Characteristics of food irradiated by 7.5 MeV X-ray

Abstract

Food irradiation, which exploits the microbicidal effect of ionizing radiation, is used in many countries to improve the microbiological safety and to extend the shelf-life of food. Among different ionizing radiations including X-rays, gamma rays, and electron-beams (e-beams), 7.5 MeV X-rays generated when electrons with 7.5 MeV energy collide with a metal target have been newly suggested as an alternative food pasteurization process to overcome the application limitation from disadvantages of conventional irradiation sources such as low energy efficiency of 5 MeV X-rays, low consumer acceptance for gamma rays, and low penetration power of e-beams. To evaluate the availability of food irradiation using 7.5 MeV X-rays, characteristics of 7.5 MeV X-ray-irradiated foods must be investigated. Therefore, this study was conducted to evaluate the microbiological, physicochemical, and toxicological characteristics of model foods (red pepper powder, chicken breast meat, and ground beef) irradiated with 7.5 MeV X-rays, which have greater conversion efficiency than 5 MeV X-rays and higher penetration power than both gamma rays and e-beams. The D10-values, which are the irradiation doses required achieving a decimal reduction in the initial bacterial population, of 7.5 MeV X-rays were ranged from 0.11 to 0.21 kGy for pathogens including Escherichia coli, Salmonella Typhimurium, Listeria monocytogenes, and Staphylococcus aureus suspended in a buffer solution and 0.22–0.41 kGy for beef-inoculated bacteria. Moreover, no significant differences in bactericidal efficiency of 7.5 MeV X-rays, gamma rays, and 10 MeV e-beams were observed for the bacterial strains inoculated on ground beef. In addition, 7.5 MeV X-ray irradiation showed the inactivation efficiency of 3 log reductions in total aerobic bacteria in red pepper powder irradiated at approximately 6 kGy. The total aerobic bacteria in chicken breast meat and ground beef were below the detection limit of 1 log CFU/g when they were irradiated at more than 4 kGy. In the bacterial reverse-mutagen, in vitro chromosomal aberration, and in vivo micronucleus assays, the irradiated chicken breast meat with 7.5 MeV X-rays at 30 kGy exhibited dose-independent responses similar to those shown by the negative control. These results suggested that the irradiated samples were not genotoxic. In acute and sub-chronic toxicity studies, mortality or any abnormal clinical signs of ICR mice were not observed during the test periods. Several hematological and serum biochemical parameters of ICR mice showed significant differences from the values in the control group

however, those values were within the normal range for hematological and serum biochemical parameters of ICR mice. No specific toxic effects were observed in male and female ICR mice upon single oral administration of X-ray-irradiated (30 kGy) chicken breast at up to 2000 mg/kg body weight. Furthermore, daily intake of X-ray-irradiated chicken breast at 2500 mg/kg body weight for 90 days did not cause in any toxicological effects on the male or female mice. Therefore, these results revealed that chicken breast irradiated with 7.5 MeV X-rays at 30 kGy was not toxic to mice under the tested conditions. Red pepper powder samples X-ray-irradiated at more than 8 kGy exhibited significantly more off-odor than did non-irradiated sample

however, there was no significant difference between non-irradiated and irradiated samples at less than 7 kGy, which is the maximum irradiation dose permitted in Korea. Characteristics of red pepper powder irradiated with 7.5 MeV X-rays including color, contents of capsaicinoids and capsanthin, and organoleptic properties except for off-odor exhibited no significant change upon an absorbed dose (p < 0.05). 2-thiobabituroinic acid reactive substance values of chicken breast meat and ground beef irradiated with 7.5 MeV X-rays increased as absorbed dose increased, whereas pH of meats did not exhibited significant changes, regardless of absorbed dose. Therefore, it is considered that 7.5 MeV X-ray-irradiated model foods at less than the upper dose limit did not exhibit quality deterioration induced by irradiation. The photo-stimulated luminescence photon counts for a minute (PCs/60 s) of red pepper powder irradiated at a dose less than 2 kGy were 700–5000 PCs/60 s, indicating the need for further confirmative analysis. In contrast, all samples irradiated at more than 4 kGy were correctly identified as irradiated (> 5000 PCs/60 s). In thermoluminescence (TL) analysis, TL ratios (TL1/TL2), which were calculated from the TL signal intensity of silicate mineral separated from red pepper powder irradiated with 7.5 MeV X-rays at 0.49–9.27 kGy (TL1) and the TL signal intensity of the TL1 mineral normalized with gamma rays at 1 kGy (TL2), of all the irradiated samples were over 0.1, resulting in positive results of all irradiated samples with 7.5 MeV at various doses. In X-ray-irradiated chicken breast meat, specific radiolytic hydrocarbons such as C16:2 and C17:1, which are used as markers for irradiated meat, were detected in the samples irradiated at > 4 kGy. Furthermore, 2-dodecylcyclobutanone and 2-tetradecylcyclobutanone, unique radiolytic products derived from lipids, were detected in all samples 7.5 MeV X-ray-irradiated at various doses and increased with a function of absorbed dose, whereas these were not detected in the non-irradiated sample. From the result, 7.5 MeV X-ray irradiation can be used as a food pasteurization process not providing the physicochemical quality deterioration induced by irradiation within the allowed upper limit dose. Moreover, it is elucidated that 7.5 MeV X-ray-irradiated food is toxicologically safe and can be supervised by using physical and chemical identification methods.