IONIZING RADIATION TREATMENT: GENERAL CONCEPT

Ionizing radiations consist of electromagnetic waves present at the most energetic end of the electromagnetic spectrum or subatomic particles, ions, and atoms that move at high speeds (Figure 1.4). The energy associated with ionizing radiations is sufficient to “ionize” the material that they pass through and it is able to move a large number of electrons of atoms and molecules converting them into charged particles, hence the term “ionizing radiation.”

Schematic representation of Ionizing radiation technology and the penetration properties of the different radiations

FIGURE 1.4 Schematic representation of Ionizing radiation technology and the penetration properties of the different radiations.

As stated by the codex general standard for irradiated foods, ionizing radiations recommended for use in food processing are the following: 1 2

  • 1. Gamma (y) Rays: The two radioactive isotopes (radionuclides), cobalt-60 or cesium-137, can produce highly penetrating (40 cm) y-rays, thus making possible to treat food in bulk or in the final packaging. Cobalt-60 is the radionuclide widely used for gamma irradiation of food (Liberty et al., 2013). The y-rays are emitted all the tune by radioactive substances. These y-ray sources, if they are not in use, are stored in such a way as to absorb the radiations so harmless and complete.
  • 2. X-Rays: These are produced by a machine that operates at an energy level equal to or less than 5 MeV. X-rays have shown to have a penetration depth into the product (20 cm) higher than electron-beam. They are generated by directing accelerated electrons towards a thin plate (gold or other metal), producing a stream of X-rays. X-rays are able to pass through thick foods and require heavy shielding for security.

In this case, the use of radioactive substances is not involved because the machine can be switched on and off easily (Liberty et al., 2013).

3. Accelerated Electrons (Electron-Beam): It consists of an accelerated electron system obtained at a level of energy equal to or less than 10 MeV. The main advantage of this ionizing radiation machine is that the whole processing system operates without any radioactive substance. Electron-beam machines can be turned on or off simply, they use linear accelerators to produce accelerated electron beams and no radioactive materials are involved in the process. However, the electron-beams have a limited penetration power (only to a depth of 8 cm) and are suitable only for foods relatively thin (Liberty et al., 2013).

The unit to measure the irradiation dose applied to a food product is kilograys (kGy). The gray (the basic unit) is the amount of irradiation energy that 1 kilogram of food receives. The maximum irradiation dose for food must not exceed 10 kGy (CODEX STAN 106-1983, REV. 1-2003).

MECHANISMS OF ACTION ON MICROBIAL INACTIVATION

Microbial inactivation occurs through the direct or indirect effect of ionizing radiation. In the first one, the ionizing radiation impairs the DNA of targeted microbes, thus preventing their cell division (Farkas et al., 2014). The indirect effect takes place because of the interaction of ionizing radiation with water (H,0) molecules that lose an electron, producing hydroxyl radicals and hydrogen peroxide (H,0,), which subsequently can provoke cell lysis (Tahergorabi et al., 2012). The susceptibility of microorganisms to irradiation differs greatly between them. Compared to vegetative cells the spores are more resistant, whereas molds are sensible to the irradiation process such as vegetative cells. Some fungi seem to be resistant like bacterial spores, while viruses generally require higher radiation doses for inactivation (Farkas, 2006). The efficacy of ionizing radiation for the inactivation of microbes mainly depends on the dose and the level of resistance of the microorganisms. However, pH, temperature, presence of oxygen, and solute concentration can also influence microbial inactivation. It was noticed that the mentioned factors could also have a correlation with the radiolytic products that are formed during the irradiation (Liberty et al., 2013).

This treatment can bring benefits to consumers in tenns of convenience and quality, prolongation of food shelf life, and greater food safety. This technology does not induce radioactivity in foods or packaging materials. Furthermore, ionizing radiation does not affect the sensory and nutritional characteristics of the treated products, and the treatment causes practically no temperature rise in the product and in packaging materials. The irradiation treatment can be performed after packaging, thus overcoming the problem of post-processing recontamination. Among the disadvantages, there is a selective loss of vitamins, comparable to those seen in other forms of storage (Dionisio et al., 2009). This technology at doses within the practical limits is not able to inactivate completely viruses, enzymes, and microbial toxins (Liberty et al., 2013).

APPLICATION OF IONIZING RADIATION TREATMENT TO DAIRY SECTOR

Ionizing radiations are an effective means to destroy both pathogenic and non-pathogenic bacteria, parasites, and in a lesser degree virus. Some reports highlighted on ‘off-flavors development in irradiated daily products,’ but better results have been noted by irradiating them with lower doses or in frozen conditions without affecting any sensory properties (Odueke et al., 2016). Encouraging results in gamma irradiation applied to daily food has been described by Hashisaka et al. (1989), Bougie and Stahl (1994), and Ennahar et al. (1994). These results were also corroborated by more recent studies. Mahmoud (2009) evaluated the microbial inactivation efficacy of X-rays on Cronobacter (E. sakazakii) in different samples (low-fat milk, skim milk, tryptic soy broth, and whole-fat milk). In the study, samples were inoculated with the Cronobacter and were treated with different X-rays doses (0.1, 0.5, 0.75, 1.0, 2.0, 3.0, 4.0, 5.0 and 6.0 kGy). Overall, a reduction of more than 7 logs in the Cronobacter population was observed. In some countries such as France and the Czech Republic, it is allowed to treat with irradiation casein and caseinates (dose of 3 kGy), while in Croatia, dried milk products are irradiated at a maximum dose of 3kGy (Odueke et al., 2016). The occurrence of Bacillus cereus, Listeria, Salmonella, and Yersinia in the ice cream was well documented (Walker et al., 1990; Farber and Peterkin, 1991). In some studies it was proved that irradiation can be an efficient method to reduce or eliminate microbial growth in ice cream, 1 kGy was sufficient (Kamat et al., 2000; Kim et al., 2005). Yogurt is a fermented daily food with a shelf life usually of 3 weeks, considered safe for the starter cultures and the low pH. The presence of health-promoting microorganisms can compromise the health of immunocompromised patients. For this reason, Ham et al. (2009) evaluated the microbial quality, sensory properties, and allergens change of gamma-irradiated plain yogurt (1, 3, 5, and 10 kGy) stored at different temperatures viz. abuse storage, refrigerated, and room temperature. This study suggested that the irradiation did not affect the chemical and sensory quality of plain yogurt. Furthermore, the treatment can extend the shelf life and reduce allergens of yogurt, providing a safer product. As regard to fresh cheese, research-documented by Blank et al. (1992) that describe Cheddar cheese slices inoculated with Penicillium cyclopium and irradiated (dose 0.21 and 0.52 kGy), shown a shelf life prolongation of 3 and 5.5 days, respectively. In addition, under the same treatment conditions inoculating the cheeses with Aspergillus ochraceus, the authors observed a shelf-life increase of irradiated samples compared to the untreated control. Tsiotsias et al. (2002) in their study evaluated the feasibility of gamma radiation for controlling L. monocytogenes on soft whey cheese. The samples were treated at doses of 0.5, 2.0, and 4.0 kGy. The effects in the irradiated samples were very interesting on molds and Enterobacteriaceae, less effective on yeast population. A reduction of aerobic mesophilic bacteria was also recorded. Further, it was determined by the authors that the irradiation at doses up to 4 kGy may be used to control L. monocytogenes without any effect on sensoiy quality. The effectiveness of electron-beam irradiation on mozzarella cheese was also investigated using five irradiation doses (0.55, 0.81, 1.51, 2.0, 2.5 kGy). The results of this study have shown shelf-life increases. Furthermore, the authors indicated that the electron-beam irradiation may inhibit the spoilage without affecting the sensory quality (Huo et al., 2013).

 
Source
< Prev   CONTENTS   Source   Next >