Food, Frozen. Freezing is one of the best ways to preserve foods. Food-processing companies freeze such foods as baked goods, orange juice, pizzas, vegetables, and complete precooked meals. Freezing preserves food by preventing the growth of microbes that spoil food and by slowing food-spoiling chemical reactions. Frozen foods should be stored at temperatures of 0 °F (–18 °C) or below.
Almost all foods frozen commercially are quickly frozen, whereas food frozen at home is slowly frozen. Quick freezing preserves most foods better than slow freezing. Slow freezing changes the structure of some foods in such a way that the foods leak fluids when they are later defrosted. This leakage results in undesirable changes in the food’s texture. For example, leakage can make vegetables mushy, and meat tough. During slow freezing, the food may not cool quickly enough to prevent spoilage by microbes or by chemical reactions.
Commercial freezing of food began in the United States before 1865. Food-processing companies originally froze food by using ice cut from ponds or rivers. In the late 1850’s, such companies began making ice by using a process that involved the mechanical compression of ammonia (see Refrigeration (Mechanical refrigeration)). The ice was used to freeze meat and vegetables for international distribution. These early methods of freezing allowed only for slow freezing. But in the mid-1920’s, Clarence Birdseye, an American inventor, developed a quick-freezing process for fish and vegetables. In 1929, the Postum Company (now General Foods Corporation) purchased his patents and began to produce frozen foods. Frozen foods became popular in the United States in the 1950’s, when freezers became widely available.
Commercial methods of quick freezing
There are several commercial methods of freezing foods quickly. These methods include (1) air-blast freezing, (2) indirect-contact freezing, (3) cryogenic freezing, and (4) liquid immersion freezing.
Air-blast freezing
uses a steady flow of cold air at 0 to –40 °F (–18 to –40 °C). The cold air is produced by passing air over coils that are cooled by a mechanical ammonia-compression system. The cold air is then blown into an insulated tunnel. In many arrangements, the food passes through the tunnel on a conveyor belt. In most cases, processing firms package the food before sending it through the tunnel. But for faster freezing, some processors freeze foods before packaging them.
Air-blast freezing is quick and efficient, but it can partially dehydrate unpackaged foods. A type of air-blast freezing called fluidized bed freezing forces air upward through a bed of food pieces. Strawberries, peas, and other kinds of fruits and vegetables are individually quick frozen (IQF) by this method.
Indirect-contact freezing,
also called plate freezing, uses adjustable metal plates that have hollow walls. A refrigerant (cooling substance) inside the plate walls cools the plate surfaces to temperatures as low as –40 °F (–40 °C). Packaged foods are placed between the plates, which are then adjusted to make contact with the upper and lower surfaces of the packages. As the plates absorb heat, the food freezes.
Cryogenic freezing
uses a cryogen (liquefied gas), such as liquid nitrogen under pressure at –280 °F (–173 °C). Carbon dioxide may also be used, either as a liquid or in a solid form called dry ice. The cryogen or dry ice is first vaporized (turned into a misty gas). The cold vapor then flows into a chamber, where it freezes the food. This method is expensive, but it freezes food faster and results in better products than the air-blast and indirect-contact methods do.
Liquid immersion freezing
uses a refrigerated solution of salt or of a type of alcohol called glycol. A mechanical conveyor moves food in cans or other packages through the solution. Processing companies use salt or glycol solutions to freeze such products as canned fruit juices and poultry that is sealed in plastic film.
In the past, food companies used a method where unpackaged foods were sprayed with, or dipped in, liquid chlorofluorocarbons (CFC’s) at –21 °F (–29 °C). CFC’s are chemicals that contain carbon, chlorine, and fluorine. Because CFC’s harm the environment, many countries have restricted their production. As a result, processing companies switched to other methods in place of liquid-CFC freezing, which had been used to freeze such foods as shrimp and corn on the cob.
Freezing food at home
People freeze foods at home by placing them in cabinet deep-freezers or in the freezer compartments of refrigerators. Both of these kinds of freezers operate at about 0 °F (–18 °C).
Freezer capacities vary considerably. A 10-cubic-foot (0.28-cubic-meter) freezer can hold up to about 25 pounds (11 kilograms) of food. Some large cabinet units hold as much as 150 pounds (68 kilograms). But regardless of a freezer’s size, it is important not to overload the unit. Overloading prevents the freezer from maintaining the temperature at or below 0 °F (–18 °C). In addition, a freezer should not be completely filled with unfrozen food, because the food will freeze too slowly.
Home freezers freeze food more slowly than do commercial freezing systems. As a result, freezing food at home results in lower food quality than does commercial freezing. However, many foods make acceptable products when frozen at home. Such foods can be stored for a year at 0 °F (–18 °C) if properly prepared, packaged, and frozen.
Preparing foods for freezing involves several steps. First, the foods should be washed, trimmed, and cut to the desired size. Vegetables and certain kinds of fruits should then be blanched—that is, steamed or boiled for one to three minutes. Blanching destroys chemicals called enzymes that can give the food a disagreeable odor, flavor, or color. Even in a freezer, enzymes may remain active.
Fruits may or may not be blanched, depending on their intended use. Fruits intended for cooking should be blanched. But blanching gives food a cooked flavor, and so fruits that are to be eaten uncooked should not be blanched. Fruits that are not blanched lose quality more rapidly in a freezer than do blanched fruits.
Meats need little preparation for freezing. In general, meat, including poultry and fish, should be frozen uncooked. Cooked meat, when frozen, spoils two or three times faster than meat frozen raw.
In general, thawed foods should not be refrozen. Refreezing food and thawing it again reduces the food’s quality. Unless the food was previously thawed in a refrigerator or microwave oven, refreezing and thawing it again may make it unsafe.
Packaging
is an important part of freezing food at home. Proper packaging protects the food while it is stored in the freezer. The food should be packed tightly in an airtight container to prevent evaporation. Evaporation can dry the food out. It also can cause snow called package ice to form inside containers that have too much air space. In addition, evaporation results in a dull or dried-out appearance called freezer burn. To speed freezing and thawing, food should be placed in small packages.
Problems with frozen foods
Frozen foods can cause food poisoning if they are not frozen soon enough or if they are not cooked soon enough after thawing. Food-poisoning organisms can grow in food if its temperature exceeds 45 to 50 °F (7 to 10 °C) for only a few hours. If the food is cooked before it is frozen, it should immediately be put in a refrigerator or freezer. Allowing warm food to cool at room temperature permits the growth of food-poisoning microbes that may survive the freezing process.
Foods should be thawed in a microwave oven or, if such an oven is not available, in a refrigerator. Both methods prevent the growth of food-poisoning organisms. However, refrigerator thawing allows physical and chemical changes that reduce food quality.
To help prevent the loss of quality that occurs in vegetables as a result of slow thawing, processing firms package many kinds of vegetables in sealed plastic pouches. Consumers can thaw the vegetables rapidly—and cook them—by transferring the unopened pouch from a freezer directly into boiling water. The tightly packed pouches also prevent freezer burn and the formation of package ice. Many frozen foods can be cooked in a microwave oven as soon as they are removed from the freezer.
Most U.S. states require that frozen foods be stored at or below 0 °F (–18 °C). However, supermarket freezer shelves are often overstocked, and many supermarket and home freezers have automatic defrost cycles that raise and lower the food’s temperature repeatedly. The temperature of commercially frozen food also may repeatedly rise and fall as the food is moved from one freezer to another during distribution. As a result, some frozen foods often exceed 0 °F (–18 °C). The repeated temperature changes lead to loss of food quality. For example, they cause ice crystals to form in ice cream, giving it a grainy texture. They also promote freezer burn and the formation of ice.
See also Birdseye, Clarence; Food preservation; Refrigeration.
