ADVANTAGES, PROBLEMS, AND EXPERIENCES OF IRRADIATED FOODS

 

EDWARD S. JOSEPHSON

 

U.S. Army Natick laboratories

[87] I appreciate having the privilege of attending the Aerospace Food Technology Conference and the opportunity to talk to you about irradiation-preserved foods. I shall cover a program supported by between $40 and $50 million of expenditures of the U. S. Government during the last 16 years and programs which, although not of this magnitude, are planned or are underway in 74 other countries.

The following quotation is from reference 1: "Soviet cosmonauts aboard the recent Soyuz 4 a 5 flights became the first men in space to eat irradiated foods. The four cosmonauts had with them radiation-preserved meats wrapped in polyethylene film, as well as dried meats in cans. On future flights, Soviet scientists expect to substitute irradiated vegetables, fruit salads, and dry soup mixes, as well as meats, for the vacuum freeze-dried foods which now constitute the basic diet of cosmonauts. "

The Soviets see the advantages of irradiation-preserved foods in support of manned flights in space. I propose to present the current status of this process for food preservation so that you can decide whether, and to what extent, irradiated foods will fit into feeding systems for individuals and small groups in isolation and where resupply is not possible. (More information on the subject is included in refs. 2 to 10. )

IONIZING RADIATION

 

Definition

Ionizing radiation for food preservation is the employment of fast-moving subatomic particles or electromagnetic waves which are energetic enough to strip electrons from atoms or molecules of matter. Although there are a number of different classes of such radiation, only beta (or electron) and gamma radiations are of interest in food processing.

The way in which ionizing radiations act is not clearly defined. There are theories calling for direct hits, and those calling for indirect hits. Both of these types probably contribute to achieving the desired effect, which may be to inhibit sprouting of tubers during storage, to slow down the ripening of fruits, or to destroy microorganisms causing food spoilage. The direct-hit theory suggests that the nuclear rays (or high-speed electrons) strike the vital spot much in the same manner as a fast-moving projectile strikes its target. The indirect-hit theory suggests that the highly energetic particle subjects the molecule(s) near which it passes to an intense, transient electrical force. The organization of electrons within each molecule is disturbed and many molecules along [88] the path of the particle become "excited" or ionized. In their highly reactive state, free ionized molecules enter almost instantly into reactions with one another and with neighboring molecules producing as their end products new substances strange to the chemistry of the cell. The unstable secondary products, notably free radicals and peroxides, relay the disturbance in turn to other molecules in the cell, thus enlarging the area and scope of injury.

Some of the more promising applications of ionizing radiation to the treatment of food are shown in table 1. At the highest irradiation doses, all food spoilage organisms and pathogens transmitted by food are killed; prepackaged meats, poultry, and seafood can keep for years without refrigeration and on the plate of the consumer will still have a degree of acceptance approximating that of fresh food freshly cooked. At the lowest irradiation doses, certain physiological functions associated with sprouting in tubers such as white potatoes and in bulbs such as onions will be disrupted; these foods will not spoil during storage for as long as 1 year because of sprouting. Exposure of fruits such as tomatoes, bananas, mangoes, and papayas to intermediate doses of ionizing radiation will slow down ripening, and give these foods an extended shelf life ranging from a few days to several weeks. One application not included in table I is the use of irradiation to shorten rehydration and cooking time of dehydrated vegetables. For example, with diced potatoes an irradiation dose of 8 megarads can shorten cooking time from approximately 20 minutes to less than 4 minutes.

The irradiation process is attractive because there is only a slight temperature rise in the foods during the course of the treatment. It is considered a "cold process. " The Irradiated foods undergo minimal changes in texture, flavor, odor, and color so that on the plate of the consumer the irradiation-preserved food is almost indistinguishable from fresh food freshly prepared. The advantage of this process is that we can put freshlike food on the plate of the consumer on land, under the waters, in the air, and in outer space.

Another advantage of the process is its flexibility; that is, the process can be used to preserve a wide variety of foods in a range of sizes and shapes ranging from crates of potatoes to prepackaged flour in 50- or 100-pound sacks, to large roasts (beef, lamb, pork), turkeys, and hams, to sandwiches of sliced meat, fish, and chicken. The variety and dimensions of products that can be preserved by ionizing radiation fit in very well with present and anticipated future processing methods of the food industry. Astronauts and personnel at the bottom of the sea can have their meals and snacks in ready-to-eat form, in the form of slices or sandwiches, or as warm-and-serve or cook-and-serve items. Foods processed by ionizing radiation are compatible with the trend for greater convenience, simplicity in preparation, and reduction of labor in the kitchen. The shelf-life extensions without refrigeration are measured in days or weeks for certain fruits and vegetables and are from 3 to 5 years and possibly even longer in the case of meat, poultry, finfish, and shellfish.

[90] With ionizing radiation we can provide foods high in nutritive value and foods high in morale value. We can provide better quality food than hitherto possible. The food can be disease free, that is, free of all pathogens associated with food-borne diseases. We can provide a larger variety of foods such as fresh fruits and shelf-stable meats and poultry which have the character of fresh food. Because the food can be prepackaged and precooked at one place prior to irradiation, the cost in money, time, and labor for food handling all the way to the ultimate consumer can be reduced. Further reductions in cost result from reducing requirements for refrigeration and refrigeration maintenance. Spoilage losses from insect infestation, sprouting, or refrigeration breakdown will be minimized. By providing a broader spectrum of foods through introduction of irradiated items, discord from food monotony, particularly during long voyages, will be reduced.

Ionizing radiation is the first entirely new method used to preserve food since Nicholas Appert discovered thermal canning in 1809. The irradiation process is the first major food-preservation method to appear since food regulatory agencies were established at the national level in many countries.

In the United States the food regulatory agency most directly involved is the Food and Drug Administration (FDA). In the case of meats and poultry, the Department of Agriculture (USDA) also has legal responsibility.

There are several statutes which control the use of ionizing radiation for food processing. Among the laws are the Food, Drug, and Cosmetic Act as amended in 1958. Under this law ionizing radiation is legally defined as a food additive. The Federal Meat Inspection Act and the Poultry Products Inspection Act have been on the books for a long time. In recent years, with the great interest in consumer affairs, we have seen passage in 1966 of the Fair Packaging and Labeling Act; in 1967, of the Wholesome Meat Act; and, in 1968, of the Wholesome Poultry Act.

The impact of the Food, Drug, and Cosmetic Act of 1958 is to outlaw all new food additives, including ionizing radiation, from commercial application. The law provides for exemption from this universal ban by petitioning the FDA for approval of new food additives. For food preservation by ionizing radiation, FDA's approval is required for each food processed in this fashion. The law also requires approval by FDA of packaging materials in contact with food during radiation processing.

Because of the high cost of developing the process for preserving foods by ionizing radiation and the uncertainty that petitions will be approved by FDA, most of the effort in the United States is sponsored by the U. S. Army and the Atomic Energy Commission (AEC). The Army's effort is primarily in the use of radiation sterilizing doses, i.e., doses above 1 Mrad. The AEC, on the other hand, is concerned primarily with applications of radiation doses below 1 Mrad.

The overall program in the United States is reviewed periodically by the Joint Committee on Atomic Energy, Congress of the United States. The Interdepartmental Committee on Radiation [91] Preservation of Foods, consisting of ten departments and independent agencies of the government (NASA is a member) assists in promoting early commercialization of radiation-preserved foods.

Ionizing radiation for food preservation is considered to be an important peaceful use of atomic energy. It is, therefore, part of the President's Atoms for Peace Program.

At the international level the following three agencies of the United Nations are concerned with preserving foods by ionizing radiation- the International Atomic Energy Agency, the Food and Agriculture Organization, and the World Health Organization.

Except for proof of wholesomeness convincing to FDA, technology is sufficiently developed to support petitions for the irradiation-sterilized products listed in table II. These foods can vary in degree of doneness from partially cooked to ready to eat. Other irradiation-sterilized foods in various stages of development are ground beef (hamburger), pork sausage, corned beef, frankfurters, turkey, lamb, fish fillets, and prefried bacon.

The AEC, which concentrates its food-preservation program on low-dose applications of radiation geared primarily to the civilian market, has successfully processed cod, haddock, shrimp, clams, chicken, strawberries, tomatoes, citrus fruits, papayas, mangoes, peaches, bananas, and mushrooms .

Packaging is another important aspect of radiation sterilization. Most of the earlier work was done with the rigid metal can with an oleoresinous or epoxy-phenolic enamel because of its reliability as an impermeable and rugged container. Now the emphasis is on lighter weight and less expensive flexible packaging materials which would not require critically short metals during a national emergency. U. S. Army and AEC researches have been successful to the extent that the following flexible packaging materials have been approved by FDA as food contactants for the irradiation process:

 

[92] Up to 1 Mrad:

Nitrocellulose-coated cellophane

Saran coated cellophane

Glassine paper

Wax-coated paperboard

Nylon 11 film

Polystyrene film

Rubber hydrochloride film

 

Up to 6 Mrads:

Vegetable parchment

Polyethylene film

Polyethylene terephthalate film

Nylon 6 film

Vinyl chloride and vinyl acetate copolymer film

 

Up to 50 000 reds:

Kraft paper for wheat flour only

In radiation sterilization the need is for flexible materials which can withstand the stress of high radiation doses and low temperatures down to -40° C without loss of flexibility or impairment in functioning as an impermeable barrier to moisture, gases, and microorganisms. These materials must be sufficiently stable during irradiation processing that they do not impart off-odors, of-flavors, or toxic products to the food. Their all-around reliability must approach that of the rigid metal can. In order to reinforce strength of the material and keep out light that can accelerate adverse color changes, the food contactant materials are laminated to aluminum foil and other barrier materials. Two of the more promising laminates are shown in table III.

The proof of success in radiation processing of foods is in the eating. We use expert and consumer taste panelists who rate the foods on the 9-point hedonic scale developed by Peryam and Pilgrim (ref. 11):

[93] 9 Like extremely

8 Like very much

7 Like moderately

6 Like slightly

5 Neither like nor dislike

4 Dislike slightly

3 Dislike moderately

2 Dislike very much

1 Dislike extremely

In table [V are shown the scores given by volunteers at Fort Lee, Va., who tested irradiated foods as components of meals of the type served in mess halls in the United States. An irradiated food is considered to be satisfactory if it receives a score above 5 on the 9-point scale. Although the irradiated foods scored slightly lower than their nonirradiated fresh counterparts (the control in the experiment), they scored well within the acceptable range and are considered to be satisfactory for incorporation into Army rations.

In table V are preference scores for irradiation-sterilized hams that have been served at experimental luncheons. The preference scores for these hams are in the same range as are those for apple pie and ice cream.

For most applications it is important to use good quality ultrafresh food as starting material. Radiation cannot reverse deterioration and spoilage of food once it has begun; it can only arrest or prevent these conditions. Nor should radiation be used as an excuse for poor sanitation practices; its intended use is for insurance against contamination which might occur in spite of all reasonable precautions.

[95] Fruits and vegetables are irradiated in boxes or crates to minimize excessive and extraneous handling and to keep processing costs to a minimum. Meats, poultry, and fish fillets to be given pasteurizing doses to extend refrigerated shelf life should be wrapped and chilled without delay prior to irradiation.

For prepackaged meats, poultry, and seafood which are to be given sterilizing doses to promote long-term shelf stability without refrigeration, the first step is to remove as much of the inedible material as possible by deboning and trimming off gristle and excess fat. The next step is to inactivate the proteolyte enzymes in these foods. This is done by treating (blanching) to an internal temperature between 65° C and 75° C. The foods are then vacuum packaged and sealed while still hot in rigid metal cans or flexible packaging materials. The foods are then frozen without delay by blast freezer or liquid nitrogen to a temperature of -30° C and are exposed while held at -30° + 10° C either to gamma rays (from Cobalt-60 or Cesium-137), X-rays, or electrons from an electron linear accelerator. Irradiation in the frozen state minimizes adverse chemical and physical changes which may occur so that the quality of the product (taste, color, odor, texture, and vitamins) is maintained.

Clostridium botulinum is the most radiation-resistant of all the microorganisms of concern in food preservation. A dose high enough to destroy the most radiation-resistant strain of this bacterium will automatically destroy all other organisms in food which are of food spoilage or public health importance. In determining the minimum radiation dose (MRD) for sterilization, we aim for a dose high enough to reduce in number by a factor of 1 x 1012 the most highly resistant strain of C1. botulinum spores. This dose is different for each food and must be determined in every case by laboratory experiments.

Under existing statutes In the United States and in many other countries proof convincing to the appropriate health-regulating officials of safety for consumption (wholesomeness) of foods processed by ionizing radiation must be provided before these foods will be approved. In our research to appraise wholesomeness, the field is divided into four categories: Absence of induced radioactivity, microbiological safety, nutritional adequacy, and absence of carcinogens and other toxic products which may be formed by the exposure to ionizing radiation.

Under existing statutes FDA has interpreted the law concerning absence of induced radioactivity as absence of measurable induced radioactivity above the background radioactivity in food and packaging material in contact with the food. The maximum energy of the gamma rays from Cobalt-60 and Cesium-137 is below the threshold level for activation of elements normally occurring in food. Accordingly, foods processed by these two radioactive isotopes are universal! regarded as free from induced radioactivity at the highest radiation doses shown in table I. Use of X-rays at energies below 5 million electron volts (MeV) at the radiation sterilizing doses show, in table I will not induce measurable radioactivity. In the case of electrons, an expert committee convened by FAD/IAEA/WHO in Rome, Italy, in April 1964, established 10 MeV as the maximum energy level generally regarded as below the threshold level for inducing measurable radioactivity [96] in radiation-sterilized foods (ref. 2). The United Kingdom, however, has set the maximum figure for electrons at 5 MeV.

Microbiological safety in radiation-sterilized foods has been discussed previously. The use of doses required for reduction in numbers of the most radiation-resistant strains of C1. botulinum by a factor of 1 x 1012 provides a wide margin of safety. In the radiation-pasteurization range, the problem of microbiological safety is complicated by the possibility of inducing radiation-resistant mutants and upsetting the ecological balance by eliminating vegetative food spoilage organisms associated with off-odor and color, thereby permitting Clostridia to germinate and produce toxin. The current thinking for radiation pasteurization is to use radiation doses low enough to permit microorganisms associated with obvious spoilage to survive in sufficient, though reduced, numbers to give the consumer ample warning.

The use of radiation-sterilizing doses is limited to meats, poultry, finfish, and shellfish because none of the other mayor classes of foods can withstand the high doses required. At the maximum radiation doses shown in table I, there is little or no impairment in the nutritional quality of the protein or in its availability and digestibility. Similar results have been reported for essential fatty acids. For most foods of animal origin, man does not depend upon skeletal muscle as a significant source for his daily vitamin needs. The mayor exception is pork which is a rich source of thiamine. At the request of USDA the percentage retention of thiamine in irradiation-sterilized canned pork loin and ham was investigated and compared with that of pork and ham from the same lots which had been made shelf stable by heat. The figures for the processed meats were compared with the untreated pork loin and ham from the same lots. The study was expanded to include riboflavin, niacin, and pyridoxine in addition to the thiamine. The data are shown in tables Vl and VIl and indicate that the four B vitamins studied are generally less susceptible to destruction by sterilization treatment at a 4.5 to 5.6 Mrad dose at -30° ± 5° C then by the conventional thermal treatment. It is concluded that the radiation-sterilization process as developed for those foods shown in table II will not significantly impair their nutritional quality. Similar studies for foods subjected to substerilizing doses are being conducted by the AEC and by investigators abroad.

The fourth aspect of wholesomeness--the freedom from carcinogenic or toxic products formed in food by irradiation--has been extensively studied by the U. S. Army Medical Department. Twenty-one foods representing all the major food classes in the diet of North Americans were fed to rats, mice, dogs, or monkeys for 2 years and, in the case of the rodents, for 4 generations. The level of irradiated food in the daily diet on a dry-weight basis was 35 percent. In reference 4 the U. S. Army Surgeon General reported that foods irradiated up to absorbed doses of 5.6 Mrads with a Cobalt-60 source of gamma radiation or with electrons with energies up to 10 MeV have been found to be wholesome, i.e., safe, and nutritionally adequate. Feeding studies sponsored by the AEC and by scientists abroad have not uncovered evidence to indicate that foods processed by ionizing radiation are not wholesome.

This issue, the ability to demonstrate that the irradiation process does not produce carcinogenic or toxic products which will harm the consumer, is the number 1 problem which must be solved before this process can be established commercially.

I am optimistic that these irradiated foods will ultimately become commonplace on our dining-room tables because of their generally excellent quality. I base my expectation that the wholesomeness question can be resolved not only on the results of the Surgeon General's research but on data from wholesomeness studies sponsored by the AEC and by reports from other countries. I am further encouraged because of the outcome of a meeting of experts convened by the World Health Organization in April 1969. From the deliberations of this group, the World Health Organization will recommend to all its member countries that irradiated potatoes and irradiated wheat and wheat flour be given iterim approval until June 30, 1974. This will allow time to accumulate sufficient additional wholesomeness data to support final approval for these foods.

Now, what are we doing to prove wholesomeness ? The U. S. Army Medical Department is planning to resume animal feeding studies of ham, beef, chicken, pork, frankfurters, and luncheon meats to assess their safety for consumption. We expect this work to be completed by the middle 1970's when petitions will be submitted to FDA and to USDA for approval. The AEC is conducting wholesomeness studies on irradiated bananas, strawberries, and papayas to be followed by those on several varieties of fish.

Those foods which have been approved for commercial production and sale an restricted consumption are, by country:

Canada:

Potatoes

Onions

Wheat, wheat flour

 

Israel:

Potatoes

Onions

 

U. S. A.:

Potatoes

Wheat, wheat flour

 

USSR:

Potatoes

Grain

Dried fruits

Dry food concentrates

Fresh fruits and vegetables

The Soviet Union has the greatest number of approvals.

[99] Those foods which have been approved for testing of experimental lots or for market testing, by country, are:

Denmark:

Potatoes to Greenland

 

Holland:

Potatoes

 

West Germany:

Potatoes

 

USSR:

Onions

Dressed poultry packaged in plastic

Partially processed raw beef, pork, and rabbit products packaged in plastic

Kitchen-ready meat products (fried meat, steak) packaged in plastic

 

United Kingdom:

Foods for hospital patients requiring sterile diet as essential factor in medical treatment (e. g., organ transplant recipients)

Here too, the Soviet Union has the longest list of approvals.

(1) Ionizing radiation opens a new era for food preservation-the means to extend the shelf life of foods which, on the plate of the consumer, closely resemble fresh food.

 

(2) The irradiation process lends itself very readily to the concept of convenience. Irradiation-preserved foods can be offered as components of meals or as snacks in ready-to-eat form, in the form of slices or sandwiches, or as warm-and-serve or cook-and-serve foods. Foods processed by ionizing radiation require no preparation or simple preparation with reduction of labor in the kitchen.

 

(3) The irradiation process can provide high quality, nutritious, wholesome food of great morale value to individuals and small groups who are isolated, or where supply is difficult or impossible on land or sea, in the air, in space, or under the ocean.

 

(4) Proof of wholesomeness convincing to health authorities on the national level remains the number 1 problem to be resolved before ionizing radiation can be used commercially.

1. Anon. Nucleonics Weekly, Jan. 30, 1969.

2. Anon.: The Technical Basis for Legislation on Irradiated Food. Rept. of Joint FAD/IAEA/WHO Expert Comm. (Rome, Apr. 21-28, 1964). FAO Atomic Energy Ser. No. 6, Food and Agric. Organ. of the United Nations (Rome, Italy), 1965. (This report is printed in its entirety in the 1968 hearings of ref. 3. )

3. Subcommittee on Research, Development, and Radiation of the Joint Committee on Atomic Energy of the Congress of the United States: Published Hearings on the Food Irradiation Program. U.S. Govt. Printing Office (Washington, D.C.), July 18 and 30, 1968; September 12, 1966; June 9-10, 1965; May 13, 1963; March 6-7, 1962; Jan 14-15, 1960 (Part 1) and Mar 30, 1960 (Part 2); June 4-8, 1956; and May 9, 1955.

[100] 4. Anon.: Food Irradiation. Proc. Symp. Jointly Organized by IAEA and FAO (Karlsruhe, June 6-10, 1966), Int. Atomic Energy Agency (Vienna, Austria), 1966.

5. Anon.: Radiation Preservation of Foods. Advances in Chem. Ser. 65, Am. Chem. Soc., 1967.

6. Anon.: Chemical and Food Applications of Radiation. Nuclear Eng., Pt. XIX, Chem. Eng. Progress Symp. Ser., vol. 64, no. 83, 1968.

7. Anon.: Radiation Preservation of Foods. Proc. Int. Conf. (Boston, Mass., Sept. 27-30, 1964). Publ. 1273, Natl. Acad. Sci., Natl. Res. Council (Washington, D.C.), 1965.

8. Anon.: Radiation Research. Proc. Int. Conf. (U. S. Army Natick Labs., Natick, Mass., Jan. 14-16, 1963), Off. Tech. Services, U. S. Dept. Commerce, 1963.

9. Anon.: Report on Meeting on Wholesomeness of Irradiated Foods (Brussels, Oct. 23-30, 1961). Food and Agric. Organ. of the United Nations (Rome, Italy), 1962.

10. Anon.: Radiation Preservation of Food by the Quartermaster Corps, U.S. Army Res. and Development Ser. No. 1, U.S. Govt. Printing Office (Washington, D. C. ), Aug. 1957.

11. Peryam, D.R.; and Pilgrim, F.J.: Hedonic Scale Method of Measuring Food Preferences. Insert entitled "Studies in Food Science and Technology, Pt. I. " Food Technol., vol. 11, no. 9, Sept. 1957, pp. 9-14.

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