Freeze-Dried Food

Dehydrated foods, never much admired, may be headed for kitchen fame. This week Dr. A. Copson of Raytheon Manufacturing Co. showed dried shrimp, lobster tails, strawberries, etc. that actually taste fresher than many fresh ones.

A Raytheon dried shrimp is no shriveled, leathery remnant. It is nearly as big as a fresh peeled shrimp and made of a strange, brittle material with the consistency of popcorn or puffed cereal. Taken out of an airtight plastic envelope, it smells like raw shrimp, and its color is about the same. When one of these brittle ghosts is dropped into tepid water, it softens quickly and swells a little. After half an hour of soaking and two minutes in boiling water, the shrimp is firm, sweet and tastes like a shrimp that has been carefully preserved by freezing.

Waves to the Center. Dr. Copson explained that it is all done by freeze-drying. When a material that contains water is frozen and placed in a vacuum chamber, the ice crystals in it sublime, i.e., turn directly into water vapor without melting to water. Pharmaceutical manufacturers use freeze-drying to preserve sensitive drugs, but the process is difficult, and it had never been successfully adapted to low-cost materials like foods. Another difficulty is that a considerable amount of heat (heat of sublimation) is required to evaporate the ice crystals. This heat must reach the center of the material, and in the case of most foods the evaporation of crystals near the surface forms a layer of corklike stuff that is an excellent insulator. It keeps heat of sublimation from reaching the interior unless the surface temperature is raised so high that the food spoils.

Raytheon gets around this problem by putting frozen foods in a vacuum chamber and shooting through them a powerful blast of ultrahigh-frequency radio energy. The waves agitate the molecules in the interior of the food and generate just enough heat to make the ice crystals turn directly into water vapor. If the job is handled properly, the food loses up to nine-tenths of its weight and turns into a brittle sort of substance while staying far below the freezing point. Chemical changes, which would damage flavor, cannot take place. Even unstable vitamins are preserved.

Dr. Copson has successfully freeze-dried mushrooms, carrots, beef rib and sirloin steak, veal cutlets, pork chops, lobster, shrimp, strawberries and several kinds of fish. Uncooked green peas keep their shape but become as light as miniature ping-pong balls. Freeze-dried chicken breasts look like balsa wood. For gourmets, freeze-dried foods offer some interesting possibilities. Chicken or fish could be made to soak up several times their weight of wine or other flavorsome liquid.

New Cookery. Raytheon regards its new process as experimental, and it does not know yet how long freeze-dried foods will keep at room temperature. They can be stored in plastic envelopes filled with nitrogen to prevent oxidation, but in the case of meat that contains fat there may be a tendency to deteriorate with time. Elaborate tests are now in progress to find the best ways to package and store them.

Cautious Raytheon men do not want to predict what effect freeze-drying will have commercially. But they point out that freeze-dried foods can be shipped without costly refrigeration and stored on grocers' or housewives' shelves. The armed services are interested because of the possibility of supplying troops in the field with food that tastes as good as if it were fresh but weighs about one-fifth as much.

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