PRESERVATION OF FOOD BY IRRADIATION

1.      Introduction

Irradiation of food is the use of ionizing radiations from radioactive isotopes of cobalt or cesium or from accelerators that produce controlled amounts of beta rays or x-rays on food. The food does not become radioactive. Irradiation can be used: to destroy insects and parasites in grains, dried beans, dried fruits and vegetables, and meat and seafood; to inhibit sprouting in crops such as potatoes and onions; to delay ripening of fresh fruits and vegetables; and to decrease the numbers of microorganisms in foods. Hence, the incidence of food borne illness and disease can be decreased and the shelf life of food can be extended.

2.      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. 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.

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. 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, 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 radio nuclides 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.

3.      Ionizing Radiation Used For Food Irradiation

Energy exists in the form of waves and is defined by its wavelength. As the wavelength gets shorter, the energy of the wave increases. Electric power, radio and television, microwaves (radar) and light have longer wavelengths and lower energies. They cause molecules to move but cannot structurally change the atoms in those molecules.

Ionizing radiation (gamma rays, x-rays) has a very short wavelength and higher energy, high enough to change atoms by knocking off an electron from them to form an ion, but not high enough to split atoms and cause exposed sources to become radioactive. Therefore, the sources of radiation allowed for food processing (Cobalt-60, Cesium-137, accelerated electrons and x-rays) cannot make food radioactive.

3.1   Electron beam irradiation.

High-voltage electron beams (accelerated electrons) generated from linear accelerators are an alternative to radioisotope generators. 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, and to pasteurize frozen mechanically separated meat products with 10 MeV at a processing facility.

3.2   X-rays

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.

3.3   Gamma 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% over dose) 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.

3.4   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.

Figure 1.1 Irradiation unit

4.        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.

The bactericidal efficacy of a given dose of irradiation depends on the following:

• The kind and species of the organism. [Approximate Killing Doses of Ionizing Radiations in Kilorays (kGy) ]
• The numbers of organisms (or spores) originally present. The more organisms there are the less effective a given dose will be.
• 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.
• 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.
• The physical state of the food during irradiation. Both moisture content and temperature affect different organisms in different ways.
• The condition of the organisms. Age, temperature of growth and sporulation, and state (vegetative or spore) may affect the sensitivity of the organisms.
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1.4.1      Approximate killing doses of ionizing radiations in Kilograys (kGy)

Organism	Approximate lethal dose (kGy)
Insects	0.22 to 0.93
Viruses	10 teo 40
Yeasts (fermentative)	4 to 9
Yeasts (film)	3.7 to 18
Molds (with spores	1.3 to 11
Bacteria (cells of pathogens):
Mycobacterium tuberculosis        Staphylococcus aureus         Cornybacterium diphtheriae         Salmonella spp.	1.4 1.4 to 7.0 4.2 3.7 to 4.8
Gram-negative:
Escherichia coli        Pseudomonas aeruginosa        Pseudomonas fluorescens         Enterobacter aerogenes	1.0 to 2.3 1.6 to 2.3 1.2 to 2.3 1.4 to 1.8
Gram-positive:
Lactobacillus spp.          Streptococcus faecalis          Leuconostoc dextranicum          Sarcina lutea	0.23 to 0.38 1.7 to 8.8 0.9 3.7
Bacterial spores:
Bacillus subtillus         Bacillus coagulans         Clostridium botulinum (A)         Clostridium botulinum (E)         Clostridium perfringens         Putrefactive anaerobe 3679         Bacillus stearothermophilus	12 to 18 10 19 to 37 15 to 18 3.1 23 to 50 10 to 17

Table 1.1              Approximate killing doses of ionizing radiations in Kilograys (kGy)

5.        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%. 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.

6.        Applications of food irradiation

Type of Food	Radiation Dose in kGy	Effect of Treatment
Meat, poultry, fish, shellfish, some vegetables, baked goods, prepared foods	20 to 71	Sterilization. Treated products can be stored at room temperature without spoilage. Treated products are safe for hospital patients who require microbiologically sterile diets.
Spices and other seasonings	Up to a maximum of 30	Reduces number of microorganisms and insects. Replaces chemicals used for this purpose.
Meat, poultry, fish	0.1 to 10	Delays spoilage by reducing the number of microorganisms in the fresh, refrigerated product. Kills some types of food poisoning bacteria and renders harmless disease-causing parasites (e.g., trichinae).
Strawberries and some other fruits	1 to 5	Extends shelf life by delaying mold growth.
Grain, fruit, vegetables, and other foods subject to insect infestation	0.1 to 2	Kills insects or prevents them from reproducing. Could partially replace post-harvest fumigants used for this purpose.
Bananas, avocados, mangoes, papayas, guavas, and certain other non-citrus fruits	1.0 maximum	Delays ripening.
Potatoes, onions, garlic, ginger	0.05 to 0.15	Inhibits sprouting.
Grain, dehydrated vegetables, other foods	Various doses	Desirable changes (e.g., reduced rehydration times).

Table 1.2              Applications of food irradiation

7.         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. Irradiation is no more destructive to vitamins than other food preservation methods.

It was noted that there were small losses of vitamin E and thiamin. 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.

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.

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.

8.        Advantages and disadvantages of food irradiation

Benefits of food irradiation are,

• Disease causing germs are reduced or eliminated.
• The nutritional value of food is preserved.
• Decreased incident of food borne illness.
• Reduced spoilage in global food supply.
• Increased level of quality assurance in international trade of food products.

Disadvantages of food irradiation are,

• It reduces the content of several key nutrients such as Vitamin E (~15-30 %), Thiamin (~10-25%), Vitamin C (5-15%), Riboflavin (~7-10%), Pyridoxine (~10-20%), Vitamin B12 (~15-20%). Other nutrients are also affected however the results are less consistent.
• It creates radiolytic products with unknown short term or long term safety effects.
• Some of the organoleptic properties are affected especially for herbs, spices, essential oils.
• Formation of cholesterol oxides and fatty acid epoxidation and other oxidation products (aldehydes, esters, ketones etc.) posing safety concerns.
• Aggregation of certain proteins has been found for high protein commodities.
• The method is 90-95%% effective in killing microorganisms. The remaining 5-10% remain unaffected and may proliferate thus negating the irradiation steps. The methods can result in 95-100% effectiveness but will substantially affect the quality of the food item (taste, nutrient contents, radiolytic products,denaturing proteins, fatty acids etc.).
• Is ineffective against viruses.

9.        references

• http://www.hi-tm.com
• http://www.physics.isu.edu/radinf/food.htm
• http://uw-food-irradiation.engr.wisc.edu/materials/food_irradiation/sld021.htm
• Food preservation by irradiation by J.van Kooij