Janet Schweitzer, age 12, of Keen, N.H., for her question:

HOW IS FOOD FROZEN?

Freezing is one of the most important methods used to preserve foods. In the United States, food companies freeze over 10 billion pounds of food each year.

Almost all frozen foods are quick frozen and stored at temperatures of zero degrees Fahrenheit. Scientists have no exact definition of quick freezing as compared to slow freezing. Freezing food quickly, however, seems to prevent foods from leaking fluids during later defrosting.

There are several methods of quick freezing foods commercially. The chief methods include air blast freezing, indirect contact freezing, nitrogen freezing and dry ice freezing.

Air blast freezing uses a steady flow of cold air to freeze foods moving on a conveyor belt or to freeze packaged foods on trays and racks. The conveyor belt and trays and racks move through an insulated tunnel. Usually the foods are packaged before they are frozen. But some processors expose the food to cold air in the tunnel before placing it in packages.

Indirect contact freezing is done in several ways. One system uses a series of adjustable hollow walled metal plates. A refrigerant or cooling substance inside the walls cools the plate surface to about minus 28 degrees.

Workers next place packaged foods between the plates, which are then adjusted to make contact with the upper and lower surfaces of the packages. The cold plates absorb heat and the food freezes solid.
Another system uses refrigerated alcohol solutions to freeze foods, chiefly fruit juices, in cans. A screw type conveyor automatically moves the cans through the freezing solution. This movement shakes the food and the shaking speeds freezing.

In the nitrogen freezing process, nitrogen under pressure vaporizes and flows through nozzles into the food freezing chamber. With this method, food freezes very rapidly.

Dry ice freezing resembles nitrogen freezing, except that it uses dry ice as the refrigerant.

The dry ice vaporizes, or turns into a misty gas, and the resulting cold vapors freeze the food quickly. This relatively simple process is used by some commercial foods firms that do not have the complex equipment needed for other freezing methods.

The dry ice method is cheap and it freezes food as fast as the nitrogen method. However, most companies prefer to use nitrogen for extremely fast freezing because it is easier to handle than the solid dry ice.

More on the nitrogen freezing method: the nitrogen vapors, which are extremely cold, take heat from the food and the food freezes quickly. The vapors are then collected, recompressed, cooled and returned to the chamber to be used again.

This method costs less and freezes food faster than the air blast and indirect contact methods. Its high speed greatly reduces the chance of bacterial growth, cell destruction and loss of nutrients during freezing.