FOOD IRRADIATION TODAY

by O.P. Snyder and D.M. Poland
Copyright 1995
Hospitality Institute of Technology and Management
 St. Paul, MInnesota

History
The discovery of x-rays by W.K. Roentgen in 1895 and the discovery of radioactive substances by H. Becquerel in 1896 led to intense research of the biological effects of these "radiations." Initially, most of the irradiations made use of x-rays, which are produced when electrons from an electron accelerator are stopped in materials. These early investigations laid the foundation for food irradiation (Brynjolfsson, 1989). Ionizing radiation was found to be lethal to living organisms soon after its discovery. The use of this lethality to control spoilage and other organisms that contaminate foods was demonstrated in the early decades of the 20th century. However, no commercial development of this use occurred then, due to the inability to obtain ionizing radiation in quantities needed and at costs that could be afforded (Urbain, 1989).

In the mid 1940s, the interest in food irradiation was renewed when it was suggested that electron accelerators could be used to preserve food. However, the accelerators in those days were rather costly and too unreliable for industrial application. From 1940 through 1953, exploratory research in food irradiation in the United States was sponsored by the Department of the Army, the Atomic Energy Commission, and private industry (Thayer, 1986). Early research in the late 1940s and early 1950s investigated the potential of 5 different types of radiation (ultraviolet light, x-rays, electrons, neutrons, and alpha particles) for food preservation. Researchers concluded at that time that only cathode ray radiation (electrons) had the necessary characteristics of efficiency, safety, and practicality. They considered x-rays to be impractical because of the very low conversion efficiency from electron to x-ray that was possible at that time. Ultraviolet light and alpha particles were considered to be impractical because of their limited ability to penetrate matter. Neutrons exhibited great penetration and were very effective in the destruction or inactivation of bacteria, but were considered inappropriate for use because of the potential for inducing radioactivity in food.

In the 1940s, as described by Urbain (1989), sources of proper kinds of ionizing radiation became available. The first sources were machines that produced high energy electron beams of up to 24 million electron volts. This energy was sufficient to penetrate and sterilize a 6-inch No. 10 can of food when electron beams were "fired" from both sides of the can. Also in this same decade, man-made radionuclides such as Cobalt-60 and Cesium-137 (which in their radioactive decay emit gamma rays) became available through the development of atomic energy. The availability of these sources stimulated research in food irradiation aimed at the development of a commercial process.

Proctor and Goldblith (1951) concluded that food could be sterilized by ionizing radiation. They reported a number of important observations.

1. The medium in which microorganisms were irradiated was a factor in determining the correct dose of radiation for bacterial inactivation.
2. Enzymes were more resistant to ionizing radiation than were bacteria.
3. Irradiation in the frozen state minimized the development of off-flavor in milk and orange juice.

Research was continued by the U.S. Army when a food irradiation facility was built at the Army's research laboratories in Natick, Massachusetts in 1962. The U.S. Army maintained its interest in high-dose irradiation sterilization of meat products. The responsibility for low-dose pasteurization (2) applications development was transferred to the AEC (Atomic Energy Commission). The Army sponsored studies for the development of shelf-stable bacon, ham, pork, beef, hamburger, corned beef, pork sausage, codfish cakes, and shrimp. In 1980, the residual Army food irradiation program (chicken) was transferred to the U. S. Department of Agriculture (USDA). This agency assigned the responsibility to the Eastern Regional Research Center, Philadelphia, Pennsylvania (Skala, 1986).
[(2) Pasteurization is best defined as reducing the existing numbers of vegetative pathogenic cells to an undetectable level, less than 1/gram.  Since the maximum load of vegetative pathogens such as Salmonella spp. is on the order of 10³/gram, this is the standard that is usually used.]
 

Regulations for Food Irradiation
The 1958 Food Additive Amendment to the Food, Drug and Cosmetics Act required advance approval from the Food and Drug Administration (FDA) before any particular irradiated food could be sold publicly. At this time, irradiation was legally defined as an additive, not a process (IFT Expert Panel, 1983).

The FDA, in 1963 and 1964, approved the use of low-dose ionizing radiation for bacon, for killing insects in wheat and wheat flour, and for the inhibition of sprouting in potatoes. In 1983, the FDA approved sterilization of spices with ionizing radiation. Low-dose irradiation can also be used to inhibit sprouting of onions, garlic, and ginger, and to inhibit the ripening of bananas, avocados, mangoes, papayas, and guavas.
 

In 1985 and 1986, the USDA Food Safety and Inspection Service and the FDA approved the processing regulations for treatment of pork meat and products with a minimum dose of 0.3 kGy and a maximum of 1.0 kGy (1 kGy = 100 kilorads) of ionizing radiation to control Trichinella spiralis.

From 1990 through 1992, the U.S. government announced approval of ionizing radiation treatments of poultry to eliminate foodborne pathogens. The regulation for irradiation of poultry products from the USDA Food Safety and Inspection Service requires minimum and maximum doses of 1.5 and 3.0 kGy respectively.

Most recently, the Food Safety and Inspection Service of the USDA has indicated acceptable levels of ionizing radiation for processing red meat products (beef, veal, and lamb) to the FDA. A maximum level of 4.5 kGy is proposed for unfrozen red meat, and 7.5 kGy for frozen red meat. Approval of radiation treatment for ground meat products (and other meat items) is expected early in 1995.

Canadian Regulations
On March 23, 1989, Food and Drug Regulations of Canada were amended to create a new division for the specific control of food irradiation (Food Production and Inspection Branch, Agriculture Canada, 1989). Previous to this time, food irradiation had been regulated for more than 20 years under food additive provisions of Division 16. The new division, Division 26 is called Food Irradiation. It recognizes food irradiation as a process and contains regulations that are meaningful and specific.

The new regulations define "ionizing radiation" and control the sources used in the process; set out requirements for record-keeping, thereby strengthening inspection and compliance programs; and detail the pre-clearance requirements to be met before regulatory consideration is given to any additional or extended uses of food irradiation or changes in radiation sources or dosages. Canada agrees with the Codex Alimentarius Commission, which in 1983 took the position that foods irradiated below 10 kGy present no toxicological hazard.

To ensure that consumers have the information necessary to choose between irradiated and non-irradiated food, Canada's food irradiation regulations state that: "Irradiated foods be clearly marked with the international symbol for irradiated foods, and carry a statement to the effect that the food has been irradiated." When an irradiated food is used as an ingredient constituting 10% or more of a finished product, it must be described as "irradiated" in the ingredient list.

International Trade
Work on food irradiation has spread to other countries throughout the world. Because of the lack of research facilities in many third world countries, specialized agencies of the United Nations became actively involved in international food irradiation research programs. The illustration, Food Irradiation Facilities Around the World (Loaharanu, 1989), depicts the number and location of food irradiation facilities throughout the world.

In 1981, the Joint Expert Committee on Wholesomeness of Irradiated Foods convened by the World Health Organization (WHO) reviewed all food safety information regarding irradiated foods. They concluded that any food irradiated to an average dose of 10 kGy (1 Mrad) [see  Glossary for definitions] or less is wholesome for humans and therefore should be approved without future testing. The table, Partial Listing of Some Countries and Foods Approved for Irradiation in 1988 or Earlier, highlights 19 countries and some foods that have been approved for irradiation through 1988 in those countries.
 

There is a need to standardize regulations regarding food irradiation throughout the world. Harmonizing national regulations regarding food irradiation will facilitate international trade (Loaharanu, 1989). The upsurge in the interest in food irradiation by national authorities and industry may be attributed to:

1. 1. Increasing concern over foodborne diseases and uses of certain chemicals in food.
2. 2. High post-harvest food losses from infestation, contamination, and microbial spoilage.
3. 3. Stringent regulations related to quality and quarantine in international trade in food products.

Electron beam irradiation. High-voltage electron beams (accelerated electrons) generated from linear accelerators are an alternative to radioisotope generators (Best, 1989). They lack the penetration depth of gamma irradiation (about 0.5 cm.) per 1,000,000 electron volts (MeV) of energy, however, they require much shorter exposure times (seconds vs. hours for gamma irradiation) to be effective. Electron beam irradiation is currently being used to disinfest grain at 1.4 MeV at a grain loading facility in Odessa, Russia, and to pasteurize frozen mechanically separated meat products with 10 MeV at a processing facility in France. Irradiation research facilities at Iowa State University's Meat Irradiation Technology Center features a 10 MeV linear accelerator than can switch from electrons to x-ray beams.

X-rays generated when electrons from an electron beam bombard a heavy metal target such as tungsten, have a greater penetration depth but are less desirable because of the low energy conversion efficiency of electrons to x-rays.

Gamma rays used for irradiation processing of food are radioactive fission products of Cobalt-60 and Cesium-137. Gamma rays have good penetration, as do x-rays. With "cross-firing", they can easily deliver a uniform (less than 25% overdose) of energy to a 6-inch No. 10 can of food.

Cobalt-60 is not a waste product from the nuclear industry. It is specifically manufactured for use in radiotherapy, sterilization of medical products, and the irradiation of food. Cesium-137 is one of the fission products contained in used fuel rods. It must be extracted in reprocessing plants before it can be used as a radiation source. Currently, almost all radiation facilities in the world use Cobalt-60 rather than Cesium-137 (WHO, 1987).

Comparison of gamma rays, x-rays and cathode rays. Gamma rays, x-rays, and electron beams are equally effective in sterilization for equal quantities of energy absorbed. The greatest drawback at present to the use of x-rays in food preservation is the low efficiency and consequent high cost of their production. For this reason, most research has concentrated on the use of gamma photons and electron beams. Gamma rays have a maximum of 10 to 25% utilization efficiency, while the maximal efficiency of electrons from electron beam generators ranges between 40 and 80% (depending on the shape of the irradiated material). Radioactive sources of gamma rays (Cobalt-60 or Cesium-137) decay steadily and hence weaken with time, which is another cost. They must be constantly replenished.

The use of electrons from electron beam generators presents fewer health problems than the use of gamma rays, since electron beams are directional and less penetrating, can be turned off for repair or maintenance work, and present no hazard of radioactive materials after a fire, explosion, or other catastrophe. Gamma rays are emitted in all directions, are penetrating, are continuously emitted, and come from radioactive sources. Gamma rays require more shielding to protect workers.

The one overriding requirement for an energy source to be employed in food irradiation is that the energy levels must be below those that could possibly cause the food to become radioactive. After that requirement is met, sources are considered on the basis of their practical and economic feasibility. Machine sources must produce radiation with relatively simple technology. Isotopes must be sufficiently long lived and emit penetrating radiation (Jones, 1992).

Effects on Microorganisms
Lethality due to ionizing radiation, as proposed by the target theory, occurs when the irradiated microorganisms are destroyed by the passage of an ionizing particle or quantum of energy through, or in close proximity to, a sensitive portion of the cell. This direct "hit" on the target causes ionization in this sensitive region of the organism or cell and subsequent death. It is also assumed that much of the germicidal effect results from the ionization of the surroundings, especially water, to yield free radicals, some of which may be oxidizing or reducing and therefore helpful in the destruction of the organisms. This effect is reduced if food is irradiated in the frozen state. Irradiation may also cause mutations in the organisms present.

Bacterial spores are more resistant to ionizing radiation than are vegetative cells. Gram-positive bacteria are more resistant than gram-negative bacteria. The resistance of yeasts and molds varies considerably, but some are more resistant than most bacteria.

According to Frazier and Westhoff (1988), the bactericidal efficacy of a given dose of irradiation depends on the following:

1. The kind and species of the organism. [See the table, Approximate Killing Doses of Ionizing Radiations in Kilorays (kGy) ]
2. The numbers of organisms (or spores) originally present. The more organisms there are, the less effective a given dose will be.
3. The composition of the food. Some constituents [e.g., proteins, catalase, and reducing substances (nitrites, sulfites, and sulfhydryl compounds)] may be protective. Compounds that combine with the SH groups would be sensitizing.
4. The presence or absence of oxygen. The effect of free oxygen varies with the organism, ranging from no effect to sensitization of the organism. Undesirable "side reactions" are likely to be intensified in the presence of oxygen and to be less frequent in a vacuum.
5. The physical state of the food during irradiation. Both moisture content and temperature affect different organisms in different ways.
6. The condition of the organisms. Age, temperature of growth and sporulation, and state (vegetative or spore) may affect the sensitivity of the organisms.

 
 

Importance of Surviving Bacteria in Low-dose Irradiated Food
It is common knowledge that some microorganisms grow in foods and cause them to "spoil" or deteriorate in quality before they are eaten or discarded. The growth of these microorganisms also inhibits the growth of some pathogenic microorganisms. Therefore, there must be some "spoilage survivors" to inhibit the outgrowth of pathogens from surviving spores. For example, radiation survival curves were determined for 7 strains of Enterococcus faecium, 10 strains of Enterococcus faecalis, and 8 strains of the proteolytic variety of E. faecalis (Huhtanen, 1990). The D values (i.e., the doses giving 90% reduction of viable counts) ranged from 5.0 to 47 kGy for E. faecium strains, 3.5 to 21 kGy for the E. faecalis strains, and 3.0 to 4.5 kGy for the proteolytic variants of E. faecalis strains. The survival curves were linear for most strains but some exhibited non-linear trends. The results of this study indicate useful radiation resistant strains of group D streptococci, which may find application in low-dose irradiated foods for preventing toxin formation by pathogenic microorganisms such as Clostridium botulinum in bacon and other foods.

Uses of Food Irradiation
Ionizing radiation can be used to process food. Its effect on the food is dependent on the dose level (amount) of irradiation to which the food has been subjected. High-dose levels of irradiation (20 to more than 70 kGy) can be used to sterilize foods by eliminating all vegetative microorganisms and spores in the food. Very low doses of irradiation (less than 0.1 kGy) can be used to inhibit sprouting in potatoes, onions and garlic.

Low doses have also been shown to be as effective as pesticide fumigants for deinfesting grain products prior to shipment and storage, and for reducing microbial and insect contamination on fresh fruits and vegetables. For example, grapefruit grown in Mexico, Central America, and South America frequently are infested with larvae of the Mexican fruit fly, Anastrepha ludens. To prevent entry of this insect into the United States, grapefruits must be quarantined and treated with ethylene dibromide. A study reported that 20 grays for 0.25, 0.5, 1.0, or 100 minutes reduced adult emergence of Mexican fruit flies from larvae by more than 99% (Lester and Wolfenbarger, 1990). Therefore, once a quarantine security treatment for the Mexican fruit fly is established, a low irradiation dose rate can be used to reduce adult emergence and should impart little damage to grapefruit peel tissue.

Not all fresh produce is suitable for irradiation. The shelf life of mushrooms, potatoes, tomatoes, onions, mangoes, papayas, bananas, apricots, strawberries, and figs can be extended with low-dose irradiation with no loss in quality. However, the quality of some foods (some citrus fruits, avocados, pears, cantaloupes, and plums) is actually degraded by irradiation.

Pasteurizing doses of irradiation can kill or reduce the populations of both food spoilage and pathogenic microorganisms in food. For example, Salmonella spp. and Campylobacter jejuni can be eliminated from poultry, and trichinae from pork (ACSH, 1988). Escherichia coli O157:H7, Salmonella, C. jejuni, Listeria monocytogenes, and Staphylococcus aureus can be eliminated in uncooked ground beef (Beuchat et al., 1993).

Different doses (levels) of radiation are used for different purposes as shown in the table Applications of Food Irradiation below.
 

Irradiated foods are used by astronauts during space travel. In a review of the use of irradiated foods, Karel (1989) stated that, "On earth, food irradiation will most likely be used in combination with other preservation techniques. In space, how irradiation will be used will depend on the length of the voyage."

Effect of Ionizing Radiation on Nutrients in Food
When foods are exposed to ionizing radiation under conditions envisioned for commercial application, no significant impairment in the nutritional quality of protein, lipid and carbohydrate constituents was observed (Josephson et al., 1979). Irradiation is no more destructive to vitamins than other food preservation methods (ACSH, 1988).

It was noted that there were small losses of vitamin E and thiamin. According to Thayer et al., (1993a) thiamin in pork is not significantly affected by the FDA-approved maximum radiation dose to control Trichinella, but at larger doses, it is significantly affected. Protection of nutrients is improved by holding the food at low temperature during irradiation and by reducing or excluding free oxygen from the radiation environment. This is accomplished by irradiating vacuum-packaged foods at temperatures below 0°C (32°F).

The effect of irradiation on retention of vitamin E (alpha tocopherol) in chicken breasts was determined when the chicken breasts were irradiated in air with a Cesium-137 source at 0, 1, 3, 5.6, and 10 kGy at 0° to 2°C (32.0° to 35.6°F). The fresh muscle tissue was saponified and the total tocopherols were isolated and quantitated using normal phase high performance liquid chromatography with a fluorescence detector. Gamma irradiation of the chicken resulted in a decrease in alpha tocopherol with increasing dose. At 3 kGy and 2°C, the radiation level approved by the FDA to process poultry, there was a 6% reduction in alpha tocopherol level. No significant changes were observed for gamma tocopherol (Lakritz and Thayer, 1994).

Free radical scavengers were tested for their ability to reduce the loss of thiamin and riboflavin in buffered solutions and in pork during gamma irradiation. In aqueous solution, the tested compounds were twice as effective for the protection of riboflavin as for the protection of thiamin. The presence of nitrous oxide doubled the rates of loss for thiamin and riboflavin in solution, indicating a predominance of reactions with hydroxyl radicals. In buffered solutions, niacin was not affected by gamma radiation unless either thiamin or riboflavin was present, in which case, the niacin was destroyed rather than the other vitamin. Ascorbate, cysteine, and quinoid reductants were demonstrated to be naturally present in sufficient quantities to account for the lower rates of loss of thiamin and riboflavin observed during irradiation of pork meat, as compared to irradiation in buffered solution (Fox et al., 1992).

A study was made of thiamin content of the skeletal muscles and livers of pork, chicken, and beef after gamma irradiation. Gamma irradiation from a Cesium-137 source was used to irradiate the samples with doses of 0, 1.5, 3, 6, and 10 kGy at 2°C (35.6°F). Samples were also titrated with dichlorophenoindophenol to determine the reducing capacity of the tissue. The rate of loss of thiamin upon irradiation was found to be about 3 times as fast in skeletal muscle as in liver and to be a function of the reducing capacity of the tissues, the loss decreasing with increasing reductant titer. For the same amount of thiamin loss, liver could be irradiated to 3 times the dose as could muscle (Fox et al., 1993).

Wholesomeness of Irradiated Foods
The World Health Organization (WHO) (as cited by Lee, 1994) released the following updated policy statement on September 23, 1992: "Irradiated food produced under established Good Manufacturing Practices is to be considered safe and nutritionally adequate because:

1. The process of irradiation will not introduce changes in the composition of the food which, from a toxicological point of view would impose an adverse effect on human health.
2. The process of irradiation will not introduce changes in the microflora of the food which would increase the microbiological risk to the consumer.
3. The process of irradiation will not introduce nutrient losses in the composition of the food which, from a nutritional point of view, would impose an adverse effect on the nutritional status of individuals or populations."

In a feeding trial in China, 21 male and 22 female volunteers consumed 62 to 71% of their total caloric intake as irradiated foods for 15 weeks (Chi et al., 1986). The diet included rice irradiated to 0.37 kGy and stored for 3 months; rice irradiated to 0.4 kGy and stored for 2 weeks; meat products such as pork sausage irradiated to 8 kGy and stored at room temperature for 2 weeks; and 14 different vegetables irradiated to 3 kGy and stored at room temperature for 3 days. A double-blind design was used and included measurement of total caloric intake, monthly biochemical and physical exams and sensory evaluations of the food. The diet was well received, and there were no adverse findings associated with the consumption of the irradiated foods.

Bhaskaram and Sadasivan (1975) reported that children suffering from kwashiorkor developed a 1.8% incidence of polyploidy (3) after being fed irradiated wheat. It was also reported that there was 0% polyploidy in controls and a test group of children after the removal of the treated diet, even though polyploidy is not unusual in human populations. Experts and governmental agencies (The Government of India; FDA; IFT Expert Panel on Food Safety and Nutrition; WHO/FAO). looking at the safety of food irradiation have discredited this study because of its inconsistencies.
[(3) Polyploidy describes cells, tissues, or individuals in which there are 3 or more sets of chromosomes.]

There is a concern that ionizing radiation creates free radicals, and that they may be present in the food at the time of ingestion. Free radicals are also formed when food is fried, baked, ground, and dried. More free radicals are created during the toasting of bread than through ionizing radiation. In foods with a high moisture content, free radicals disappear within a fraction of a second; in dry foods, the free radicals are much more stable and do not dissipate as quickly (ACSH, 1988; Jones, 1992).

The assessment of the safety for human consumption of irradiated foodstuffs has involved basically four aspects (Elias, 1989).

1. Radiological safety
2. Microbiological safety
3. Nutritional adequacy
4. Toxicological safety

Recent Support for the Irradiation of Foods in the United States
In an editorial commentary in the Journal of the American Medical Association, Dupont (1992) discussed the potential for transmission of bacterial enteropathogens (Salmonella, Shigella, Campylobacter, Vibrios) through food vehicles destined for human consumption. It was recommended that the microbiological safety of food in the United States be improved through the use of irradiation. Future health hazards could be reduced if this technique is employed widely for certain high-risk items, including poultry potentially contaminated with Salmonella and Campylobacter.

The Assistant Secretary for Health, Director, U.S. Public Health Service, Dr. Phillip R. Lee pointed out the importance of using irradiation to prevent foodborne illness in another recent issue of the Journal of the American Medical Association (Lee, 1994). "Foodborne illness is one of the largest preventable public health problems in the United States. Studies by the Centers for Disease Control and Prevention show that foodborne diseases caused by pathogenic bacteria, such as Salmonella, Campylobacter, E. coli, and by Vibrio, Trichinella, tapeworms, and other parasites, cause an estimated 9000 deaths and from 6.5 million to 81 million cases of diarrheal disease annually. Unfortunately, it has taken a crisis to raise consumer, industry, and lawmakers' awareness. Media scrutiny about salmonella in poultry and the highly publicized deaths of four children and severe illness of 600 people linked to E. coli-tainted hamburger have raised serious questions about food safety."

"There is a need and responsibility for U.S. Public Health Service to use what is known to protect and improve the health of the public. Each modern food-processing advance -- pasteurization, canning, freezing -- produced criticism. Food radiation is no different." (Lee, 1994).

The Wall Street Journal noted that: "Irradiating beef may help save lives, but the meat industry is still waiting for the green light" (Gibson, 1994). Agriculture scientists report that small doses of irradiation could wipe out "nearly 100%" of the E. coli strain in beef, and has been shown to sterilize meat and produce against practically all bacteria that cause spoilage. Yet, the Department of Agriculture is holding back because of the concern that the public might link beef irradiation with the government radiation experiments of the 1950s. Proponents of Beef Irradiation include: The National Food Processors Association, the American Medical Association, the World Health Organization, and the American Meat Institute. All say beef irradiation will help reduce deaths from E. coli.

In December, 1993, the American Medical Association (AMA) endorsed food irradiation as "a safe and effective process that increases the safety of food when applied according to governing regulations." The AMA also affirms the principle that "the demonstration of safety requires evidence of a reasonable certainty that no harm will result, but does not require proof beyond any possible doubt [i.e., 'zero' risk does not exist]" (Marsden, 1994).

An article in the Washington Times (Fumento, 1994) reported that irradiation will give food producers another line of defense in preventing foodborne illness, and to prevent its use is to deny them a valuable tool in protecting public health. The U.S. Secretary of Agriculture asked the Food and Drug Administration to approve the use of irradiation on beef. There are many activists who oppose the use of radiation for food. "Instead of using irradiation to enhance food safety, opponents say the answer is increased government regulation, especially increasing the size of the government's meat inspection force. At this time, the government employs only about 8,000 inspectors, including supervisors, for about 32 million head of slaughtered cattle annually." However, "inspectors cannot see bacteria and other spoilage organisms and no microbiological tests currently exist that would make it practical to perform routine laboratory analysis on raw meat." (Fumento, 1994)

Gallager and Kwittken (1994) reported conclusions of an E. coli O157:H7 Consensus Development Conference sponsored by the American Gastroenterological Association Foundation. "Protection of the public's health requires the immediate implementation of currently recognized scientific technology for ensuring food safety. An emphasis should be placed on science-based monitoring and verification of the nation's slaughter plant operations. The current inspection-based system should be replaced by a science-based risk assessment process with government verification." The 14-member nonadvocate panel was comprised of professionals and public representatives from gastroenterology, epidemiology, public health, microbiology, food science, industry and consumer affairs. Speakers included scientific investigators, government representatives, industry officials, and consumers. One of the major recommendations made was as follows: "Irradiation pasteurization is a safe and effective intervention strategy, especially in ground beef and should be implemented as soon as possible."

Effect of Ionizing Radiation of Meats
Various parasites cause human illnesses with medical costs and productivity and disability losses totaling billions of dollars annually. Food is an important vehicle for some of these parasitic diseases. In the United States, congenital toxoplasmosis is estimated to cost up to $5.3 x 109 annually. Half of the incidence of toxoplasmosis can be attributed to food sources. Irradiation of fresh pork could decrease cases of congenital toxoplasmosis. Similarly, other parasitic diseases (Cyticerci, Anisakindinae, Trichinella spiralis, and Giardia lamblia) could be reduced by irradiating beef, pork or fish (Roberts and Murrel, 1993).

Research completed in 1993 for the American Meat Institute Foundation found that very low doses of irradiation are sufficient to kill a host of harmful bacteria in frozen, boxed hamburger patties. The dosage equivalent to that approved for poultry (1 to 3 kGy) is ample and this amount of radiation can be used for this purpose when irradiation is approved for beef. Research was conducted at the Center for Food Safety and Quality Enhancement, University of Georgia in collaboration with Vindicator, Inc., Mulberry, Florida. Experiments were done to determine the D kGy of E. coli O157:H7, Salmonella, C. jejuni, L. monocytogenes, and S. aureus in uncooked ground beef (Beuchat et al. 1993). The influence of 2 levels of fat (8 to 14% and 27 to 28%) and temperature (frozen [-17° to -15°C (1.4° to 5.0°F)] and refrigerated [-3° to 5°C (26.6° to 41.0°F)] during gamma irradiation (Cobalt-60) was studied. Cells of all test pathogens were in the stationary phase of growth when inoculated into ground beef before subjecting it to 7 target irradiation doses ranging from 0 to 3.0 kGy.

The use of ionizing (gamma) radiation can be used as an alternative to thermal processes for the preservation of food. Thayer et al. (1986) reviewed research studies of the uses of ionizing radiation to extend the safety of processed meats. Radiation research studies of meat products included: bacon, ham, frankfurters, corned beef and pork sausage, and beef, chicken and pork. Beef, chicken and pork were organoleptically acceptable after irradiation in vacuo at -30°C (+/-10°C) [-22.0°F ()18°F)]. 12D doses for the process were 4.12, 4.27, and 4.37 Mrad for beef, chicken, and pork loin, respectively. It was also reported that while sublethal radiation doses enhance the safety and storage life of raw beef, chicken, and pork, it may actually increase the spoilage rate in cured meat products.

Gamma radiation doses of 0.26 kGy and 0.36 kGy, administered in vacuo at 0°C (32°F), destroyed 90% of log-phase and stationary-phase colony forming units (CFU) of S. aureus, respectively, in mechanically deboned chicken meat (MDCM). Samples inoculated with 103.9 CFU/g of S. aureus were treated with gamma radiation in vacuo at 0°C (32°F) and then held for 20 hours at 35°C (95.0°F) (abusive storage). Enterotoxin was not detected in irradiated MDCM. A predictive equation was developed for the response of S. aureus in MDCM to radiation dose and irradiation temperature (Thayer and Boyd, 1992).

Treating fresh or frozen meats with ionizing radiation is an effective method to reduce or eliminate several of the foodborne human pathogens such as Salmonella, Campylobacter, Listeria, Trichinella, and Yersinia. It is possible to produce high-quality, shelf-stable commercially sterile meats. Irradiation dose, processing temperature, and packaging conditions strongly influence the results of irradiation treatments on both microbiological and nutritional quality of meat. These factors are especially important when irradiating fresh non-frozen meats. Radiation doses up to 3.0 kGy have little effect on the vitamin content, enzyme activity, and structure of refrigerated non-frozen chicken meat, but have very substantial effects on foodborne pathogens (Thayer et al., 1993a).

E. coli O157:H7 had a significantly higher D value when irradiated at -17° to -15°C (1.4° to 5.0°F), compared to treatment at 3° to 5°C (37.4° to 41.0°F) At a given temperature, the level of fat in beef did not have an effect on D values. Salmonella behaved similarly to E. coli O157:H7 in low fat beef, but temperature had no effect on D values when the pathogen was irradiated in high-fat beef. Significantly higher D values were calculated for C. jejuni in frozen compared to refrigerated low-fat beef. The pathogen was more resistant to irradiation in low-fat beef when treatment was done at -17° to -15°C (-1.4° to 5.0°F). Neither the level of fat nor the treatment temperature significantly affected the D values for L. monocytogenes and S. aureus. Considering all combinations of fat level and treatment temperature, the ranges of D values (kGy) were, in ascending order of irradiation resistance:
 

Campylobacter jejuni	0.175 to 0.235 kGy
Escherichia coli O157:H7	0.241 to 0.307 kGy
Staphylococcus aureus	0.435 to 0.453 kGy
Listeria monocytogenes	0.507 to 0.610 kGy
Salmonella	0.618 to 0.800 kGy

 

Thayer and Boyd (1994b) reported a study of the sensitivity of E. coli O157:H7 suspended in beef or mechanically deboned chicken meat (MDCM) to gamma irradiation and also to determine the influence of processing parameter such as atmosphere or temperature on that sensitivity. Undercooked and raw meat has been linked to outbreaks of hemorrhagic diarrhea due to the presence of E. coli O157:H7; therefore, treatment with ionizing radiation was investigated as a potential method for the elimination of this organism. Response-surface methods were used to study the effects of irradiation dose (0 to 2.0 kGy), temperature [-20° to +20°C (-4.0° to 68.0°F)], and atmosphere (air and vacuum) on E. coli O157:H7 in mechanically deboned chicken meat. Differences in irradiation dose and temperature significantly affected the results. 90% of the viable E. coli in chicken meat was eliminated by doses of 0.27 kGy at +5°C (41.0°F) and 0.42 kGy at -5°C (23.0°F). Small but significant differences in radiation resistance by E. coli were found when finely ground lean beef rather than chicken was the substrate. Unlike non-irradiated samples, no measurable verotoxin was found in finely ground lean beef that had been inoculated with 104.8 CFU E. coli O157:H7 per gram, irradiated at a minimum dose of 1.5 kGy, and temperature abused at 35°C (95.0°F) for 20 hours. The authors concluded that radiation is an effective method to control this foodborne pathogen.

Research from the USDA, Agricultural Research Service which defines the effect of atmosphere and irradiation temperature on control of the foodborne pathogens (Aeromonas, Listeria, Salmonella, and Staphylococcus) was reviewed by Thayer (1993d). Results indicate that both the temperature and atmosphere during irradiation of meats are important, and that these pathogens can be greatly reduced in population by radiation doses of 3 kGy or less.

Vacuum-packaged ground fresh pork samples absorbed gamma radiation doses of 0, 0.57, 3.76, 5.52, or 7.25 kGy at 2°C (35.6°F). Samples were analyzed after 1, 7, 14, 21, 28, or 35 days storage at 2°C (35.6°F) for presence and number of aerobic and anaerobic mesophiles and endospore formers and aerobic psychrotrophs. Conventional plate counts did not detect surviving microflora in any sample that received an absorbed dose of 1.91 kGy or higher, even after refrigerated storage for up to 35 days. The microflora in the control were predominantly Gram-positive for the first 21 days; however, Serratia predominated at 28 and 35 days. Staphylococcus, Micrococcus, and other yeast species predominated in samples that received 0.57 kGy (Thayer, 1993e).

The gamma radiation resistance of 5 enterotoxic and 1 emetic isolate of Bacillus cereus vegetative cells and endospores was tested in mechanically deboned chicken meat (MDCM) ground turkey breast, ground beef round, ground pork loin, and beef gravy. The D values for B. cereus were 0.184, 0.431, and 2.56 kGy for logarithmic-phase cells, stationary-phase cells, and endospores at 5°C (41.0°F) on MDCM, respectively. Neither the presence nor absence of air during irradiation significantly affected radiation resistance of vegetative cells or endospores of B. cereus when present on MDCM. Irradiation temperature [-20° to +20°C (-4.0° to 68.0°F)] did affect the radiation resistance of stationary-phase vegetative cells, and to a limited extent that of spores on MDCM. Impedance studies indicated that surviving vegetative cells were severely injured by radiation. A dose of 7.5 kGy at 5°C (41.0°F) was required to eliminate a challenge of

4.6 x 103 B. cereus from temperature-abused MDCM [24 hours at 30°C (86.0°F)]. The radiation resistance of a mixture of endospores of 6 strains to gamma radiation was 2.78 kGy in ground beef round, ground pork loin, and beef gravy, but 1.91 kGy in turkey and MDCM. The results indicate that irradiation of meat or poultry can provide significant protection from vegetative cells but not from endospores of B. cereus (Thayer and Boyd, 1994a).

The effects of water content, activity, NaCl, and sucrose content on the survival of Salmonella typhimurium on irradiated MDCM and ground pork loin were investigated. The effects of NaCl and sucrose concentration were investigated by adding various amounts to MDCM or ground pork loin with NaCl solutions with various degrees of saturation. The effects of water content were investigated by rehydrating freeze-dried ground pork loin with different quantities of water. Inoculated samples were irradiated at 5°C (41.0°F) in vacuo to doses up to 6.0 kGy. The survival of S. typhimurium was effected by water content, water activity, and NaCl content, but not by sucrose content. The failure of sucrose to provide the same protection for S. typhimurium in meat against radiation argues against reduced water activity being a primary mechanism of protection. The results indicate that the survival of foodborne pathogens on irradiated meat with reduced water content or increased NaCl levels may be greater than expected (Thayer et al., 1994b).

Longissimus dorsi from beef, pork, and lamb, and turkey breast and leg meats were inoculated with E. coli O157:H7, L. monocytogenes, Salmonella spp. and S. aureus, and the gamma radiation resistance of the pathogens were determined under identical conditions. At a temperature of 5°C (41.0°F), the respective radiation D value for the mixture of E. coli O157:H7 and L. monocytogenes was not affected by the suspending meat. The D value for a mixture of Salmonella spp. was significantly lower on pork than on beef, lamb, turkey breast, and turkey leg meats. The D value for S. aureus was significantly lower on lamb and MDCM than on the other meats. All values were, nevertheless, within the ranges expected for these pathogens (Thayer et al., 1994d).

Research has demonstrated that ionizing radiation can inactivate parasites, eliminate or greatly reduce the populations of microbial pathogens, and extend the shelf life while preserving the desired nutritional and sensory properties of refrigerated poultry and red meat. Foodborne pathogens can be greatly reduced in population and sometimes completely eliminated from foods by low doses of ionizing radiation. The shelf life of poultry, pork, and beef can be significantly extended by treatment with ionizing radiation. Combination treatments with vacuum packaging or modified atmosphere packaging and ionizing radiation have produced better than predicted results. Additional research is needed on the combined processes (Thayer, 1993c).

The American Meat Institute (AMI) is actively involved in the investigation of viable pathogen-preventing technologies which can be applied to the meat and poultry industry. Irradiation is one of those technologies and it is one that has gathered support from governments and scientists worldwide (Marsden, 1994). In AMI's view, reducing pathogens in the food supply and preventing foodborne illness will demand a multi-faceted, farm-to-table approach. While irradiation may be helpful, it alone will not solve public health problems related to foodborne pathogens.

Labeling
FDA requires labeling if the entire product or major ingredient has been irradiated (ACSH, 1988). The current version of the FDA low-dose irradiation rule requires that the retail label consist of the internationally agreed-upon symbol  (International Food Irradiation Symbol), and the phrases "treated with radiation" or "treated by irradiation."

 Irradiated spices in foods are considered minor ingredients and do not require labeling in combination products (Pszczola, 1990).

Schutz et al. (1989) reported a nationwide study on the influence of label statements on the perception of quality, safety, and willingness to buy. The label statements used were "Irradiated to control microorganisms", "Irradiated for quarantine control", "Irradiated to extend shelf life", and "Irradiated to control spoilage". Consumers thought the products bearing the label "Irradiated to extend shelf life", and "Irradiated to control spoilage" would stay fresh longer. They also responded that a food with a label "Irradiated to control microorganisms" was an indication of higher quality than a non-irradiated food. People thought it would probably be safer and more expensive than the non-irradiated counterpart; about 50% said they would like to buy the product.

Consumer Acceptance
Irradiated foods carry a value-added perception of safety among consumers in both the Netherlands and South Africa, where irradiation is in commercial use. However, in the United States, consumers' acceptance of irradiated foods is being demonstrated slowly.

Bruhn and Schutz (1989) reported a study concerning consumer acceptance or rejection of irradiated food products. Rejecters are opposed to irradiation for personal beliefs, such as the desire to eat only natural, unprocessed, or "organic" foods. About 5 to 10% of the population are irradiation rejecters. About half of the population are undecided. These people are unsure in their knowledge and understanding of the irradiation technology. Their concerns include food's safety, quality and nutritional value. The acceptors, estimated to be from 25 to 30% of the population, believe they understand the process of irradiation and accept that irradiated foods are safe and nutritious. Accepting consumers trust the FDA and food manufacturers. To be acceptable, irradiation must offer to the consumer an advantage. This advantage could be higher quality, greater safety, longer shelf life, wide product availability, or lower cost.

All consumers wanted irradiated products labeled for various reasons. Some, so they can avoid them, while others wanted irradiated products labeled so they could choose them and realize the advantage they thought the process could offer. Others simply wanted to know what they were buying. Some people believe that non-labeling implies that information about the product is hidden.

The authors (Bruhn and Schutz, 1989) concluded that consumers need to be educated and informed. Information may not reduce consumer concern, but it allows choice to be based on fact, rather than suspicion. Consumer attitude and marketing studies show that, given information about irradiation, the majority of consumers will choose irradiated foods. Others object to irradiation and will never select it. The opportunity for choice in the marketplace should exist. "The eventual utilization of this technology will depend upon the safety of alternative technologies, industry's perception of potential success, responsible media coverage, and consumer information."

Clarke and Riley (1993) reported that consumers lack knowledge, but support irradiation once they understand it. A study conducted by The Gallup Organization (1993) confirms that the more consumers know about food irradiation, the more likely they are to desire it for destroying bacteria in poultry, pork, beef and seafood. Although 73% of Americans have heard about food irradiation, only 24% claim to have any knowledge of the process. After the benefits of irradiation are explained and endorsements from health organizations are mentioned, over half (54%) of those interviewed say they would purchase irradiated meat rather than non-irradiated meat. Further, 60% said they would be willing to pay a 5% premium for irradiated hamburger. Men and those who had experienced foodborne illnesses are most accepting of irradiated foods.

Consumers generally do not understand the food safety benefits of most food preservation technologies, including pasteurization, canning, and freezing. In a simulated supermarket setting study conducted by the University of Georgia and reported by Clarke and Riley (1993), 50% of consumers tested, chose irradiated ground beef over regular ground beef. After all consumers tested learned more about the irradiation process and how it affects raw meats, those choosing irradiated beef increased to 70% of the sample size (Clarke and Riley, 1993).

There is the perception that the public, reacting to a distrust of anything even remotely related to nuclear energy, will never buy or accept irradiated food. In response to this perception of consumer fear, no industry has been willing to sell irradiated food until recently (Conley, 1992). Consumers are not likely to demand food irradiation because they do not know enough about it, but they are demanding safer food. Food safety is an area of consumer, industry, and government concern. Irradiation of meat, poultry, fruits, and vegetables at appropriate levels could control many organisms that cause foodborne illness and product spoilage. Thus, food irradiation could be a viable process that could improve safety and extend shelf life.

Market tests in Florida indicate consumer acceptance of irradiated mangoes, papayas, apples, and strawberries. A 1988 study conducted by a USDA Economic Research Service economist and a University of Florida professor found that consumers are willing to pay more for a safer product and would buy irradiated chicken (Conley, 1992).

Anti-food Radiation Activists
However, there are anti-food radiation activists. Food and Water, an anti-irradiation group based in Marshfield, Vermont, is leading a nationwide campaign to keep food irradiation from winning public acceptance (Katzenstein, 1992). Other groups that look upon food irradiation with suspicion include Ralph Nader's Public Citizen Consortium and Michael Jacobsen's Center for Science in the Public Interest. At this time, some supermarket chains and grocery retailers are reluctant to offer irradiated foods for sale, fearing customer boycotts.

Consumer Education and Responsible Information Concerning Food Irradiation
Education is the key. Proponents of food irradiation firmly believe that these foods will be accepted when offered for sale. But it will not be easy. If half the population is undecided about the process, and studies show that information and education can increase consumer acceptance, why are massive amounts of information unavailable? The answer is that the industry, not wanting to appear prejudiced and fearing reprisals from interest groups, has been reluctant to produce consumer-oriented materials. The federal government thus far has taken the position that, while the process is safe and should be available to the industry and consumers, the government should not be viewed as a proponent of the process. As a result, there is very little consumer-oriented material available about irradiated food or the process. Into this void step activist groups, some of which provide information and material than can increase fear of irradiated food products.

Responsible government should provide information to help consumers make educated choices, not choices based on limited information and fear tactics. Conley (1992) advocates that the FSIS and the National Agricultural Library in cooperation with other food-related agencies such as the FDA should provide education materials to consumers regarding food irradiation.

Food Irradiation Standards
Food irradiation will be successful only if regulators, industry, and consumers agree that it is safe and effective. Appropriate control and inspection procedures are important aspects of that agreement. ASTM (American Society for Testing Materials) standards are being developed using "Codes of Good Irradiation Practice" developed by the International Consultative Group on Food Irradiation (ICGFI). This will facilitate acceptance of irradiated foods in international trade (Derr, 1993).

Commercialization of the Food Irradiation Process
Decisions by food processors to invest (or not to invest) in radiation processing depends on many factors (Hall, 1989). Some of these rarely receive much attention in discussions of the subject. In the Netherlands, commercial food irradiation is routinely applied to a broad range of products; however, nowhere else has food irradiation achieved this goal. Regulatory and safety aspects of the process, equipment and cost factors, and obtaining consumer acceptance are factors which have contributed to the slowness of commercialization of food irradiation in the United States.

When deciding whether or not to develop a food irradiation process, the processor must ask:

1. Will irradiation serve my particular needs?
2. How much will it cost?
3. Is it safe for consumers, customers, employees?
4. What regulations will apply, how much trouble will they be, and what will they cost me?
5. What public relations aspects, including community attitudes, will I need to address? What will I have to do address them? How much will it cost?
6. What will consumers of my product think? How will they react? How can I find out in advance?
7. What liability questions are involved?

1. Radiation treatment of foods shall be carried out in facilities licensed and registered for this purpose by a competent national authority.
2. The facilities shall be designed to meet the requirements of safety, efficacy and good hygienic practices of food processing.
3. The facilities shall be staffed by adequate, trained and competent personnel.
4. Control of the process within the facility shall include the keeping of adequate records including quantitative dosimetry.
5. Premises and records shall be open to inspection by appropriate national authorities. (In the United States, the authorities would include the Nuclear Regulatory Commission, the FDA and the USDA.)

1. The food should comply with the provisions of Recommended International Code of Practice—General Principles of Food Hygiene and, where appropriate, with the Recommended International Code of Hygienic Practices of the Codex Alimentarius relative to a particular food.
2. The irradiation of food is justified only when it fulfills a technological need or where it serves a food hygiene purpose, and should not be used as substitute for Good Manufacturing Practices.

1. Preserves food to a varying extent as determined by the treatment. Food irradiation is particularly effective in controlling foodborne spoilage microorganisms. All organisms present in the food can be inactivated to secure long-term preservation, or a fraction of them can be inactivated to secure limited extension of product life. Meats, seafood, fruits, vegetables, cereal grains, and legumes are some of the foods than can be preserved.
2. Decontaminates food of pathogenic bacteria, yeast, molds, and insects. This decontamination can improve the hygienic quality of the foods and prevent the potential health hazards. Meats and seafood can be decontaminated of bacteria and parasites; cereal grains, legumes, fruits, and dried fish of insects; spices and vegetable seasonings of bacteria and insects.
3. Controls maturation, senescence, and sprouting of fresh fruits and vegetables.
4. Alters chemical composition for quality improvement. The chemical composition of cereal grains and legumes can be altered so as to improve their quality. This is regulated by the dose (i.e., amount of radiation absorbed by the food).
5. Produces no toxic residues in foods. This is accomplished by limiting the energy level of the radiation employed, and also be selecting the type of radiation. The lethal action of ionizing radiation on living organisms was traced to alteration of the DNA molecule. Products formed in foods by irradiation were identified and determined to be of no toxicological significance to the consumer of irradiated foods.
6. Maintains full nutritive value of foods. Studies have shown no changes in macronutrients and only insignificant ones in the micronutrients (vitamins). Irradiated foods were shown to be wholesome.
7. Maintains sensory quality. Knowledge of radiation chemistry has guided the development of means to prevent undesired sensory changes. An example of this is the irradiation of meats in the frozen rather than unfrozen state in order to avoid a flavor change.

1. Foods can be treated after packaging.
2. Irradiation processing permits the conservation of foods in the fresh state.
3. Perishable foods can be kept longer without noticeable quality loss.
4. The cost of irradiation and the low energy requirements compare favorably with conventional food processing methods. Irradiation treatment up to the prescribed dose leave no residue; changes in nutritional value (i.e., loss of some vitamins) are comparable with those produced by other processes and during storage.
5. Foods processed under prescribed conditions for irradiation do not in any way become radioactive, a fact that many people do not understand.

"Unfortunately, irrational fears about irradiation too often stand in the way of rational consideration of its benefits and realistic consideration of its risks when it is used to treat some foods that people eat.

"Several marketing surveys have shown that when consumers learn more about the process and its uses in food, acceptance of the theory and practice of irradiating food rises markedly. .....Everyone agrees that the nation's food supply needs cleaning up so that consumers can be spared the costly, debilitating and sometimes deadly illnesses and the enormous waste associated with foods that spoil before they can be eaten and the potential hazards of pesticides now used to control spoilage and insect contamination."

In closing, it can be stated that food irradiation is not a miracle process that can convert spoiled food into high-quality food. It is equally true that not all foods are suitable for radiation treatment, just as not all foods are suitable for canning, freezing, drying, etc.

Food irradiation has two main benefits to the health and well-being of humans: the destruction of certain foodborne pathogens, thus making the food safer; and prolongation of the shelf life of food by killing pests and delaying the deterioration process, thus increasing food supply.

Glossary*

Electron radiation.  Streams or beams of electrons accelerated to energies of up to 10 MeV. There are a number of machine sources available for electron radiation.

Gray (Gy).  The unit (or level) of energy absorbed by a food from ionizing radiation as it passes through in processing.
            1 Gray (Gy) = 100 rads
            1,000 Gy = 1 Kilogray (kGy)

X-rays.  Electromagnetic radiation of a wide variety of short wavelengths. They are usually produced by a machine in which a beam of fast electrons in a high vacuum bombards a metallic target. X-rays are sometimes called "Roentgen rays", after their discoverer, Lord von Roentgen.

Gamma Radiation.  Electromagnetic radiation of very short wavelength, of the same nature as "short" x-rays. Gamma rays are emitted by isotopes of such elements as cobalt and cesium as they disintegrate spontaneously.

Rad.  Another name or unit for "radiation energy absorbed" by food being processes with radiation.
            1,000 rads = 1 Kilorad = 10 Grays
            1,000,000 rads = 1 Mrad = 10 kGy
            (The rad has been superseded by the Gray.)

* Adapted from: Institute of Food Technologists' Expert Panel on Food Safety and Nutrition. 1983. Radiation preservation of Foods. A scientific status summary. Food Technol. 37(2): 55-60.

Abbreviations
FAO = United Nations Food and Agriculture Organization
WHO = World Health Organization
IAEA = International Atomic Energy Agency
FSIS = Food Safety and Inspection Service, USDA

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