Radiation Protection of Foods, and Nature of Microbial Radiation Resistance 383 The irradiation of steaks at 1.5 kGy inoculated with ∼105 /g of Escherichia coli 0157:H7 resulted in complete elimination of cells.21 Yersinia enterocolitica was reduced to undetectable levels under the same conditions Irradiation of mechanically deboned chicken meat that was inoculated with ∼400 spores of 20 strains of C botulinum types A and B at 1.5 or 3.0 kGy resulted in no samples becoming demonstrably toxic after refrigerated storage for four weeks, but samples that were temperature abused at 28◦ C became toxic within 18 hours.79 In a similar product, an initial level of Salmonella Enteritidis 3.86 log10 /g was reduced to 2 kGy.17 The O2 content of the 3◦ C-stored samples ranged from 1.5 to 3.7% after day 5, and the CO2 content was higher than in non-irradiated controls In a study of the effect of irradiation on the destruction of a 5-strain cocktail of L monocytogenes in six different ready-to-eat meats at doses of 2.0 and 4.0 kGy followed by storage at and 10◦ C, no cells could be detected when 4.0 kGy was used.20 With 2.0 kGy and 10◦ C, survivors were recovered after the second week but with 2.0 kGy-treated products stored at 4◦ C, survivors were recovered after weeks Radurization Irradiation treatments to extend the shelf life of seafoods, vegetables, and fruits have been verified in many studies The shelf life of shrimp, crab, haddock, scallops, and clams may be extended from 384 Modern Food Microbiology twofold to sixfold by radurization with doses from to kGy Similar results can be achieved for fish and shellfish under various conditions of packaging.59 In one study, scallops stored at 0◦ C had a shelf life of 13 days, but after irradiation doses of 0.5, 1.5, and 3.0 kGy, shelf life was 18, 23, and 42 days, respectively.63 The Gram-negative non-spore-forming rods are among the most radiosensitive of all bacteria, and they are the principal spoilage organisms for these foods Following the irradiation of vacuum-packaged ground pork at 1.0 kGy and storage at 5◦ C for days, 97% of the irradiated biota consisted of Gram-positive bacteria, with most being coryneforms.15 The Gram-negative coccobacillary rods belonging to the genera Moraxella and Acinetobacter have been found to possess degrees of radiation resistance higher than for all other Gram negatives In studies on ground beef subjected to doses of 272 krad, Tiwari and Maxcy86 found that 73–75% of the surviving biota consisted of these related genera In unirradiated meat, they constituted only around 8% of the biota Of the two genera, the Moraxella spp appeared to be more resistant than Acinetobacter spp., with D10 values of 0.539 and 0.583 kGy, whereas the D10 for M osloensis strains was 477 up to 1,000 krad (0.477–1.0 kGy) In comparing the radiosensitivity of some non–spore-forming bacteria in phosphate buffer at −80◦ C, Anellis et al.2 found that Deinococcus radiodurans survived 18 kGy, Enterococcus faecium strains survived 9–15 kGy, E faecalis survived 6–9 kGy, and Lactococcus lactis did not survive kGy Staphylococcus aureus, Lactobacillus casei, and Lactobacillus arabinosus did not survive 3-kGy exposures It was shown that radiation sensitivity decreased as the temperature of irradiation was lowered, as is the case for endospores The ultimate spoilage of radurized, low-temperature-stored foods is invariably caused by one or more of the Acinetobacter-Moraxella or lactic acid types noted above The application of 2.5 kGy to ground beef destroyed all pseudomonads, Enterobacteriaceae, and Brochothrix thermosphacta; and reduced aerobic plate counts (APCs) from 6.18/g to 1.78 log10 /g, but reduced lactic acid bacteria only by 3.4 log/g.57 Radurization of fruits with doses of 2–3 kGy brings about an extension of shelf life of at least 14 days Radurization of fresh fruits is permitted by at least six countries, with some meats, poultry, and seafood permitted by several others (Table 15–5) In general, shelf-life extension is not as great for radurized fruits as for meats and seafood because molds are generally more resistant to irradiation than the Gram-negative bacteria that cause spoilage of the latter products When ground beef patties were subjected to 2.0 kGy under vacuum, they remained unspoiled after 60 days in the refrigerator.55 In another study, unirradiated ground beef patties that originally contained 106 APC/g contained 108 /g after days at 4◦ C, but the samples that were irradiated at kGy (range 1.9–2.4) reached only 106 /g after 55 days at 4◦ C.56 In regards to pathogens in ground beef, it was concluded that an applied dose of 2.5 kGy would be sufficient to destroy 108.1 E coli 0157:H7, 103.1 salmonellae, and 1010.6 Campylobacter jejuni.7 Insect eggs and larvae can be destroyed by kGy, and cysticerci of the pork tapeworm (Taenia solium) and the beef tapeworm (T saginata) can be destroyed with even lower doses, with cysticercosis-infested meat being rendered free of parasites by exposure to 0.2–0.5 kGy.92 LEGAL STATUS OF FOOD IRRADIATION At least 40 countries had approved the irradiation of some foods as of mid-1989.47 At least 20 different food–packaging materials have been approved by the U.S Food and Drug Administration (FDA) at levels of 10 or 60 kGy In 1983, the FDA permitted spices and vegetable seasonings to be irradiated up to 10 kGy (U.S Federal Register, July 15, 1983) In 1985 the FDA granted permission for the irradiation of pork at up to kGy to control Trichinella spiralis (U.S Federal Register, July 22, Radiation Protection of Foods, and Nature of Microbial Radiation Resistance 385 1985) In 1986, fermented pork sausage (Nham) was irradiated in Thailand at a minimum of 2.0 kGy, and the product was sold in Bangkok.47 Puerto Rican mangoes were irradiated in 1986 at up to 1.0 kGy, flown to Miami, Florida, and sold Hawaiian papayas were treated at doses of 0.41–0.51 kGy to control pests in 1987 and later sold to the public USDA approval was granted for Hawaiian papayas in 1989 for insect control In May 1990, the USDA approved the irradiation of poultry up to 3.0 kGy, and on September 2, 1993, irradiated poultry was sold in a retail grocery store in Illinois for the first time.65 Strawberries were irradiated at 2.0 kGy and sold in Lyon, France, in 1987, and in the United States on January 25, 1992, in the state of Florida In 1995, the states of Maine and New York repealed their bans on the sale of irradiated foods Sprout inhibition and insect disinfestations continue to be the most widely used direct applications of food irradiation In 1981, a joint Food and Agriculture Organization (FAO)/International Atomic Energy Agency (IAEA)/WHO Expert Committee on food irradiation found that foods given an overall average of up to 10.0 kGy were unconditionally safe At least 40 countries have approved irradiation of one or more food products, and 29 are using food irradiation commercially For the control of salmonellae in animal feed and pet foods in the United States, 2–25 kGy was approved in 1995; and, in 1997, 4.5 kGy was approved for refrigerated raw, and 7.5 kGy for frozen, raw ground beef In the early 1970s, Canada approved for test marketing a maximum dose of 1.5 kGy for fresh cod and haddock fillets In 1983, the Codex Alimentarius Commission suggested 1.5 or 2.2 kGy for teleost fish and fish products.19 One of the obstacles to getting food irradiation approved on a wider scale in the United States is the way irradiation is defined It is considered an additive rather than a process, which it is This means that irradiated foods must be labeled as such Another area of concern is the fate of C botulinum spores (see below), and yet another is the concern that nonpathogens may become pathogens or that the virulence of pathogens may be increased after exposure to subradappertization doses There is no evidence that the latter occurs.74 As of the first quarter of 2003, over 7,000 supermarkets and other retail stores in the United States were selling irradiated ground beef, according to the American Council on Science and Health Food irradiation has been approved by the American Medical Association, the American Dietetic Association, the Institute of Food Technologists, and the United Nations The U.S Centers for Disease Control and Prevention has estimated that if only one-half of the ground beef, pork, poultry, and processed luncheon meats in the U.S were irradiated, there would be over 880,000 fewer cases of foodborne illness (Food Protection Trends, July 2003) An agency of the U.S government has approved the use of irradiated ground beef for school disricts, and this product is to be made available by the fall of 2004 When low-acid foods are irradiated at doses that not effect the destruction of C botulinum spores, legitimate questions about the safety of such foods are raised, especially when they are held under conditions that allow for growth and toxin production Because these organisms would be destroyed by radappertization, only products subjected to radicidation and radurization are of concern here In regard to the radurization of fish, Giddings22 has pointed out that the lean whitefish species are the best candidates for irradiation, whereas high-fat fishes such as herring are not, because they are more botulogenic This investigator noted that when botulinal spores are found on edible lean whitefish, they occur at less than 1/g EFFECT OF IRRADIATION ON FOOD QUALITY The undesirable changes that occur in certain irradiated foods may be caused directly by irradiation or indirectly as a result of post irradiation reactions Water undergoes radiolysis when irradiated in the 386 Modern Food Microbiology Table 15–6 Methods for Reducing Side Effects in Foodstuffs Exposed to Ionizing Radiations Method Reasoning Reducing temperature Reducing oxygen tension Addition of free-radical scavengers Concurrent radiation distillation Reduction of dose Immobilization of free radicals Reduction of numbers of oxidative free radicals to activated molecules Competition for free radicals by scavengers Removal of volatile off-flavor, off-odor precursors Obvious Source: Goldblith.23 following manner: radiolysis 3H2 O −−−−−→ H + OH + H2 O2 + H2 In addition, free radicals are formed along the path of the primary electron and react with each other as diffusion occurs.13 Some of the products formed along the track escape and can then react with solute molecules By irradiating under anaerobic conditions, off-flavors and off-odors are somewhat minimized due to the lack of oxygen to form peroxides One of the best ways to minimize off-flavors is to irradiate at subfreezing temperatures.88 The effect of subfreezing temperatures is to reduce or halt radiolysis and its consequent reactants Other ways to reduce side effects in foodstuffs are presented in Table 15–6 Other than water, proteins and other nitrogenous compounds appear to be the most sensitive to irradiation effects in foods The products of irradiation of amino acids, peptides, and proteins depend on the radiation dose, temperature, amount of oxygen, amount of moisture present, and other factors The following are among the products reported: NH3 , hydrogen, CO2 , H2 S, amides, and carbonyls With respect to amino acids, the aromatics tend to be more sensitive than the others and undergo changes in ring structure Among the most sensitive to irradiation are methionine, cysteine, histidine, arginine, and tyrosine The amino acid most susceptible to electron-beam irradiation is cystine Johnson and Moser34 reported that about 50% of this amino acid was lost when ground beef was irradiated Tryptophan suffered a 10% loss, whereas little or no destruction of the other amino acids occurred Amino acids have been reported to be more stable to gamma irradiation than to electron-beam irradiation Several investigators have reported that the irradiation of lipids and fats results in the production of carbonyls and other oxidation products such as peroxides, especially if irradiation and/or subsequent storage take(s) place in the presence of oxygen The most noticeable organoleptic effect of lipid irradiation in air is the development of rancidity It has been observed that high levels of irradiation lead to the production of “irradiation odors” in certain foods, especially meats Wick et al.94 investigated the volatile components of raw ground beef irradiated with 20–60 kGy at room temperature and reported finding a large number of odorous compounds Of the 45 or more constituents identified by these investigators, there were 17 containing sulfur-, 14 hydrocarbons, and carbonyls, and or more were basic and alcoholic in nature The higher the level of irradiation, the greater is the quantity of volatile constituents produced Many of these constituents have been identified in various extracts of nonirradiated, cooked ground beef Radiation Protection of Foods, and Nature of Microbial Radiation Resistance 387 With regard to B vitamins, Liuzzo et al.46 found that levels of 60 Co irradiation between and kGy effected partial destruction of the following B vitamins in oysters: thiamine, niacin, pyridoxine, biotin, and B12 Riboflavin, pantothenic acid, and folic acid reportedly increased by irradiation, probably owing to the release of bound vitamins Overall, the reported effects on water-soluble vitamins are not striking.84 In addition to flavor and odor changes produced in certain foods by irradiation, certain detrimental effects have been reported for irradiated fruits and vegetables One of the most serious is the softening of these products caused by the irradiation–degradation of pectin and cellulose, the structural polysaccharides of plants This effect was shown by Massey and Bourke49 to be caused by radappertization doses of irradiation Ethylene synthesis in apples is affected by irradiation so that this fruit fails to mature as rapidly as nonirradiated controls.49 In green lemons, however, ethylene synthesis is stimulated upon irradiation, resulting in a faster ripening than in controls.50 Among radiolytic products that develop upon irradiation are some that are antibacterial when exposed in culture media When 15 kGy were applied to meats, however, no antimicrobial activity was found in the meats.12 The overall wholesomeness and toxicology of irradiated foods have been reviewed.76,85 STORAGE STABILITY OF IRRADIATED FOODS Foods subjected to radappertization doses of ionizing radiation may be expected to be as shelf stable as commercially heat-sterilized foods There are, however, two differences between foods processed by these two methods that affect storage stability: Radappertization does not destroy inherent enzymes, which may continue to act, and some postirradiation changes may be expected to occur Employing 45 kGy and enzyme-inactivated chicken, bacon, and fresh and barbecued pork, Heiligman31 found the products to be acceptable after storage for up to 24 months Those stored at 70◦ F were more acceptable than those stored at 100◦ F Licciardello et al reported the effect of irradiation on beefsteak, ground beef, and pork sausage held at refrigerator temperatures for 12 years.45 These foods were packed with flavor preservatives and treated with 10.8 kGy The investigators described the appearance of the meats as excellent after 12 years of storage A slight irradiation odor was perceptible, but was not considered objectionable The meats were reported to have a sharp, bitter taste, which was presumed to be caused by the crystallization of the amino acid tyrosine The free amino nitrogen content of the beefsteak was 75 and 175 mg%, respectively, before and after irradiation storage; and 67 and 160 mg% before and after storage, respectively, for hamburger Foods subjected to radurization ultimately undergo spoilage from the surviving biota if stored at temperatures suitable for growth of the organisms in question The normal spoilage biota of seafoods is so sensitive to ionizing radiations that 99% of the total biota of these products is generally destroyed by doses on the order of 2.5 kGy Ultimate spoilage of radurized products is the property of the few microorganisms that survive the radiation treatment For further information on all aspects of food irradiation, see references 66 and 89 NATURE OF RADIATION RESISTANCE OF MICROORGANISMS The most sensitive bacteria to ionizing radiation are Gram-negative rods such as the pseudomonads; the coccobacillary-shaped Gram-negative cells of moraxellae and acinetobacters are among the ... FOODS Foods subjected to radappertization doses of ionizing radiation may be expected to be as shelf stable as commercially heat-sterilized foods There are, however, two differences between foods... STATUS OF FOOD IRRADIATION At least 40 countries had approved the irradiation of some foods as of mid-1989.47 At least 20 different food? ??packaging materials have been approved by the U.S Food and... are using food irradiation commercially For the control of salmonellae in animal feed and pet foods in the United States, 2–25 kGy was approved in 1995; and, in 1997, 4.5 kGy was approved for refrigerated