Freezing

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Freezing preserves by lowering the temperature so that all spoilage organisms and enzyme processes are slowed and then brought to a standstill. As a rule, -10oC (14oF) brings most biological and biochemical processes to a halt, which is warmer than the standard for freezers of -18oC (0oF). Of all the preservation methods, freezing causes the least effect on the food being preserved.

Although the Inuit and peoples in frozen regions of China and Russia have frozen foods, the beginning of the concept of frozen foods is attributed to "Captain" Birdseye, the American Clarence Birdseye[1][2]. Captain Birdseye came up with the idea during 1912 - 1916 when living in Labrador; he observed local inhabitants during the winter months freezing fish immediately in temperatures of -43oC (-45oF). In 1924, Birdseye after several years of experimentation built his first automatic quick-freezing machine, but his ideas could not get widespread use until freezers were installed in stores. His business went bankrupt, but was bought by General Foods in 1929, who had the capital to develop the idea. This began during the late 1930s in the USA, then in Europe after World War II.

At first, only shops had freezers. Domestically, people kept frozen food in the ice-making compartment of their fridge, but this is cold enough only for a few days storage. Gradually, the home freezer came into use, first as a luxury during the 1930s and now as a staple in most home. The home freezer has allowed the home cook to get access to different foods in a fresh format throughout the year, so is no longer dependent upon the seasons, nor general preservation methods.

Science of Freezing

Effects of Freezing

The preservative effects of cold temperatures are covered in refrigeration. When food is frozen, other effects also occur.

The liquids in foods are not pure water, but are mixtures with salts and sugars, so they are still liquid at freezing points, i.e. 0oC (320F). The stronger the solution, the lower the freezing point. As the liquid begins to freeze, so pure ice forms and the dissolved chemicals pass into the remaining liquid, which becomes the stronger solution with a lower freezing point. The liquids, therefore, only freeze little-by-little and no water solution completely freezes, even in the coldest freezer, because there is always a residue of solution somewhere within the food. At a certain point, the solution becomes too concentrated for spoilage and disease micro-organisms to function, except for a few moulds that will still grow slowly even in the freezer.

Freezing does not stop the action of enzymes within the food, which still react slowly in the unfrozen solutions in the food. This is why blanching fruits and vegetables is important to denature these proteins, so stopping their actions. If these foods are frozen without blanching, they still have a good preserved life in the freezer, but not as long as those that have been blanched. Some foods - e.g. crustaceans - must be cooked before freezing because their enzymes are so reactive that freezing them raw is a disaster. Fruits that are to be eaten raw, and so are not being blanched, are often dipped in syrup before freezing to halt enzyme action.

Freezing has little effect on the nutrition of your food. There may be a small loss of vitamin C in fruit and vegetables from the blanching prior to freezing; peas though have more vitamin C than fresh peas. Meat may lose some of its vitamin B1, but as with fruit and vegetables, this is really in the initial processing rather than the freezing.

Methods of Freezing

Rapid freezing processes are designed to quickly bring temperatures from 0oC (320F) to -5oC (230F), or commercially to -18oC (00F). These processes are designed to be rapid to minimise the chance of larger ice crystals forming, so reducing their damage on the cell structure of the food cells and their mushiness when unfrozen later.

Using a blast freezer is the most common method. Blast freezing freezes smallish pieces, e.g. packets of frozen vegetables, quickly and uniformly. The food is frozen on refrigerated trays by blasting with cold air at -12oC (100F), after which it is cooled more slowly to -18oC (00F), the temperature required in commercial freezers. For very small objects, fluid bed freezing is used, where ingredients like peas move along conveyors wit holes in the belt, through which cold air is blasted; this lifts the food which freezes in mid air.

Contact freezing can be used for larger ingredients that do not require rapid freezing, e.g. fish fillets. Here, the fillets are frozen between two refrigerated plates that are pressed lightly against the food.

Immersion freezing is used for whole chicken and turkeys. In this case, the whole chicken or turkey is immersed in very cold liquid, e.g. brine or sugar solution. After freezing, the frozen object is cleaned by centrifuging to spin off the liquid. Related to this, spray freezing sprays food with cold liquids as they pass along a wire mesh conveyor belt.

Luxury foods that are damaged by freezing are sometimes frozen by dipping in liquid nitrogen at -196oC (-321oF). Freezing is instantaneous, so only tiny ice crystals develop.

Freeze Drying

Freeze drying uses the concept that ice can "sublime", i.e. bypass the liquid stage in passing from solid to water vapour. It is a fairly expensive process, so is used for higher-value foods - coffee, Kaffir lime leaves, peas and prawns. These are quickly frozen, then placed into a vacuum chamber and slightly warmed to encourage sublimation. The final product may have a moisture content as low as 2%. It is then packed in airtight containers to prevent them from absorbing moisture. This preserves the volatile oils and precious vitamins and minerals in delicate foods.

How to Freeze Various Foods

Vegetables

Frozen vegetables require half the cooking time of fresh vegetables, because they have already been blanched. Most are best cooked from frozen in a minimum of water that is then brought to the boil. Corn on the cob should be defrosted before boiling. Vegetables will keep for 9 - 12 months in the freezer, except onions that last for only 2 - 3 months.

If preparing your own vegetables, you should prepare the vegetables by washing, trimming and slicing them as for cooking, then blanch them. Blanching is vital as it deactivates the enzymes and so stops deterioration that will spoil the flavour and colours. Blanching times vary from 1½ minutes for peas to 10 minutes for large corn. Most vegetables are in the range of 2 - 4 minutes. there are two ways to blanch vegetables:

  • Immersion in Boiling Water: place a small quantity of vegetables in a wire basket and immerse in a pan of boiling water. Bring back to the boil and count the blanching time from the moment the water returns to the boil. Shake the basket to ensure an even cook. Remove from the basket and plunge into ice cold water for 1 - 2 minutes. Drain well, dry the vegetables and then pack.
  • Steam Blanching: this is better for French beans, turnip and cut kernel corn, preserving the vitamins and minerals better than boiling for these. Bring 2½ cm (1 in.) water to the boil in a pan, place the vegetables in a wire basket above the water and cover tightly. Dry the vegetables and pack in rigid containers or plastic bags, sealing so airtight, removing all the air before sealing the bags.

Fruit

Most fruit freezes well, but not bananas or pineapples. Use fresh and ripe fruit, but never overripe fruit. Do not wash them as this makes them mushy - especially soft fruits like raspberries and strawberries.

There are three ways to freeze fruit:

  • Open Freezing: is best for soft fruit. Spread the fruit on trays and freeze at the lowest temperature possible until solid (30 - 60 minutes), then put into rigid containers, seal and store in freezer.
  • With Sugar: is suitable for harder fruit, e.g. gooseberries, rhubarb and citrus fruit. Pack the fruit into rigid containers, alternating fruit with a layer of sugar. Allow 1 cm (½ in.) headspace for the fruit to expand, seal and freeze.
  • With Sugar Syrup: works well for pears and peaches that discolour when peeled, and for fruit salads (do not include pineapple or bananas). An average syrup should contain 275 g sugar to 500 ml water (10 oz sugar to 1 pt water). Place the sugar and water in a pan, add teaspoon of lemon juice or ¼ teaspoon of ascorbic acid, then bring to the boil, stirring occasionally, simmering for 5 minutes. Cool it before adding to the peeled, chopped or sliced fruit. pack in a rigid container, allowing 1 cm (½ in.) headspace. Seal and freeze.

Cooked and puréed fruit may be prepared as usual, put into cartons and then frozen.

Frozen fruit thaws in 3 - 4 hours at room temperature. Frozen fruit will keep in the freezer for 9 - 12 months.

Fish

Prepare fish as for cooking., The fresher the better. Wrap each fish, fillet or steak in paper or foil and then pack into plastic bags. Once frozen, white fish will store for 6 - 9 months, while oily fish will last for 4 months. Fish should be allowed to thaw fully in the fridge for 14 hours before cooking. Small fish, fillets and steaks can be cooked from frozen, with some extra cooking time to ensure that the centre if thoroughly heated through.

Shellfish

If possible, buy lobster, crab and crayfish live and cook by plunging in boiling water for 10 - 15 minutes. Let them cool, then remove the meat and pack into rigid containers, seal and freeze. Shrimp is best frozen uncooked and in their shells, but with heads and tails removed. Wash shrimps in cold salted water before freezing them.

Oysters, clams and scallops are best frozen uncooked. Remove them from their shells, keeping the natural juices. Throw away any that have opened. Pack them into rigid containers with their juices. Fill any headspace with crumpled foil, seal and freeze.

Shellfish last for about 2 months. Shellfish should be allowed to thaw fully in the fridge for 14 hours before cooking.

Poultry and Game

Game should be hung before freezing. Game birds and poultry should be plucked and gutted, with head and tail removed, and the insides washed and cleaned in cold water. They may be frozen whole or in pieces; they should be stored in plastic bags. Do not freeze stuffing inside the birds, because the stuffing has a much shorter life than the frozen poultry. Birds must be thoroughly thawed overnight in the fridge before cooking.

Venison should be hung for 5 - 8 days and then frozen like other meats.

Hare should be hung for 5 - 6 days, then skinned and gutted. Freeze hare whole or jointed, packed in airtight plastic bags. The blood, which is needed for jugged hare, can be frozen in ice-cube moulds. Rabbit does not need to be hung; otherwise treat as for hare.

Meat

Before freezing meat, remove as much bones as possible, because this wastes freezer space. Wipe meat clean, prepare as for cooking, wrap in plastic, seal and freeze.

Prepare stews and casseroles as for cooking, then pack into plastic bags, removing as much air as possible.

Steaks, chops and cutlets should be trimmed, separated with pieces of foil and packed in plastic bags for freezing. Sausages and sausagemeat should also be packed in plastic bags.

Meat is best thawed overnight in the fridge, so it then cooks more evenly.

Dairy Products

Butter, margarine, lard and cheese freeze well. You should overwrap them in plastic bags before storing in the freezer. Dairy products keep for up to 6 months, except soft cheese that will last for 4 months only.

Heavy Cream, clotted and whipped cream can be frozen, but light cream does not freeze well. Adding a tablespoon of sugar per 500 ml (1 pt) of cream helps increase its storage life. Cream can be stored in its original container.

Frozen ice cream and frozen yoghurt store well.

Bread and Cakes

Breads, buns and cakes freeze very well, and can be stored for up to 6 months. Cakes are best frozen before they are iced, because the icing will smudge when packed. Pack bread and cakes in plastic bags. Wrap sandwiches in foil or plastic film and then freeze in bags.

Pastry

Uncooked pastry freezes very well and keeps for up to 6 months. Shape the pastry into a black, wrap it in plastic film and put into a plastic bag. Commercially prepared frozen pastry is really handy, especially puff pastry, vol au vents cases and even pizza dough. Cooked pastry, e.g. flan cases, is very flimsy, so should be packed in rigid containers.

Tips

The quicker food is frozen, the better it tastes. If it is frozen slowly, large crystals can form and rupture the cells. This is why commercial freezers use a blast freezer. In contrast, the slower food is thawed the better. Hot or warm food should never be put into domestic freezers as they raise the temperature which damages the other stored foods.

For cooked foods, do not season them unless it is leftovers that you wish to prolong. Freezing can change the taste of seasonings, e.g. cloves and garlic become stronger during freezing.

Some foods do not freeze well: hard-boiled eggs, light and sour cream, mayonnaise, custard and cooked potatoes. So don't freeze them!

Associated Pages

References

  1. Biography.com Editors (undated) Clarence Birdseye, A&E Television Networks, accessed 8 June 2016 [1]
  2. Editors of Encyclopaedia Britannica (undated) Clarence Birdseye, Encyclopædia Britannica Online, accessed 8 June 2016 [2]

Further Reading

  • Davidson, A. (1999) The Oxford Companion to Food, Oxford University Press. ISBN 0192115790.
  • Stobart, T. (1981) The Cook's Encyclopedia: ingredients & processes, Harper & Row. ISBN 0060141271.