-bottled tomato sauce. Tomatoes sit near pH 4.3–4.6 - borderline for C. botulinum, which is anaerobic inside the jar, low-acid tolerant and spore-forming. Reasoning: adding lemon juice to bring pH to 4.6 or below, then a full boiling-water process, is what makes the jar safe; boiling later does not reliably destroy pre-formed toxin, so prevention is the control.
Traps
- Believing freezing kills bacteria - it only stops growth.
- Cooling stock in one deep pot instead of shallow containers.
- Reheating more than once, or below 75 °C.
- Confusing water activity with moisture content.