Chapter I. Literature review
I- 1. Food irradiation by X-ray
Food irradiation is a physical treatment, in which the food is subjected to a defined dose of ionizing radiation to extend the shelf life and inactivate harmful organisms (Arvanitoyannis & Tserkezou, 2010). Decades of research have conclusively shown that food irradiation can have myriad beneficial applications, including the insect disinfestation of fruits and grains, inhibition of sprouting in potatoes and onions, the delayed ripening of fresh fruits and vegetables, and the enhanced safety and sterilization of fresh and frozen meat products, seafood, and eggs (Diehl, 2002;
Kume et al., 2009b).
Food irradiation technology has been permitted for more than 250 food items in 56 countries, with 400000 tons of food irradiated globally in 2005. China accounted for 36% of the total global irradiated food, which included the irradiation of 80000 tons of garlic to inhibit its germination, and 52000 tons of dried vegetables and spices. In the US, a total of 92000 tons of food was irradiated, including 80000 tons of spices, 8000 tons of ground beef and chicken meat, and 4000 tons of fruits and vegetables. In addition, Ukraine irradiated 70000 tons of wheat and barley using an electron accelerator. While the EU has shown a decrease in food irradiation due to the enforcement of regulations on labeling violations, food irradiation in Asian countries has increased (Kume et al., 2009a).
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Irradiation sources for food treatment include the following: gamma rays from radionuclides 60Co or 137Cs; electron-beams (e-beams) generated from machine sources operated at or below a nominal energy level of 10 MeV; and X-rays generated from e-beam machines operated at below a nominal energy level of 5 MeV (Code of Federal Regulations, 1986; Codex Alimentarius Commission, 2003).
The US Food and Drug Administration has approved an increase of the limit for e-beam energy used for generating X-rays to 7.5 MeV in response to a petition from IBA (Code of Federal Regulations, 2004).
X-rays are electromagnetic radiation with a wide energy spectrum, and generated by colliding accelerated electrons with a dense material (target) such as tantalum or tungsten in a process known as bremsstrahlung-conversion (Koch & Motz, 1959;
Miller, 2005). The efficiency for converting electron beam power to X-ray power increases with the energy of the incident electrons and with the atomic number of the target material. Increasing the energy improves also the X-ray penetration;
however, higher energy of incident e-beams than the threshold energy of target material can induce radioactivity of the material by emission of neutrons (Grégoire et al., 2003). Because the threshold energies of tungsten, tantalum, and gold are 6.19, 7.58, and 8.07 MeV, respectively, tantalum or gold is used as a target material for 7.5 MeV X-rays, whereas the maximum energy of tungsten for generating X-rays is 5 MeV (Cleland & Stichelbaut, 2013). In a tantalum target, the conversion efficiency from incident electron beam power to emitted X-ray powder in the forward direction is about 8–9% at 5 MeV and 12–13% at 7.5 MeV (Meissner et al.,
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2000). The broad energy spectrum of bremsstrahlung photons emitted from tantalum target extends from a few tens of keV up to the maximum energy of the incident electrons. However, the peak of the photon energy spectrum occurs at about 0.3 MeV in the electron energy range from 5.0 to 10 MeV (Seltzer et al., 1983).
X-rays have a deep penetration power comparable to gamma rays from 60Co sources, with the added advantage of using an electronic source that stops radiating when switched off (Lazurik et al., 2007). A bremsstrahlung radiation from a 5.0 MeV electron beam penetrates slightly more than the gamma radiation (Seltzer &
Berger, 1987). This greater penetration is partly caused by the higher photon energy of X-rays and the angular distribution. The narrow angular distribution of X-rays at above 5 MeV increases the penetration in materials because the most intense zone of the emitted radiation is perpendicular to the surface of the irradiated products, whereas gamma radiation has a wide angular distribution (Meissner et al., 2000).
The optimum thickness of material irradiated with 7.5 MeV X-rays is approximately 38 g/cm2, whereas those of 10 MeV e-beams, gamma rays, and 5.0 MeV X-rays are 8, 31, and 34 g/cm2, respectively (Cleland & Stichelbaut, 2013).
Since the first discovery by Wilhelm Conrad Rőntgen in 1895, X-rays have been mainly used for medical radiography, scanning for security, and analyzing instruments such as X-ray diffractometer. For these applications, low-energy X-rays at below a few hundreds of keV were used. As the energy of electrons for generating X-rays increased for keV to MeV, sterilization of packaged food and
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medical devices was proposed. Although the greater penetration, improved conversion efficiency, and radiological safety, the commercial application of 7.5 MeV X-rays is still scarce, owing to the economic issues such as lower throughput than e-beam irradiation and higher cost than gamma irradiation. However, recent comparisons have shown that the capital and electric power costs for electron accelerators with energies of 5–7 MeV equipped with X-ray targets can be lower than the capital and source replenishment costs for 60Co source loadings greater than 6 MCi based on the recent prices of 60Co at $2.5 US per curie and average cost of electric power at 12 cents per kilowatt hour (Cleland & Stichelbaut, 2007). IBA has reported that cost of energy for sterilizing 1 m3 of products is 8.6 € for 7.5 MeV X-ray irradiation, 9.1 € for gamma irradiation, and 4.1 € for e-beam irradiation, based on the same throughput at 64,000 m3/year (Mullier, 2015).
Different characteristics of X-rays with gamma rays and e-beams are summarized in Table 1. Gamma rays are a type of electromagnetic radiation emitted from radioactive isotopes such as 60Co and 137Cs (Woods & Pikaev, 1994). This type of radiation has the highest frequency and energy with the shortest wavelength, and can easily remove an orbital electron from an atom in the irradiated medium (Laughlin et al., 1989). This ionizing property imparts significant damage to a living cell and thus gamma radiation is often used to inactivate living organisms in food.
137Cs is not suitable for commercial use due to its lower energy compared to 60Co, and is used only in small hospital units to treat blood before transfusion for the prevention of Graft-versus-host disease (Ehlermann, 2005). Food irradiation using
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60Co is the preferred method by most processors, as its deeper penetration enables the administration of treatment to entire industrial pallets. The main characteristics of gamma irradiation include the treatment of food with photons of well-defined energies (1.17 and 1.33 MeV) emitted from 60Co sources, and the irradiation of large amounts of food in bulk through its high penetration power. However, gamma irradiation also presents a number of disadvantages. These include the low consumer acceptance through fear of the radioisotope, and high construction costs due to the requirement for shielding (World Health Organization, 1988). However, despite of its disadvantages, gamma irradiation is used predominantly in food industries to inactivate microorganisms in spices, dried vegetables, and meat products (Kume et al., 2009a).
E-beam irradiation uses electrons accelerated in an electric field to a velocity close to the speed of light. High-energetic electrons are particulate ionizing radiation. As electrons have a significantly larger cross section than photons, they generally do not penetrate the product beyond a few centimeters, depending on product density (Bhat et al., 2012). The maximum energy of e-beams approved for food irradiation is 10 MeV, because higher energetic e-beams than 10 MeV can result in the subject becoming radioactive. The optimum penetration depth of such 10 MeV e-beams is approximately 8 g/cm2 with double sided irradiation. However, the use of e-beam irradiation is growing in the food irradiation market, as food can be treated quickly with e-beams at high dose rates. For example, frozen food such as meat and seafood can be treated with e-beam without any quality deterioration
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induced from thawing, which is proceeded during gamma irradiation process (Brown, 2015).
Conclusively, food irradiation by 7.5 MeV X-rays has a few advantages compared to gamma and e-beam irradiation. Similarly with gamma irradiation, food product in bulk can be irradiated with 7.5 MeV X-rays, due to the highest penetration depth. The recent development of high-energy and high-power electron accelerators has made X-ray processing a practical alternative to gamma-ray processing in applications requiring greater penetration than can be provided by energetic electron beams. Moreover, the increased conversion efficiency makes 7.5 MeV X-ray irradiation competitive to gamma irradiation in terms of economic. The abilities to turn the radiation source on and off and to control the X-ray intensity are attractive features of an accelerator facility. Despite of the advantages, characterization studies and commercial application of 7.5 MeV X-ray irradiation to food commodities are still scarce. Therefore, further studies to evaluate microbiological, physicochemical, and toxicological characteristics of irradiated food by 7.5 MeV X-rays are needed.
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Table 1. Characteristics of three irradiation sources for food treatment
Characteristics
Irradiation sources
X-ray Gamma-ray E-beam
Radiation source Electric machine Radionuclide Electric machine
Energy type Photon Photon Electron (particle)
Maximum energy 7.5 MeV 1.33 MeV 10 MeV
Dose rate 24 kGy/h 1 kGy/h 8000 kGy/h
Penetration depth
(double sided) 38 g/cm2 31 g/cm2 8 g/cm2
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