The very nature of the technology of irradiating food to reduce microbiological bio-burden and increase shelf-life has given rise to concerns about the effects of the process on the food itself. This article reviews the considerable body of experimental data that have been generated on the subject and shows that, at least at low doses, irradiation of food does not have a significant effect on the food and is harmless to the end-consumer.
by Prof. I. S. Arvanitoyannis and Dr A. Stratakos
Food irradiation is a process for the treatment of food products to enhance their shelf life and to improve microbial safety. A number of articles and books over the years have reviewed most aspects of food irradiation [1].
Sources of irradiation used in the food industry
The radionuclides approved for food irradiation include 137Cs and 60Co whose radiation can destroy rapidly growing cells but do not leave the product radioactive [2]. Another way to treat food is using e-beam irradiation, which can be produced by accelerating electrons so that high energy, accelerated electrons are propelled in a stream out of an electron gun. In e-beam radiation, no radioactive source is required to produce accelerated electrons. The main advantage of e-beam irradiation is that the electrons can penetrate 5-10 cm into food. Moreover, X-rays can be produced by accelerating electrons into a thin metal plate [3].
Effect of irradiation on lipids
In response to the continuously growing role of irradiation in food preservation, several reviews and research studies have been published on the irradiation of foods of both animal and plant origin over the past years [4,5,6].
The application of ionizing radiation results in the radiolysis of water, which is present in most foods such as meat and fish products. This triggers the development of species such as OH−, hydrated electron and H+, which can then induce several chemical reactions with food constituents. Studies show that the quantity of radiolysis products varies as a function of fat content and fat composition, as well as with the temperature during the irradiation process and the actual dose of radiation used [7]. When fatty acids are exposed to high-energy radiation they undergo preferential cleavage in the ester-carbonyl region giving rise to certain radiolytic compounds that are specific for each fatty acid [8]. The strong oxidizer ozone is produced from oxygen during food irradiation and can promote the
oxidization of lipids and myoglobin [9].
Many research studies have been carried out in recent years on meat and fish irradiation and its impact on lipids. Experiments carried out on chicken revealed no significant difference in total saturated and unsaturated fatty acids between irradiated (1, 3, 6 kGy) and non-irradiated frozen (−20°C) chicken muscle [10]. Other studies showed that e-beam irradiation (2.5 kGy) seemed to increase the levels of thiobarbituric acid-reactive substances (TBARS) in ground beef, but the difference between irradiated and non-irradiated samples was not statistically significant [11]. The results of Yilmaz and Gecgel [12] showed that irradiation in ground beef induced the formation of trans fatty acids. However, the ratio of total unsaturated fatty acids to total saturated fatty acids was 0.85, 0.86, 0.87, and 0.89 in irradiated ground beef samples (1, 3, 5, and 7 kGy, respectively) whereas for the control samples it was 0.85. Fish lipids are more unsaturated than lipids in red meats and therefore are more susceptible to oxidation [13]. An examination of the effect of irradiation at 10 kGy on the linoleic and linolenic acid contents of grass prawns found that irradiation resulted in 16% decrease in linoleic acid content, whereas linolenic acid was not affected significantly [14]. In the case of Spanish mackerel, C16:0 and C16:1 fatty acids decreased when irradiated at 1.5 to 10 kGy [15]. No changes were reported in the fatty acid composition of two species of Australian marine fish irradiated at doses up to 6.0 kGy [16], and the levels of fatty acids in oil remained stable in the irradiated fish samples whereas they decreased in non-irradiated fish. The extent of lipid oxidation was dependent on the irradiation dose. An analysis of the literature concluded that when lipids are irradiated under conditions which are met in commercial food processing (≤7 kGy), there is no significant loss of nutritional value [17].
Effect of irradiation on proteins and amino acids
Damage caused to protein by ionizing radiation includes deamination, decarboxylation [18], reduction of disulfide linkages, oxidation of sulfydryl groups, cleavage of peptide bonds and changes of valency states of the coordinated metal ions in enzymes [19]. Other studies indicated that there was no significant destruction of cystine, methionine and tryptophan up to a dose of 71 kGy [20]. The majority of amino acids in minced lean beef or pork and chicken breast muscle are stable up to a dose of 5 kGy [21]. Irradiation does not generally affect the stability of amino acids and proteins in situ. The stability to irradiation at 2 to 45 kGy of tryptophan of shrimp muscle was measured after storage under different temperature and moisture conditions. The results revealed that the loss of tryptophan was small under all the conditions applied [22]. Essential amino acids were not affected in electron- beam processed (53 kGy) haddock fillets [2]. Data obtained from the literature indicate that irradiation of meat at commercial doses (2–7 kGy) has no significant effect on the nutritional value of proteins or amino acids [23].
Effect of irradiation on vitamins
Many authors have studied the effect of irradiation on the stability of vitamins in foods [24]. No loss of riboflavin is found in pork chops and chicken breasts irradiated at temperatures between −200°C and 200°C at doses up to 6.6 kGy. Some irradiated samples even exhibited an increase in riboflavin concentration of up to 25% [25]. Pork chops irradiated at different temperatures with doses up to 5 kGy displayed no loss in niacin. A loss of 15% was observed with a dose of 7 kGy when irradiation was applied at 0°C [26]. Furthermore, in the case of pantothenic acid, it has been shown that there is no loss in many foods irradiated at doses of ≥10 kGy [27]. The application of gamma irradiation (1, 2, and 6 kGy) on fillets of Black Bream (Acanthopagrus australis) and Redfish (Centroberyx affinis) resulted in vitamin E loss but this could not be correlated with the treatment dosage. All irradiated fillets were found to have vitamin E muscle contents above the levels considered to be desirable for human consumption [16]. No loss of vitamin B12 was observed in haddock fillets irradiated up to 25 kGy. Similarly, there was no loss of niacin in cod irradiated at 1 kGy [28]. Irradiation of shrimps at 2.5 kGy induced a 15% loss of riboflavin in air, 8% in vacuum, and 20% in nitrogen [29].
Effect of irradiation on organoleptic characteristics
Textural alterations and development of off-flavors are not considered a problem with irradiation at doses lower than 2 kGy. Any sensory changes at lower radiation doses are similar to those associated with thermal processing [30]. It was found that e-beam irradiated (0 or 4.5 kGy) raw pork patties produced more volatiles than did non-irradiated patties, and the proportion of volatiles varied with the irradiation conditions [31]. Irradiation produced many unidentified volatiles that could be responsible for the off-odor in irradiated raw meat. The results of an experienced testing panel showed that there was no significant differences in odor and taste between irradiated (4 kGy) and non-irradiated ground beef patties (23% fat) during 7 days of storage at 4oC [32]. Irradiation at 2.5 kGy extended the shelf life of carp, but at doses above 2.5 kGy, the cooked meat had an unacceptable odor and flavor. Other studies showed that the color of brook char (Salvelinius fontinalis) was affected negatively by irradiation and the effect was more pronounced with 3 kGy than with 1 kGy treatment [33]. However, the flavor of fish was not affected by irradiation.
There are several methods that can be employed in order to decrease such detrimental effects of irradiation. These include oxygen exclusion, the replacement of oxygen with inert gases, the addition of protective agents such as antioxidants, and post-irradiation storage to allow the flavor to return to near-normal levels [34].
Effect of irradiation on microorganisms
A large amount of data is available on the sensitivity of microorganisms to irradiation processing; this varies greatly from micro-organism to micro-organism and is also dependent on other extrinsic factors. Vegetative cells are less resistant to irradiation than spores, whereas moulds have a susceptibility to irradiation similar to that of vegetative cells. However some fungi can be as resistant as bacterial spores [35]. Compared to bacteria, viruses generally require higher radiation doses for inactivation [36]. Studies have shown that irradiation doses of 2 and 3 kGy destroyed Yersinia spp. and Listeria spp., respectively, with the microorganisms being undetectable during storage of irradiated fish [39]. Irradiation (1, 2, and 3 kGy) significantly improved the microbiological quality of the chicken by reducing the total bacterial count (TBC), with the decrease in TBC being dose-dependent. In all the irradiated samples, no fecal coliforms were detected [38]. Table 1 provides a brief overview of some of the research studies on applying irradiation on foods of animal origin.
Conclusions
Although there has been a heated debate over the “harmlessness” of irradiation on food and whether irradiation should actually be applied, most studies reported that irradiation at lower than 10kGy is not dangerous for humans. However, it must also be stressed that consumers must be able to choose whether they would really like to consume an irradiated item. The latter is feasible only through the appropriate labeling of foods.
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The authors
Ioannis S. Arvanitoyannis and
Alexandros Stratakos
University of Thessaly
School of Agricultural Sciences
Department of Agriculture Icthyology & Aquatic Environment
Fytokou str.,
Nea Ionia Magnessias,
38446 Volos, Greece
