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Preserved Foods - Part 2

( Originally Published 1939 )



Refrigeration. As a result of the general observation that perish-able foodstuffs can be maintained in edible condition for a longer time if they are kept cold, much effort has been devoted to developing refrigeration methods which can be applied economically and effectively. In the earlier days, ice was the refrigerant. Since the invention of the compression refrigerating machine in 1834 and the beginning of the cold-storage industry in about 1865, the commerce in refrigerated food has so increased that there are now some 1400 cold-storage ware-houses. A new phase of this industry has more recently been created by the quick-freezing of foodstuffs, with all its many new problems of production, manufacture, and distribution.

There are three fairly distinct degrees of refrigeration applied to food preservation. Some plants are maintained at temperatures of about freezing up to 40-45 F. (4.5-7 C.). These handle products which would be spoiled and denatured if frozen (such as eggs and milk). In another class are the hundreds of regular cold-storage warehouses over the country operated at temperatures between 0 F. (18 C.) and 45 F. (7 C.). These handle a great variety of food-stuffs, such as butter, meats, poultry, and also various frozen foods. The more recent quick-freezing methods use temperatures of 0 F. (18 C.) or below. The first group may be called chilled storage, the second group cold storage, and the third group sharp or quick-freezing storage. In the second group, many of the products are, in fact, frozen, but the designation of this range of temperature as de-fining cold storage has the sanction of time, and particularly is defined by the legal statutes which control cold-storage warehouse operations.

Cold storage. The effect of refrigeration is to retard deterioration to the extent of prolonging the commercial life of the product from a few months up to a year at most, depending on the nature of the product. Cold-storage warehouses do not carry their products from year to year or hold foodstuffs indefinitely. They could not maintain the quality of the product or pay the carrying charges. Their great service is to carry over seasonal food from the period of peak production to months when it would otherwise be unobtainable, without excessive cost, and to extend its availability over the ensuing period of consumer demand. Many states have enacted legislation which places time limits on their warehouse holdings; for example, Maryland specifies that any food held at a temperature below 45 F. (7 C.) for 30 days must be reported monthly to the Food and Drug Commissioner, and may not be held longer than 12 months.' Proctor brings out the idea that the quality of refrigerated food is dependent rather on its biochemical age than on its chronological age.' Improvements in cold-storage operations make it possible to deliver a cold stored product of several months' holding in better condition and quality than often obtains with the same kind of commodity produced fresh within a few days of delivery.

Conditions of temperature and humidity vary with the foods that are stored. Dried products like prunes, figs, and apricots are kept just above freezing at a relative humidity of about 60 percent, eggs at 85 percent humidity, and the general run of fruits, vegetables, and meats at 85-95. Low temperatures, near freezing, afford better protection from the standpoint of keeping down bacterial spoilage, but they also remove some moisture from the air with consequent drying out and loss of weight of the stored product. High humidity reduces this moisture loss but favors mold growth. Some warehousemen introduce ozone into the air (a few parts per million) to help reduce mold infections, but this cannot be used with products that are easily oxidized or which take up flavor. Carbon dioxide has been used to some extent, particularly in the storage of eggs, meats, and fruits.

When refrigerated products are brought out of cold storage, they may condense moisture from the atmosphere. This sweating induces mold growth with consequent decay. Fluctuations in temperature or humidity of the warehouse may have a similar effect.

Cold storage is largely a wholesale business. By the time the food is sold to the consumer, it has reached atmospheric temperature (except special products like butter), and few persons ever know that it was refrigerated. Early prejudice against cold-storage foods has largely disappeared, owing to the supervision of food-control officers, competition among distributors to produce quality products, and a greatly improved refrigeration technology which is the result of years of extensive research by governmental and commercial agencies.

Freezing. Certain foods, like fish, which possess particularly powerful enzymic systems, cannot be successfully preserved by the relatively high temperatures of chilling refrigeration. Their autolytic processes must be retarded by subjecting them to temperatures of 0 F. or below. Other products such as poultry, meat cuts, sea food, fruits, and vegetables are also successfully preserved by this sharp-freezing method.

When food is slowly frozen, there is a tendency for ice crystals to form throughout the tissue. The range of maximum crystal formation has been shown to lie between 31 and 25 F. Large crystals are formed with more or less rupturing of the cellular structure, greater in plants than in animals because of the relative inelasticity of their cell walls. Furthermore, the colloidal stability of the cell contents is reduced so that a more or less irreversible flocculation or coagulation of the protoplasm occurs. One fraction is a more concentrated gel, and the other is a thinner, free-flowing liquid phase. When the product is defrosted, these fractions are not completely restored to their original condition, with the result that the liquid phase "drips" away. This loss can be greatly reduced in flesh foods by proper salt adjustment, but it is more persistent in vegetable products. This denaturation is minimized in quick-freezing.

Slow-and quick-freezing. In the older process of freezing, food was stored in a "sharp" freezer or cold room where the air was maintained at a temperature of about 15 F. down to -20 F. Heat was abstracted so slowly by convection currents that sometimes a period of 3 to 72 hours was required for a product to freeze. A sharp freezer is merely a cold-storage room kept at a particularly low temperature. When the air of such a room is rapidly circulated, the rate of freezing is hastened so that the product is more quickly frozen. The actual time is largely dependent on the size of the package. Several types of quick-freezing operations have been developed which in general may be classified as follows:

1. Cold air, such as forced circulation of air in sharp freezers, or freezing in the cold air out-of-doors. This process is used mostly for fish.

2. Direct contact of the food in cold brine. One process immerses the fish in a cold brine solution; the Taylor process sprays the brine over the fish; and the Zarotschenzeff or "Z" process conveys the food through a fog of refrigerated brine spray.

3. Indirect contact with the refrigerant. The "Z" process is in this class when the food is packaged. Other processes are the ordinary manufacture of ice whereby water in cans is lowered into a refrigerated brine bath, the freezing of food on metal plates which are chilled by brine, or the freezing of food in shallow pans which are immersed or float in cold brine. One of the best known of these processes is the method of Birdseye.

The Birdseye process. This process was originally devoted to the freezing of fish but has been developed so that it now freezes poultry, meat, oysters, crabmeat, sausage and a wide variety of fruits and vegetables. The earlier type of machine consisted of two endless belts, one placed close above the other to travel in the same direction. Brine sprays chilled the upper side of the upper belt and the lower side of the lower belt. The food placed between the two belts was protected from the brine splash by the greater overhanging width of the upper belt, which allowed the brine to drop into the underlying trough, clear of the lower belt.

A newer development uses a stacked series of plates, through each of which anhydrous ammonia is circulated. These plates are mounted in a vertical frame so that packages of food can be placed between two successive plates, and all can be brought into close contact by means of an hydraulic piston which rises from the bottom of the frame and raises the plates against the intervening packages of food. These machines are self-contained units, each with its own refrigerating system and mounted individually for operation from any source of adequate electric power. This arrangement gives mobility to the business of following the available supply of fruits and vegetables as the season advances at different places over the country.

Fresh foods are packed in their final containers and frozen in them. This facilitates production of a uniform, attractive product, compactly packaged, and sealed against contact with air, thereby minimizing loss of moisture and oxidative change. The best conditions for handling different products are specific for each one. Careful selection of varieties is necessary since some food cannot be frozen as satisfactorily as others. Many animal and plant products are now packed by this process, and are to be found in wide commercial distribution. Its uniqueness lies in its adaptability in packing the sea-, vine-, or tree-fresh product in the final moisture-proof container, then freezing it quickly, and retailing the original packages directly to the consumer at a price within reach of the more limited purse.

Some of the enzymes which are present in vegetables are active at 0 F. In order to prepare vegetable foods satisfactorily for freezing, it has been found necessary to give them a preliminary cook or blanch. This involves heating the food at approximately the boiling point of water until the product reaches a temperature throughout of at least 180 F. (82 C.). Some workers think that treatment at a lower temperature for a longer period or the use of only dry steam entails less loss of nutrients and gives a better product. It removes gases, saturates tissues with water, inactivates enzymes, minimizes discoloration, and reduces deterioration in storage.

Fish fillets, shellfish, crustacea, poultry, cuts of muscle meat, vegetables, and fruits are frozen in a variety of containers, and in many different sizes. Some are wrapped in vegetable parchment, paper or glassine, and moisture-proof Cellophane, and packaged in tin, wooden boxes, paper cups, or cardboard cartons, sometimes under a vacuum. The package products are held at temperatures below 0 F. Unless specially protected, they dehydrate, like products in ordinary refrigeration, by the moisture evaporating into the drier atmosphere of the storage room, with consequent loss of weight and flavor, and inability to return to their original natural condition when defrosted. This is minimized by coating the food with a thin film or glaze of ice or oil, or by packaging it in tins or moisture-proof wrappings. Use of the can should be discouraged because the consumer is inclined to treat it like any other canned product which does not need refrigeration to preserve it, and to find that it spoils easily when left on the shelf. Vacuum containers, sealed or air-tight containers, sugar, and salt have all been used more or less successfully to inhibit discoloring and other oxidative changes, but the food often darkens on defrosting. Shipment is made in special boxes of corrugated strawboard, with several layers of additional insulating board or Kraft paper, cemented by hot asphalt and sealed as air-tight as practicable. They are shipped in refrigerated cars, although protected packages can be shipped long distances, when solidly packed, without special refrigeration.

Other methods. There are several other methods of quick-freezing which are in extensive commercial use in the production of quick-frozen foods of high quality. The Taylor process sprays brine directly onto fish suspended from an overhead conveyor in an insulated chamber. The fish may be enclosed in aluminum molds or packed in cans, and the brine sprayed over the outside of the containers.

The Zarotschenzeff process conveys meat, fish, and poultry through an insulated tunnel where brine in the form of a finely divided mist impinges directly on the outside of the packages. The food passes out of this freezing chamber into another where the brine is washed off. It has been suggested that this process may be used also for the quick-freezing of fruits and vegetables.

The Murphy process utilizes both direct cold air and indirect contact with the brine refrigerant to produce quick-frozen foods. An insulated chamber is equipped with coils of rectangular-shaped pipe with flat surfaces, installed in the form of shelves one over another. Cold air is introduced at the top and bottom of the chamber and distributed by blower fans. The food, packaged or loose, is placed in pans or trays, and set directly on the shelves. Intimate contact of the trays with the flat surfaces conducts heat from the product, and the cold air removes the heated air.

Defrosting. Although some frozen products, especially poultry, are defrosted by the dealers before retail sale, their wholesomeness is improved if they are delivered to the consumer while frozen, and then thawed out in the home refrigerator just before being used. Frozen products may often be cooked directly without giving them a preliminary defrosting. This method, when possible, is the more desirable inasmuch as products that are defrosted and held deteriorate more quickly than the untreated fresh food.


Epidemiology. Although the amount of food that is preserved by refrigeration is so enormous,''' these foods have been free from incrimination in food-poisoning outbreaks. The very nature of commercial refrigeration precludes the development of any pathogenic organisms, even if some were present on the food. The perishable nature of the food itself, when withdrawn from refrigeration, warns the consumer to use it at once or suffer loss from its spoilage. These factors cumulatively operate to preclude the holding of foodstuffs under conditions which might allow a slight initial infection to grow to the proportions that would cause an outbreak. Absence of any epidemiology of refrigerated foods attests their safety.

Microbiology. Fresh food carries a miscellaneous flora of many types of microorganisms. These are the usual ones encountered in the soil and water where the product was grown. On fruits and vegetables, the organisms are spread over the surface and do not penetrate the tissue, unless it is broken down by decay, insect infestation, or rough treatment. Microorganisms usually penetrate tissue of flesh food as soon as the animal dies. All these organisms secure their nutriment from the juices of the food which they infect. When conditions of production, transportation, and handling in the plant are not sanitary, food may accumulate an additional load of microorganisms, some of which may be contributed by employees.

All vegetables and animals contain the biological agencies, enzymes, which bring about the life processes. They exist within the ' cellular structure, and carry on their biological function of control of growth, digestion, and ripening, even after the plant is harvested or the animal has died. These processes are called autolytic.

Microorganisms attack the food from without. Enzymes decompose it from within. Action of both is retarded at low temperatures, facilitated at moderate temperatures, and destroyed at high temperatures. When a foodstuff is chilled by refrigeration, the biochemical processes of both these agencies are so greatly reduced that the food-stuff may remain edible for a much longer time than if unprotected. Generally speaking, the colder the treatment, the greater is the retardation of these microbiological processes. Bacterial decomposition is stopped and enzymic changes are markedly reduced at temperatures below 0 F. Most of the microorganisms die off under the severe conditions of refrigeration, although many hardy types do survive, and some may even multiply at 20 F. (6.5 C.) , if they can secure the necessary food.' When the foodstuff is withdrawn from refrigeration, it warms up to atmospheric temperature. Microorganisms which had lain dormant find soluble nutriments in juices of the food, and begin to multiply. Enzymic reactions respond to the higher temperature so that autolytic changes begin to manifest themselves. If conditions of refrigeration have been so severe on texture of tissue that the normal structure of cells and the colloidal stability of cellular contents are damaged, then the defrosting operation will release some of the nutrient juices from the ruptured cells, and not only provide abundant nutriment for surface microbic development but also open channels for their invasion of the tissue. (Under some conditions, this exudation of juice, called "drip," may take place from undamaged cellular structure.) For this reason, the rate of spoilage of food withdrawn from frozen storage may be much greater than that from merely chilled storage.

In order to determine the effect of freezing refrigeration and the subsequent handling of vegetables which are infected with the spores of Cl. botulinum, Straka and James 6 inoculated 1200 containers of peas in tin, cardboard, and glass, with and without vacuum, with the washed spores of this organism. Some of the peas were washed in cold water; others were blanched. They were frozen within a few hours in a storage room at 10 F. (12 C.). The authors state that neither the period of storage nor the rates of freezing ("quick-freezing" or "slow-freezing") affected the spore population. The samples were later defrosted by four common methods, and examined for the presence of toxins and cultures of the organism. Their results are summarized in Table XLVI. No toxin was found in tin, glass, or cardboard when the samples were examined immediately after they had been defrosted and not held, or when defrosted and held in an icebox at about 50 to 60 F. for 3 days. On the other hand, toxin was found in samples which had been defrosted at room temperature and also by heating, and then held at room temperature for 3 days. All the last were also badly spoiled organoleptically.

A similar test on the effect of freezing on the Cl. botulinum, con-ducted by Wallace and Park, showed that the toxin is not destroyed when food is held for 1 year at 3.2 F. (16 C.). Some toxin was formed from massive infections. They state that no danger from botulism is to be feared from properly frozen foods, provided that they are not defrosted and allowed to stand for several days at room temperature.

Tanner and Wallace 8 have shown that the spores of Cl. botulinum survived freezing at -16 C. (3 F.) for 14 months, and that there was no decrease in toxin at -79 C. (110 F.) for 2 months, or at -16 C. (3 F.) for 14 months. Vegetables and detoxified spores held at -14 C. (7 F.) for 14 months became toxic in 3 to 6 days when thawed and stored at room temperatures. Although acid fruits are unfavorable media for the germination of botulinum spores, a few in-stances are known where some toxin was formed in some acid fruits, believed to have been made possible by local bacterial or mold action. Presence of other organisms in frozen foods might increase such a possibility.

Although numerous investigators have shown that about 99 per-cent of typhoid organisms, suspended in water, are killed on freezing, freshly isolated strains have much greater resistance to freezing than laboratory strains that have been grown on artificial media.' Inoculation and storage experiments have shown that a few survivors may remain alive at 0 C. (32 F.) for 3 to 5 weeks, although their virulence would be unknown. In the presence of proper food, this period may be increased. The absence of any reported outbreaks of typhoid fever traced to frozen foods (except ice cream) indicates that there is little danger from this source unless the infection should be massive.

Prescott and Tanner reports that studies with several strains of Salmonella indicated that refrigeration at 5 C. (41 F.) or less must be employed to insure that these organisms will not grow on foods. Below this temperature, the numbers of organisms decrease sharply.

Great care must be exercised in observing sanitation in plant operations, including the use of potable water, or handling by a carrier, to avoid infecting the food with dangerous organisms. Frozen foods are not given any subsequent treatment that kills any disease germs that they may have picked up.

Nutritive value. The effect of refrigeration of food at temperatures just above freezing is merely to retard the biological processes without bringing about any appreciable change in the structure or composition of the product. There is no indication that the nutritive values of properly frozen products are substantially impaired, except possibly some of the vitamins,' but even these are probably retained better than when the food is not refrigerated. There may be some slight loss of moisture, and the flavor may have acquired some of that of the foods stored nearby. The "drip" from the improperly frozen product entails some loss of its aliquot content of nutrient. The retardation, not cessation, of enzymic action possibly affects the vita-min C content to some extent. It is known that fats may oxidize and become rancid, that the skin may darken from oxidative effects, and that the phenomenon of "rusting" occurs in fatty fish, the hemoglobin changing to methemoglobin with consequent darkening of the tissue. These effects are being minimized by improvements in freezing technology. The frozen meats, fish, eggs, poultry, fruits, and vegetables can be nearly as attractive and substantially as sound, palatable, and nutritious as when fresh.'

The blanching that is necessary for treatment of vegetables before freezing extracts some of the soluble constituents, particularly if this is done for several minutes in boiling water. This loss varies according to intensity of treatment. Scalding with steam entails less loss than with water.

The freezing of fruits and vegetables is not injurious to their con-tent of vitamin A. The amount of vitamin C present may be fully up to the amount present in the fresh product, or it may be materially lower, depending upon varietal, agronomic, and harvesting effects, but particularly in the processing methods used in the factory. For ex-ample, Jenkins, Tressler, and Fitzgerald show that, when peas are properly blanched and frozen, they retain their original vitamin C content. Vitamin C is also unimpaired in frozen grapefruit, in orange juice, in cranberries, and in certain apples. There is no evidence that vitamins D and G are adversely affected by freezing.


Where state laws obtain, the records of the warehouses must be examined for compliance with the periods that storage is lawful. Sanitation and cleanliness are always necessary. Accumulation of rubbish is intolerable. Storage plants should be given a thorough cleaning every year, and the walls and ceilings painted or sprayed with a light-colored, fungicidal dressing. No foul odors should be allowed to exist, especially where they might be detrimental to the food.

The control officer should carefully inspect the methods of handling frozen foods, whether they are being manufactured, stored, or retailed in his territory. Cleanliness and sanitation are necessary in the plant, and the stock must be free from dirt and other materials foreign to a wholesome food. Delay in refrigeration entails the beginning of de-composition. Fruits and vegetables must be free from mold, fermentation, decayed parts or insect infestation. Fish and marine products must be thoroughly cleaned, uncontaminated with fecal bacteria, and free from odor and incipient spoilage. Meats and poultry should be under adequate ante- and post-mortem inspection. The storage boxes and showcases of retailers should be kept clean, and provided with adequate refrigeration to protect the food from defrosting. No product that has once thawed should be refrozen.


1. Food and Drug Laws of Maryland, State Department of Health, Baltimore, Md.

2. "Committee Report," American Public Health Association Year Book 1935-36, p. 47.

3. D. K. TEESSLER and C. F. EvERS, The Freezing Preservation of Fruits, Fruit Juices, and Vegetables, The Avi Publishing Co., New York, 1936.

4. C. R. FELLERS; Am. J. Pub. Health, 22, 601 (1932).

5. E. HEss, Contr. Canadian Biol. Fish., 8 (32) 461 (1934), quoted from Biol. Abs., 9, 7534 (1935).

6. R. P. STRAKA and L. H. JAMES, Am. J. Pub. Health, 22, 473 (1932) ; 23, 700 (1933).

7. G. I. WALLACE and S. E. PARK, J. Infectious Diseases, 52, 150 (1933).

8. F. W. TANNER and G. I. WALLACE, Proc. Soc. Exptl. Biol. Med., 29, 32 (1931).

9. S. C. PRESCOTT and F. W. TANNER, Food Research, 3, 189 (1938).

10. G. A. FITZGERALD and C. R. FELLERS, ibid., 3, 109 (1938).

11. R. R. JENKINS, D. K. TRESSLER, and G. A. FITZGERALD, ibid., 3, 133 (1938).

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