Food Science, Food Preservation and Food Safety
Questions here combine the chemistry of food changes, the principles and methods of preservation, and food safety practice including adulteration testing.
Food science basics
- Colloidal systems: sol, gel, foam and emulsion - milk is oil-in-water, butter is water-in-oil.
- Starch gelatinises on heating with water and retrogrades on cooling; dry heat causes dextrinisation (toast); sugar gives caramelisation; sugar with protein gives Maillard browning (bakery crust).
- Protein denatures and coagulates with heat or acid - the reason lemon curdles milk.
- Fat turns rancid by hydrolysis or oxidation; prevented by airtight, cool, dark storage and antioxidants.
- Cut apple or brinjal browns because polyphenol oxidase acts on phenols; lemon juice, salt water or blanching slows it.
Preservation principles
Two aims: prevent micro-organisms entering (asepsis, packaging, hygiene) and destroy or inhibit them. Methods include drying, canning, pasteurisation (about 72 °C for 15 seconds, HTST), freezing at −18 °C, pickling with salt, vinegar or oil, sugar preservation (jam, squash at roughly 65–68% total soluble solids), chemical preservatives (sulphur dioxide in dried fruit, sodium benzoate in squash), irradiation and fermentation.
Food safety
WHO five keys: keep clean; separate raw and cooked food; cook thoroughly; keep food below 5 °C or above 60 °C; use safe water and raw materials. FSSAI licence or registration is mandatory for food businesses; HACCP identifies critical control points. Home adulteration tests: iodine turns blue in milk adulterated with starch; metanil yellow in turmeric; sawdust in chilli powder.
Worked example
"Why does a cut apple brown, and why does lemon juice help?" Answer: cutting ruptures cells, releasing polyphenol oxidase, which oxidises phenols to brown melanins. Lemon supplies ascorbic acid and lowers pH, so the enzyme is inhibited and the vitamin C is itself oxidised first. Apply this to a fruit salad: toss in lemon juice, chill, and cover to limit oxygen.
Traps
- Treating pasteurisation as sterilisation.
- Saying freezing kills all bacteria - it only stops multiplication.
- Canning low-acid vegetables without pressure sterilisation.
- Confusing FSSAI with AGMARK or ISI.