Irradiating food with γ-rays, X-rays and accelerated electrons can kill microorganisms responsible for both spoilage and foodborne illness. However, the high energy input, which results in the radiolytic cleavage of water and kills microbes, can also initiate oxidation of fats and break down of sulfur-containing amino acids resulting in off-odours and flavours.
by Professor S. Brewer
Irradiation
The potential for inactivation of foodborne and spoilage bacteria using irradiation (γ-rays, X-rays and accelerated electrons) has long been known. Soon after the discovery of X-rays (1896), patents were filed to use irradiation in order to improve the condition of food. Most food spoilage organisms are eliminated with a 1.0 kGy dose. A 1.5 kGy dose of irradiation can result in a 6-log reduction in E. coli O157:H7 [1]. Recently, Ramamoorthi, Toshkov and Brewer [2] reported that irradiating fresh (inoculated) beef at 1.0 – 1.5 kGy reduced initial total plate counts from >5 CFU/g to <2 CFU/g. These reductions were maintained over the 21 day storage period. Irradiation at 2.0 kGy resulted in no detectable microorganisms.
Raw meat, especially raw ground meat, has been the vector of a variety of pathogenic bacteria which has resulted in outbreaks of foodborne illness in recent years (e.g. E. coli O157:H7, Campylobacter). The maximum irradiation dose permitted for meat depends on the type (poultry vs. red meat), and form in which it irradiated (chilled vs. frozen). Irradiation of fresh and frozen poultry was approved in 1992 [3] and is published in the Code of Federal Regulations○ [4]. Irradiation of red meat was approved in 1997 [3]. Up to 4.5 kGy is permitted for uncooked, chilled red meat for pathogen reduction; up to 7.0 kGy is permitted for uncooked, frozen meat; up to 3.0 kGy is permitted for fresh or frozen poultry.
The radionuclides approved for food irradiation include 60Co and 137Cs. They decay to nonradioactive nickel and barium, respectively by emitting γ-rays and α or β particles. The γ-rays kill rapidly growing cells (microbes) but do not leave the product radioactive. γ-rays are highly penetrating, so they can be used to treat packaged food. High energy particles can be produced by ‘accelerating’ electrons using electricity, then propelling them out of an electron gun in a stream (e-beam). The electrons can penetrate 5-10 cm into food. Treating packaged food is advantageous in preventing post-processing contamination.
Development of off flavours and odours in meat
Irradiating fresh meat for safety, even at low doses, can result in off-odours and flavours (rotten egg, wet dog, blood, fish, barbecued corn, burnt, sulfur, metallic, alcohol, acetic acid). The odours produced vary depending on the type of meat, composition of the fat (highly saturated vs. polyunsaturated), temperature during irradiation, exposure of the product to oxygen during and/or after the irradiation process, packaging, and the presence or addition of antioxidants.
‘Flavour’ results from the combined effects of the basic tastes (salt, sour, bitter, sweet, umami) and odours, which are derived from substances originally present in the food product or are produced via various reactions [5]. The precursor compounds depend on the chemical composition of the fresh product. A wide array of flavour- and odour-active volatiles occur in meat (acids, alcohols, aldehydes, aromatic compounds, esters, ethers, furans, hydrocarbons, ketones, lactones, pyrazines, pyridines, pyrroles, sulfides, thiazoles). Compounds that elicit various tastes and odours have widely different thresholds for perception and a particular compound may taste/smell different at different concentrations. The ultimate flavours of meat products subjected to irradiation can vary widely. However, most of the irradiation-induced odour/flavour changes in meat are a result of lipid oxidation, breakdown of sulfur-containing amino acids or both.
Irradiation energy can cause atoms/molecules of biological materials to eject electrons. Ultimately, radiolysis of water by highly energised electrons into free radical species (•OH, •H, H30+, eaq- ) may be the initiator of both lipid oxidation and sulfur-containing volatiles responsible for irradiation odour. γ-rays can provide the activation energy required for radiolysis. Most chemical changes in irradiated meat are associated with free radical reactions. Hydroxyl radicals (•OH ) tend to react with conjugated systems and are often considered to be the initiators of lipid oxidation in muscle tissue. Unsaturated fatty acids (linoleic, linolenic, arachidonic) are of primary concern. They are electron-deficient at the carbonyl groups and at the carbon-carbon double bonds making them particularly likely to form free radicals. Once free radicals form, autoxidation then proceeds via traditional pathways [Figure 1]. Reducing the temperature during the irradiation process reduces free radical generation and dispersion reducing the effects on odour/flavour. Freezing increases the viscosity, which also reduces free radical dispersion.
Irradiation generally accelerates lipid oxidation, especially the unsaturated fatty acids [6]. The most common fatty acids occurring meat are oleic, linoleic, arachidonic, palmitic and stearic acids. Turkey and chicken dark meat contain similar amounts of linoleic acid (18:2; 1.75 and 1.87 g /100g lipid, respectively; Table 1) and substantially more than is found in beef (0.12 g), pork (0.30 g) and Atlantic salmon (0.67 g). The total amount of fatty acids 16:1, 18:1 and 18:2 are highest in chicken dark meat (5.33 g). It is similar in turkey white and dark meat, and Atlantic salmon (3.84, 3.34, and 3.49, respectively), while it is lower in beef and pork (1.54 and 1.82, respectively). These unsaturated fatty acids are the primary source
materials for lipid oxidation.
Irradiation can induce the formation of volatile compounds (1-heptene and 1-nonene) resulting in fatty, tallowy odours, and aldehydes (propanal, pentanal, hexanal) resulting in cooked, pungent, and grassy odours from the fat fraction of the meat. Increasing the irradiation dose increases the volatiles, while cooking reduces them. Once formed, aldehydes are generally influenced most by packaging type (aerobic vs. vacuum). Ramamoorthi, Toshkov, and Brewer [2] reported that the beef odour of raw beef irradiated over a range of doses (0.5 to 2.0 kGy) was higher initially and after seven days of refrigerated storage than after days 14 to 21 days. Aerobically packaged samples had lower beef odour scores than those in modified atmosphere containing carbon monoxide. Irradiation slightly decreased sour/acid odour. Storage time had a significant effect on acid/sour, grassy, sweaty and rancid odours. Acid/sour odour increased and rancid odour decreased on days 14 and 21. Grassy and sweaty odours increased on day 21. Changes in grassy, sweaty and rancid odour are indicative of oxidation, which often occurs over time. Loss of beef odour and increases in acid/sour odour can also be indicative of spoilage over time. Ultimately, they concluded that storage time was the major factor affecting odour, decreasing raw beef odour, and increasing acid, grassy, sweaty and rancid odour. Ramamoorthi, Toshkov, Tucker, Stetzer & Brewer [7] reported similar changes in acid/sour and rancid odours or irradiated beef.
Irradiation can also affect the protein fraction of meat. Aqueous electrons produced from radiolysis of water promote the release of sulfur from sulfur-containing compounds such as cysteine, methionine, glutathione, taurine, and thiamine in fresh meat [8].
Fishy, putrid odours are the result of formation of dimethyltrisulfide, while sulfurous odour is often the result of bismethylthiomethane. Sulfur-containing compounds generally have very low thresholds for human sensory detection; the odour detection threshold for 2-methyl-butanethiol is <0.0001 ppb. For this reason, these compounds can have significant negative impacts on aroma. The concentration of sulfur-containing amino acids in meat prior to irradiation might be expected to have significant effects on the odour after irradiation. When expressed in terms of total sulfur-containing amino acids (cysteine + methionine), chicken contains 1.58 g/100 g, about twice that beef, lamb, perch and salmon (0.87, 0.75 g , 0.97 and 0.89 g/100g, respectively; [9]).
The volatile compounds responsible for the off-odour in irradiated meat produced by the impact of radiation on protein and lipid molecules are different from those of lipid oxidation alone. Increasing lipid peroxidation products (especially hexanal) in combination with the loss of desirable meaty odourants (4-hydroxy-2,5-dimethyl-3(2H)-furanone) results in development of ‘warmed over flavour’, the stale flavour of re-heated meat. However, irradiation produces alkanes and alkenes that appear to be the result of both unsaturated fatty acid and amino acid breakdown producing the rotten egg, wet dog, bloody, fishy, barbecued corn, burnt, sulfur, metallic, alcohol, and acetic acid odours commonly attributed to irradiated meat.
The irradiation-induced oxidation increases as storage time increases. However, if the product is stored in an aerobic environment, the low molecular weight sulfur compounds, which are highly volatile, can dissipate - allowing the meat to regain some of its original flavour.
Prevention of adverse effects
Oxygen exclusion (vacuum packaging), replacement of oxygen with inert gases (nitrogen), addition of protective agents (antioxidants), and post-irradiation storage to allow flavour to return to near-normal levels (re-packaging or double packaging in oxygen permeable film), and their combinations, are effective methods to decrease the detrimental effects of irradiation.
References
1. Olsen DG. Irradiation of foods. Food technology 1998; 52 (1): 56-62.
2. Ramamoorthi L, Toshkov S and MS Brewer. Effects of carbon monoxide-modified atmosphere packaging and irradiation on E.coli K12 survival and raw beef quality. Meat Science 2009; 83: 358–365.
3. FSIS (Food Safety and Inspection Service). Irradiation of Meat Food Products. Federal Register 2009; 64. (264): 72150-72166. 9 CFR Parts 381 abd 424. Docket No. 97-076F.
4. Code of Federal Regulations. CFR (21 CFR 179.26). Federal Register, Washington DC.
5. Brewer MS. Irradiation effects on meat colour—a review. Meat Science 2004;
68: 1-17.
6. Ahn DU and Lee EJ. Mechanisms and prevention of off-odour production and colour changes in irradiated meat. Irradiation of food and packaging: recent developments. American Chemical Society Symposium Series 2004; 875: 43-76.
7. Ramamoorthi L, Toshkov S, Stetzer AJ, Tucker E and Brewer MS. Effects of antioxidants on irradiated beef colour. J Food Qual 2009 (In press).
8. Motohashi N, Mori I, Sugiura Y and Tanaka H. Modification of gamma-irradiation induced change in myoglobin by mercaptopropionylglycine and its related compounds and the formation of sulfmyoglobin. Radiation Research 1981; 86: 479-487.
9. Brewer MS. Irradiation effects on meat flavour: a review. Meat Science 2009; 81: 1-14.
The author
Susan Brewer
Professor of Food Science
Agricultural Bioprocess Laboratory
University of Illinois, USA.
Email: msbrewer@illinois.edu
