Low-temperature preservation and refrigeration
Cold slows microbes, enzymes and chemical changes but does not kill. Refrigeration (chilling) keeps food at about 0-5 °C for days to weeks.
How cold preserves
Low temperature slows the growth of microbes, the action of enzymes and chemical reactions such as rancidity. As a rough rule these rates halve with every 10 °C fall (Q10 about 2). Cold is bacteriostatic: most microbes survive and grow again when the food warms up. Psychrotrophs, cold-tolerant microbes such as Pseudomonas and the pathogen Listeria monocytogenes, still grow slowly in the fridge, so chilled food has a limited life.
Three levels of cold storage
| Level | Temperature | Keeps for | Examples |
|---|---|---|---|
| Cellar (common) storage | About 15 °C, not much below outside air | Weeks to months | Potatoes, onions, root vegetables, some apples |
| Refrigeration (chilling) | About 0-5 °C | Days to weeks | Milk, curd, meat, fish, eggs, fruits, vegetables, leftovers |
| Freezing | −18 °C or colder | Months | Frozen peas, prawns, meat, parathas, ice cream |
Draw it: the refrigeration (vapour-compression) cycle
- back to Compressor
Refrigerants: ammonia in large cold stores; ozone-safe HFCs and hydrocarbons such as isobutane (R-600a) in home fridges. CFCs (Freon-12) were phased out because they damage the ozone layer.
Good practice in cold storage
- Keep the fridge at or below about 5 °C; do not overload it, so cold air can circulate.
- Humidity: fresh vegetables need high humidity (the crisper drawer) to stop wilting; dry foods need dry air.
- Chilling injury: tropical produce is damaged by cold above freezing. Bananas below about 12-13 °C turn black and fail to ripen; mango, cucumber, brinjal and green tomato are also sensitive.
- Potatoes kept too cold turn sweet (starch changes to sugar) and fry dark.
- Cover foods to stop drying and smell transfer: milk and butter pick up onion and fish odours.
- Controlled-atmosphere (CA) storage: cold plus less oxygen and more carbon dioxide slows the breathing of apples and other fruits for months.
- Cold chain: pre-cooling at the farm → refrigerated trucks → cold store → shop chiller → home fridge. Village bulk milk coolers chill milk soon after milking.
Advantages and limitations of refrigeration
In a hospital or canteen, cold storage is the main shield for perishables: cooked food should go into the fridge within about two hours, and raw meat is kept below ready-to-eat food so it cannot drip on it. Chilled greens keep their vitamin C far better than greens left at room temperature.
2 marks: “What is chilling injury? Give an example.” Damage to tropical fruits and vegetables stored below their safe temperature, though above freezing, e.g. banana skin blackens and the fruit fails to ripen below about 12-13 °C.
5 marks: “Explain the working of a refrigerator with a diagram.” Vapour-compression cycle: compressor, condenser, expansion valve, evaporator; the refrigerant takes heat from the cabinet and gives it out at the back; draw the cycle; name refrigerants.
10 marks: “Discuss preservation by refrigeration with its advantages and limitations.” Principle (cold slows microbes and enzymes; Q10); levels of cold storage; refrigeration cycle; storage practice, chilling injury, CA storage, cold chain; advantages and limitations.
Fridge cycle C-C-E-E: Compressor squeezes, Condenser gives off heat, Expansion valve drops pressure, Evaporator takes heat from the food.