among all the processes for preservation of food, has the advantage of causing relatively little change in the food.
It is thought of as a recent innovation, but has been used since antiquity, although only in places where the climate is cold enough for it to be possible by natural means. The Inuit (Eskimos) and other peoples of the far north have always stored food by simply burying it in the snow. This natural freezing has been practised in the northern parts of Russia, China, and Japan.
In more clement regions, ice could be brought down from the mountains to assist in preserving food; but the use of ice by itself was not freezing and is dealt with separately—see also refrigeration. It was not until the early 16th century, in Italy, that the discovery was made that if enough salt is mixed with ice its temperature falls to −18 °C (0 °F), and that this mixture can be used to freeze things. For a long time this technique was used only to make ice cream and the like, not for preserving food.
The first food other than such desserts to be frozen artificially for sale was fish, in the 19th century. Dr Kitchiner (1817), mentioned frozen fish, presumably prepared in an ice and salt mixture, but the first patent for this method of freezing fish was not granted until 1842, to Henry Benjamin and Henry Grafton in Britain. The first mechanical freezing plant was built in Sydney, Australia, in 1861. By 1870 chilled meat—that is, a couple of degrees below freezing point, but not deep frozen—was being shipped from the USA to Europe in insulated holds cooled with ice and salt; 1880 saw the first successful shipment of chilled meat on the longer voyage from Australia to England.
By the end of the century the method was being applied to fish and poultry, and early in the 20th century experiments were made in freezing fruits. These were at first unsuccessful; the fruits were mushy when thawed, since they had been frozen too slowly so that large ice crystals formed which disrupted their delicate tissues. Similar troubles were encountered with vegetables, the more so since the need for initial blanching to destroy enzymes was not realized. Animal products generally emerge better from a process of slow freezing because of the greater elasticity of their tissues.
The problem was overcome by Clarence Birdseye, an American who lived in Labrador between 1912 and 1916. There, in the intense winter cold, he observed the effect of very rapid freezing not only on meat and fish, which the inhabitants customarily froze, but also on vegetables. He began experiments with artificial quick freezing and in 1923 set up a company to prepare and sell frozen fish. This soon went bankrupt, but he did manage to develop the first specially designed quick freezer for vegetables and fruits. In 1929 he was bought out by General Foods and the era of frozen food began in earnest.
At first only shops had freezers. At home, people kept frozen foods in the ice-making compartment of a refrigerator, which is just cold enough to store them for a few days. The home freezer came in gradually during the 1930s as an expensive luxury, at first mainly in the USA.
The preservative effects of cold are described under refrigeration. When food is frozen rather than merely chilled, other effects come into play. The liquids in food are not pure water, but solutions of salts and sugars; so they are still liquid at 0 °C (32 °F), and only begin to freeze at lower temperatures. The stronger the solution, the lower the freezing point. Once the liquid starts to freeze, pure ice forms and the dissolved substances pass into the remaining liquid, which thus becomes a stronger solution with a lower freezing point. The liquid therefore freezes little by little, and no water solution ever freezes completely, even in the coldest freezer—there is always a residue of very strong solution. At a certain point, however, the solution will have become too strong for micro-organisms to function, with the exception of some troublesome moulds which can grow very slowly in a freezer.
Freezing does not stop the action of enzymes, which continues slowly in the residual unfrozen solution. However, the blanching of fruits and vegetables before freezing destroys the enzymes. If foods are frozen completely raw, they will still have a respectable freezer life, but less long than if blanched. There are some exceptions such as crustaceans, whose enzymes are so active that cooking before freezing is essential. Fruits that are to be eaten raw, and so cannot be blanched, are often packed in sugar or dipped in syrup before freezing, to exclude air and thus inhibit enzyme action.
Freezing does little damage to the nutritive value of food. Fruits and vegetables lose some vitamin C, mostly in processes such as blanching before they are frozen. But frozen peas—which are often frozen within minutes of being picked—still have more of the vitamin than ‘fresh’ peas which have been hauled to a wholesale market and then left sitting in a shop. Meats lose a certain amount of vitamin B1, again mostly in initial processing.
Quick freezing processes are designed to take food through the temperature range from 0 °C to −5 °C (32 °F to 23 °F) in a few minutes, since this is when most ice crystals form. The less time they have to form, the smaller they will be and the less damage they will do to the cell structure of the food. The smaller the pieces of food, the faster they freeze. Several methods are currently used for items of different sizes.
Air blast freezing is the most used technique. It freezes smallish objects, such as packets of frozen vegetables, quickly and uniformly. The food is frozen on refrigerated trays by a blast of chilled air at −12 °C (10 °F), after which it can be cooled more slowly to the storage temperature of a commercial freezer, −20 °C (−4 °F). A variant used for very small objects, such as loose peas, is fluid bed freezing, where the food travels along a belt pierced with holes through which chilled air is blown, lifting the food so that it is frozen in mid-air.
Contact or plate freezing is suitable for larger items that do not have to be frozen very quickly, such as fish fillets. These are frozen between refrigerated plates which press lightly against the food.
Immersion freezing is used for very large things such as whole chickens and turkeys. These are immersed in a very cold liquid—brine, sugar solution, or propylene glycol (a chemical also used as anti-freeze). After the food is frozen it is centrifuged to remove the liquid. A related technique is spray freezing, where food is sprayed with cold liquid as it travels on a wire mesh conveyor belt.
Luxury foods that are easily damaged by freezing, such as soft fruits, are sometimes frozen by dipping them in liquid nitrogen at −196 °C (−321 °F). Freezing is more or less instantaneous, so only tiny ice crystals are formed.
This method can produce foods that are almost as well preserved as by straight freezing, but do not need cold storage. It exploits the fact that ice can ‘sublime’, or change straight from a solid to water vapour without passing through a liquid stage. (Sublimation is a familiar phenomenon in a home freezer: the frost that forms on the inside of bags of food comes from water vapour that has sublimed out of the food and then refrozen.)
Freeze-drying of a kind has been practised for centuries in cold, mountainous areas. If small pieces of food are left to freeze out of doors and hung up in the wind, the moisture will gradually sublime. The low air pressure at high altitude speeds the process. Products prepared in this way include chuño, the dried potatoes of the Andes; and freeze-dried tofu in Japan. (See also drying.)
Modern freeze-drying is used for small items such as peas and prawns, and also for liquids such as coffee. It is a relatively expensive technique and therefore used mainly for high-value products. The food is quick frozen by an appropriate method, then put in a vacuum chamber and very slightly warmed to encourage sublimation. The final product may have a moisture content as low as 2%. It is packed in an airtight container such as a foil pouch to prevent it from absorbing moisture.
The nutritional value of foods is scarcely more affected than in conventional freezing.
Ralph Hancock is an encyclopedist with a special interest in food history and food science.
