Because individual classes of foods differ in their requirements for preservation, a number of methods have been developed over the years involving one or a combination of procedures such as dehydration, fermentation, salting, chemical treatment, canning, refrigeration, and freezing.
The basic objectives in each instance are to make available supplies of food during the intervals between harvesting or slaughter, to minimize losses resulting from the action of microorganisms and insects, and to make it possible to transport foods from the area of harvest or production to areas of consumption.

In earlier years, the preservation of food was essentially related to survival.
In the more sophisticated atmosphere of today's developed nations, food-preservation techniques have sought also to bring variety, peak freshness, and optimum taste and flavor in foods at reasonable cost to the consumer.

With the development of nuclear technology, isotopic materials, and machine radiation sources in recent years, the possibilities of applying ionizing radiation to the preservation of foods attracted the attention of investigators in the United States and throughout the world.
An early hope that irradiation might be the ultimate answer to practically all food preservation problems was soon dispelled.
Interest remained, however, in the possibility that it would serve as a useful supplementary method for counteracting spoilage losses and for preserving some foods at lower over-all costs than freezing, or without employing heat or chemicals with their attendant taste alterations.

Factors responsible for the spoilage of foods

The chief factors responsible for the spoilage of fresh foodstuffs are (1) microorganisms such as bacteria, molds, and yeasts, (2) enzymes, (3) insects, (4) sprouting, and (5) chemical reactions.
Microorganisms are often responsible for the rapid spoilage of foods.
Of special concern is the growth of bacteria such as Clostridium botulinum which generate poisonous products.
Enzymatic action in stored food produces changes which can adversely affect the appearance of food or its palatability.
Spoilage by chemical action results from the reaction of one group of components in the food with others or with its environment, as in corrosion of the walls of metal containers or the reaction of fats with oxygen in the air to produce rancidity.

Sprouting is a naturally occurring phenomenon in stored potatoes, onions, carrots, beets, and similar root vegetables.
Insect infestation is a problem of importance chiefly in stored grain.
The presence of parasitic organisms such as Trichinella spiralis in pork introduces another factor which must be dealt with in food processing.

To permit the storage of food for long periods of time, a method of preservation must accomplish the destruction of microorganisms and inhibition of enzymatic action.
The term "sterilization" applies to methods involving essentially complete destruction of all microorganisms.
Food treated in this manner and protected from recontamination by aseptic methods of packaging and containment presumably could be stored for long periods without refrigeration.
The process of "pasteurization" involves milder and less prolonged heat treatment which accomplishes the destruction of most, but not all, of the microorganisms.
Less severe thermal treatment as by blanching or scalding serves to inactivate enzymes.

General effects of ionizing radiation

Ionizing radiation can cause the destruction of microorganisms and insects involved in food spoilage or, at lower doses, can inhibit their action.
It furnishes a means of destroying insects in stored grain products as well as certain parasitic organisms present in meats.
Deactivation of enzymes is also possible, although some types require extremely heavy doses of 10 Mrad or more.
Because of undesirable flavors, odors, colors, and generally low palatability associated with radiation treatment of this magnitude, the inactivation of enzymes is best accomplished prior to irradiation by the conventional heat-processing methods of blanching.

Radiation does not retard the chemical spoilage of food.
It will, however, inhibit the sprouting of potatoes and other root vegetables.

The radiation doses required for the preservation of foods are in the following ranges: 1.

For radiosterilization, to destroy all organisms for long-term preservation -- about 4.5 Mrad for nonacid foods of low salt content.
2.

For radiopasteurization, to partially destroy microorganisms;
results vary with types of food, storage conditions, and objectives of treatment -- commonly of the order of 0.2 Mrads but up to about 0.8 Aj.
3.

For destruction of insects -- about 25,000 Aj.
4.

For inhibiting the sprouting of root vegetables -- 4,000 to 10,000 Aj.

Preserving foods with ionizing radiation leads to some undesirable side effects, particularly at the higher radiation dosages.
In this respect, the general palatability and individual acceptance of most radiosterilized foods has, to date, been found to be low in comparison with fresh and commercially processed foods.
A number of foods are quite acceptable as regards taste and palatability, however, at dosages substantially less than sterilization levels.
Moreover, the nutritive value of irradiated foods apparently undergoes little, if any, change, although some of the fat-soluble vitamins are affected by sterilization doses.

Radiation sources

For irradiation of food, the results obtained depend upon the dose rather than the specific type of radiation, and X-ray, gamma, and high-energy electron radiation are suitable.
Aside from availability and economic considerations, each has certain practical advantages;
for example, gamma rays give deeper penetration but cannot be focused or collimated, whereas unidirectional electron beams may be split and directed to both the top and bottom of the food package to be irradiated.
Selection of a source for commercial irradiation would involve consideration of numerous factors including required dose rate, load factor, throughput, convenience, safety, and most important, costs.

Of the potentially useful sources of ionizing radiations, gamma sources, cobalt-60, cesium-137, fission products, or a reactor irradiation loop system using a material such as an indium salt have received most attention for food-preservation systems.
Of the various particle accelerators, the Van De Graff machines, resonant transformers, and linear accelerators are the principal ones available for commercial use.

Costs of the effective energy produced by these sources is a major obstacle in the development of food-preservation processes.
Estimated production costs of radiation energy from machine and nuclide sources range from $1 to $10 per Aj.
Conventional energy for processing foods is available in the range of at most a few cents per kwhr for electric power and the equivalent of a few mills per kwhr for process steam.
Radiation, therefore, is at an initial cost disadvantage even though only 1 to 10 per cent as much radiation energy as heat energy is required for radiopasteurization or radiosterilization.
What are the possibilities of lowered radiation production costs for the future?
It has been estimated that for applications on a megawatt scale costs might reach values in the neighborhood of 10 cents per kwhr for large-scale accelerators or for gamma radiation generated in a reactor core.
No comparable reductions in the cost of nuclide radiation are foreseen.
Such projections, however, appear highly speculative and the capacities involved are far beyond those foreseen for food-preservation facilities.

Because agricultural activities are seasonal and the areas of production and harvest of many foods are widely scattered geographically, and because of the high cost of transporting bulk food items any substantial distance to a central processing location, the use of large central processing stations, where low-cost radiation facilities approaching the megawatt range might be utilized, is inherently impracticable.

Present status of irradiation preservation of foods

The objective of complete sterilization of foods is to produce a wholesome and palatable product capable of being stored without refrigeration for extended periods of time.
Chief interest in radiosterilization resides in the military services.
For them, providing appetizing food under battle or emergency conditions is a paramount consideration.
They require completely sterile foods capable of being stored without refrigeration, preferably items already cooked and ready to eat.
High nutritional value, variety, palatability, and appetizing appearance are important for reasons of morale.
Foods for rear stations, which require cooking, but no refrigeration, are also of interest.
Of primary interest are meats.

Radiopasteurization, which produces fewer adverse sensory changes in food products, has potential usefulness in prolonging the keeping qualities of fresh and refrigerated food items.
Thus, food so processed might reach more remote markets and permit the consumer to enjoy more produce at peak freshness and palatability.
Commercial interest is chiefly in this type of treatment, as is military interest under peacetime conditions.

The present status of food preservation by ionizing radiation is discussed by food classes in the following paragraphs.
Meats

The radiation processing of meat has received extensive investigation.
To date, the one meat showing favorable results at sterilization doses is pork.
Of particular interest to the military services is the demonstration that roast pork, after radiosterilization, is superior in palatability to available canned pork products.
Tests with beef have been largely unsuccessful because of the development of off-flavors.
A prime objective of the Army Quartermaster Corps program is to find the reasons for beef's low palatability and means of overcoming it, since it is a major and desirable dietary item.
Partly because low-level heat treatment is needed to inactivate enzymes before radiosterilization, treated fresh meats have the appearance of boiled or canned meat.

Off-flavor is a less severe problem with the radiopasteurization of meats, but problems of commercial acceptability remain.
Moderate radiation doses of from 100,000 to 200,000 rads can extend the shelf life (at 35 F) of fresh beef from 5 days to 5 or 6 weeks.
However, the problem of consumer acceptability remains.
The preradiation blanching process discolors the treated beef and liquid accumulates in prepackaged cuts.
Cooked beef irradiated in the absence of oxygen assumes an unnatural pink color.

When lamb and mutton are irradiated at substerilization doses, the meat becomes dehydrated, the fat becomes chalky, and, again, unnatural changes in color occur.

Ground meats such as fresh pork sausage and hamburger have a relatively short shelf life under refrigeration, and radiopasteurization might be thought to offer distinctly improved keeping qualities.
However, a major problem here is one of scale of processing;
ground meats are usually prepared from scrap meats at the local level, whereas irradiation at economic volumes of production would require central processing and distribution facilities.
The problems of color change by blanching and liquid accumulation within the package are the same as for solid cuts.

Specialty cooked items containing meat portions, as in "frozen dinners" might offer a potential use for radiopasteurization.
The principal potential advantage would be that the finished product could be transported and stored at lower cost under refrigeration instead of being frozen.
A refrigerated item could also be heated and served in less time than is required for frozen foods of the same type.

Competitive processes for preserving meats are by canning and freezing.
Costs of canning meat are in the range of 0.8 to 5 cents per pound;
costs of freezing are in the area of 2 to 3.5 cents per pound.
The table on page 10 shows costs of canning and freezing meat, and estimated costs for irradiation under certain assumed conditions.
Under the conditions of comparison, it will be noted that: (1)

Radiosterilization (at 3 Mrad) is more expensive than canning, particularly for the cesium-137 source.
(2)

Radiopasteurization by either the electron accelerator or cesium-137 source is in the range of freezing costs.
(3)

Irradiation using the nuclide source is more expensive than use of an electron accelerator.
Poultry

Results of irradiation tests with poultry have been quite successful.
At sterilizing doses, good palatability results, with a minimum of changes in appearance, taste, and odor.
Radiopasteurization has also been successful, and the shelf life of chicken can be extended to a month or more under refrigerated storage as compared with about 10 days for the untreated product.
Acceptable taste and odor are retained by the irradiated and refrigerated chicken.
Acceptance of radiopasteurization is likely to be delayed, however, for two reasons: (1) the storage life of fresh chicken under refrigeration is becoming a minimal problem because of constantly improved sanitation and distributing practices, and (2) treatment by antibiotics, a measure already approved by the Federal Food and Drug Administration, serves to extend the storage life of chicken at a low cost of about 0.5 cents per pound.
Seafood

Fresh seafood products are extremely perishable.
Although refrigeration has served to extend the storage life of these products, substantially increased consumption might be possible if areas remote from the seacoast could be served adequately.