Food Preservation Principles and Methods
4. Food Preservation Principles and Methods
This domain tests how preservation methods control microbial growth, enzymes and chemical deterioration. You should know the principle behind each method, typical conditions, and which methods are used for which foods.
Core rules
- Preservation principles: asepsis, heat, cold, removal of water, acidification, addition of preservatives, irradiation, fermentation and hurdle technology.
- Pasteurisation (e.g. milk 72°C/15 s) kills vegetative pathogens but not spores; sterilisation/commercial canning destroys spores.
- Blanching inactivates enzymes before freezing or drying. Freezing at −18°C stops growth but not all enzyme activity.
- Drying lowers aw; osmotic dehydration uses sugar or salt; smoking combines drying, antimicrobials and flavour.
- Permitted chemical preservatives include sorbic acid, benzoic acid, sulphur dioxide and nitrite, within FSSAI limits; nitrite is restricted because of nitrosamine risk.
- Hurdle technology combines low pH, low aw, low temperature and preservatives so no single hurdle must be severe.
Worked example
To make 1 kg fruit pulp at 30% total soluble solids (TSS) into jam at 65% TSS, let x kg sugar be added. Solids balance: (0.30 + x)/(1 + x) = 0.65. Solve: 0.30 + x = 0.65 + 0.65x; 0.35x = 0.35; x = 1.0 kg. Final mass = 2 kg at 65% TSS. Reasoning: sugar adds both solids and mass, so use the total mass denominator.
Traps
Pasteurisation is not sterilisation. Refrigeration is not a kill step. Sulphites can trigger asthma, so labelling matters. Adding sugar without checking final TSS or pH may fail jam standards.