cooling
Two basic types of cooling are available: refrigeration and freezing.
By freezing and using the fridge, cost savings can be made in the home and food waste can be reduced.
It’s important to keep food cool for many reasons, principally to stop bacteria growth so it will stay fresher for longer, or to help set foods like jellies and terrines.
Always ensure food has cooled from cooking before it goes into a refrigerator or freezer as this will stop the internal temperature of the fridge or freezer from rising.
Fridges
Fridges come in many shapes and sizes, from tall upright ones to America style “side by side” to under counter models or fridge freezers.
When the temperature of food is reduced, the activities of most microorganisms are slowed down until they go to sleep or become dormant - growth and reproduction ceases. When food is taken out of the fridge, the temperature is raised, growth of the microorganisms starts again and in turn they reproduce.
Centuries ago, primitive man tried to chill meat and other foods to keep them longer by packing them in ice, but this did not prove to be very successful. Domestic refrigeration was introduced to the UK during the 1920s.
It is only possible to store foods in a refrigerator for a short period of time. Refrigerators are kept at 5˚C so the internal temperature of the food does not rise above 8˚C. This is because at this temperature microbial activity still takes place, although it is slowed down, so food decay will still occur.
Foods to be stored in the refrigerator should be suitably wrapped and used up fairly quickly.
Types of refrigerator
There are 2 types of refrigerator:
Compression refrigerators
Absorption refrigerators
Compression refrigerators
These are the most common and are only powered by electricity. The icebox is surrounded by a coiled metal tube containing the refrigerant liquid. The heat from inside the refrigerator is absorbed form the inside of the refrigerator evaporates the refrigerant forming a vapour. An electric motor pumps the vapour round the compressor, where it is condensed back into a liquid. The liquid is cooled by circulation through pipes at the back of the fridge and then returns to the evaporator. And the whole process starts again. The temperature inside the refrigerator can be adjusted by a thermostat control.
Absorption refrigerator
These are less common and they operate silently by a small amount of heat given off by a gas or electric heater.
A note on fridge temperatures
Domestic refrigerators are designed to keep the temperature inside below 7˚C. However, the temperature varies in different areas of the fridge interior. If the fridge has an icebox, it is not designed to freeze food, only store it.
The door shelves are the warmest part of the cabinet.
Choosing a refrigerator
When choosing a refrigerator think about the following points:
Size and capacity needed.
Energy rating of refrigerator as this will affect running costs.
Storage arrangements inside the refrigerator.
Space available in the kitchen.
Star rating if there is an ice box.
Ease of cleaning the interior.
Cost of unit.
Storing food in a refrigerator
Food loses moisture as it cools down, so always wrap food or it will dry out.
Some foods absorb odours while others give off odours, so never place milk beside kippers. Meat and fish will discolour if wrapped in plastic in the fridge, so store in a glass or ceramic dish instead.
From a food hygiene point of view, place raw meat at the bottom of the fridge and cooked meat and products above. If storing eggs in a refrigerator, ensure you remove them from the fridge at least 30 minutes before cooking as a room temperature egg gives a better finished product to a cold one.
Freezers
Just like refrigerators, freezers work on the same principle of cooling a liquid to absorb heat, but they work at much lower temperatures so food will stay fresher for longer.
Freezing is a simple and effective way of preserving food. Whole meals, snacks and parts of meals can be frozen for later use and it's generally much cheaper to buy foods such as meat in bulk and store them for long-term use, likewise with home-grown or “pick your own” fruit and vegetables.
Buying a freezer
When buying a freezer, it’s useful to think of the following points:
-
How much can I afford?
The size required - allow about 55 litres per person in the household.
How much floor space is available.
Do I want a chest or an upright freezer?
Freezing and the quality of food
Deep freezing reduces the temperature in food to stop microbial growth and activity. Many foods can be frozen successfully as they contain relatively large amounts of water. When food is frozen ice crystals are formed inside it. Some foods, like fruit and vegetables can be damaged in freezing if the ice crystals are too large – this is because the cells burst as the ice crystal exceeds the cell size. Remember: water increases in volume when frozen. Once the food is thawed the structure will collapse, releasing most of its liquid. Large ice crystals are formed if the food is frozen too slowly. If food is frozen quickly the ice crystals are formed rapidly and remain within the cell walls without bursting them. Quick freezing is when food is dropped from 0˚C to -4˚C in under 30 minutes.
Some foods, like bananas which have a high enzyme content, will still discolour in the freezer.
Star markings
Freezers have star ratings on how long food can be stored inside them. Commercially frozen foods normally give this rating on the packet as a guide.
| Star rating | Temperature | Storage time |
 |
-6˚C | Up to 1 week |
|
-12˚C | Up to 1 month |
|
-18˚C | Up to 3 months |
Basic rules of freezing
Freeze only fresh foods.
Freeze food when it is at its best quality.
Once prepared or harvested, freeze food quickly to avoid deterioration.
Handle and freeze food hygienically.
Use suitable packaging materials and wrap the food properly.
Pack food in single or multiple portions according to the needs of your family.
Remove as much air as possible from the packaging before freezing to prevent oxidation of the food during storage.
Label the packs clearly with what the food is and the date it has been frozen.
Chill the food in the refrigerator before putting into the freezer so as not to raise the temperature in the freezer otherwise large ice crystals could grow and damage the food structure.
Aim to use up stocks of food before they come into season again.
Never re-freeze foods that have been thawed, unless they have undergone a process of cooking, eg, thawed meat cooked into a casserole can be refrozen.
Only freeze foods for the recommended length of time as in the table below.
Food |
Maximum length of time to be in freezer |
Vegetables |
1 year |
Fruit |
1 year |
Raw meat |
1 year |
Poultry |
1 year |
Fish |
8 months |
Cakes |
6 months |
Pastry |
6 months |
Bread |
3 months |
Soup and sauces |
4 months |
Stews |
2 months |
Ice cream |
3 months |
Thawing
Frozen food
Some food like vegetables, beef burgers, pizzas and fish can be cooked from frozen. All other foods need to be thawed before cooking. With some foods, like meat, it is very important they have been thawed completely before cooks for safety reasons.
