Irradiation of Food — Helping to Ensure Food Safety
http://www.100md.com
《新英格兰医药杂志》
In this issue of the Journal, Osterholm and Norgan (pages 1898–1901) present a convincing argument that physicians and other health care professionals, as health advocates, should also be advocates for the irradiation of foods to prevent the transmission of infection. The recent approval of irradiated hamburgers for school lunch programs in the United States has been met with unfounded claims by groups opposed to food irradiation that children are being used as experimental animals. Unfortunately, this campaign has influenced some school boards to deny their students the increased safety of irradiated foods.
Research on methods to control foodborne pathogens and the safety of irradiated foods has a long history. In 1896, Franz Minck suggested in the Munich Medical Journal that x-rays might have value as therapy for disease. Then Alan B. Green reported in the 1904 Proceedings of the Royal Society that radiation from radium inactivated Staphylococcus aureus, Vibrio cholerae, and Bacillus anthracis. In 1905, a patent was issued in Britain to the merchant J. Appleby for the use of ionizing radiation to improve the condition of foodstuffs. In 1918, in Tampa, Florida, David Gillett patented a device that used a bank of 16 x-ray tubes to preserve organic materials. He specifically suggested that it be used for the inactivation of trichinae in pork, and in 1921, a scientist at the U.S. Department of Agriculture established that encysted trichinae could be inactivated by means of x-rays. In 1927, J.K. Narat conducted perhaps the first toxicologic study of irradiated food, discovering effects in mice that were eventually attributed to vitamin deficiencies in the irradiated feed, rather than to irradiation itself. Irradiated animal feeds are now used routinely during toxicologic studies of drugs.
Research accelerated during the 1950s, with the development of commercial equipment and facilities for irradiation. Further study during the past 50 years has identified many of the factors that make the process more or less effective in controlling pathogens, as well as the effects of irradiation on nutrients and on the sensory properties of foods. Toxicologic studies have been conducted with both foods that have been pasteurized through irradiation and shelf-stable foods that have been sterilized by means of irradiation. For example, no evidence of toxic genetic or teratogenic effects has been found in mice, hamsters, rats, or rabbits that were fed radiation-sterilized chicken meat as 35 or 70 percent of their total diet (as measured on a dry-weight basis). Nor were any treatment-related abnormalities or changes observed in dogs, rats, or mice that were fed the radiation-sterilized chicken as 35 percent of their total diet during multigenerational studies. The 46-kGy dose used for sterilization in these studies far exceeded the doses used to pasteurize products such as ground beef, for which the Food and Drug Administration has set a maximum of 4.5 kGy (7 kGy for frozen beef).
Research and development have continued, and today, accelerated electrons and gamma and x-ray photons are used both in the treatment of patients and to sterilize many therapeutic products. Many people are unaware that radiation is used to sterilize or treat many of the products that they use in their own homes, such as baby-bottle nipples, personal-hygiene products, cosmetics, bandages, polymerized flooring materials, Teflon-coated skillets, and insulation on electrical wire. Most spices are contaminated with 1 million or more bacteria per gram, so many commercial facilities irradiate spices. Unfortunately, irradiated foods are in limited supply in the United States, although our astronauts have been eating steaks sterilized with 45 kGy of gamma radiation since 1960.
The radiation applied to food is much more limited than that used in radiotherapy. Only two isotopic sources of gamma rays have been approved for use — cobalt-60 and cesium-137. Electron energies are limited to a maximal acceleration of 10 MeV, and x-rays generated by the electron bombardment of a metal such as tungsten are limited to 5 MeV. None of these types of radiation are capable of generating radioactivity. The choice of the most appropriate form of technology is largely dependent on the product. Electrons with a maximal energy of 10 MeV penetrate to a depth of only 4.5 cm in water or equivalent, limiting their use to thin packages or to products with very low density; however, the required dose of radiation is delivered extremely quickly. The generation of x-rays is not very efficient, since only 6 to 12 percent of the electron energy is converted to x-rays; the remainder generates heat, which must be removed before the target melts.
The absorption of electrons or of photons produces the same effect, ionization. When a gamma or x-ray photon is absorbed, an electron is released, causing ionization. Water is the principal target for the radiation, because it is the largest component of most foods and microorganisms. Normally, approximately 70 percent of the radiation-induced ionization will occur in cellular water, and the target organisms will be inactivated because of secondary reactions, not because of a direct effect on the bacterial DNA. The same sequence occurs in frozen products, but the ice structure limits the migration of the free radicals that are generated by the ionization; therefore, a higher dose of radiation is required for frozen foods. There is much greater potential to produce adverse sensorial effects in fresh products than in frozen products.
The doses of radiation that are required to inactivate 99.9 percent of a contaminating population of a few important foodborne pathogens in meat and poultry are listed in the Table. The dose required to inactivate 99.9 percent of Escherichia coli O157:H7 in ground beef increases from approximately 0.90 kGy at 5°C to 1.35 kGy at –5°C. Food irradiation may offer the only reliable method of controlling foodborne pathogens in ground meat or poultry without cooking. Unfortunately, a high proportion of the poultry we bring into our homes or commercial kitchens remains contaminated with one or more of the pathogens listed in the Table. Cooking will kill most of these pathogens, but the problems associated with the cross-contamination of other foods remain. Some restaurants are now using irradiated poultry to prevent such contamination, and the public would benefit from greater implementation of this method of ensuring the safety of foods.
Table. Dose of Radiation Required to Inactivate 99.9 Percent of Foodborne Pathogens in Meat or Poultry with Electron, Gamma, or X-Ray Ionizing Radiation at 5°C.
Dr. Thayer reports having received consulting or lecture fees from CFC Logistics, Master Foods, and Zero Mountain.
Source Information
From Lower Gwynedd, Pa.(Donald W. Thayer, Ph.D.)
Research on methods to control foodborne pathogens and the safety of irradiated foods has a long history. In 1896, Franz Minck suggested in the Munich Medical Journal that x-rays might have value as therapy for disease. Then Alan B. Green reported in the 1904 Proceedings of the Royal Society that radiation from radium inactivated Staphylococcus aureus, Vibrio cholerae, and Bacillus anthracis. In 1905, a patent was issued in Britain to the merchant J. Appleby for the use of ionizing radiation to improve the condition of foodstuffs. In 1918, in Tampa, Florida, David Gillett patented a device that used a bank of 16 x-ray tubes to preserve organic materials. He specifically suggested that it be used for the inactivation of trichinae in pork, and in 1921, a scientist at the U.S. Department of Agriculture established that encysted trichinae could be inactivated by means of x-rays. In 1927, J.K. Narat conducted perhaps the first toxicologic study of irradiated food, discovering effects in mice that were eventually attributed to vitamin deficiencies in the irradiated feed, rather than to irradiation itself. Irradiated animal feeds are now used routinely during toxicologic studies of drugs.
Research accelerated during the 1950s, with the development of commercial equipment and facilities for irradiation. Further study during the past 50 years has identified many of the factors that make the process more or less effective in controlling pathogens, as well as the effects of irradiation on nutrients and on the sensory properties of foods. Toxicologic studies have been conducted with both foods that have been pasteurized through irradiation and shelf-stable foods that have been sterilized by means of irradiation. For example, no evidence of toxic genetic or teratogenic effects has been found in mice, hamsters, rats, or rabbits that were fed radiation-sterilized chicken meat as 35 or 70 percent of their total diet (as measured on a dry-weight basis). Nor were any treatment-related abnormalities or changes observed in dogs, rats, or mice that were fed the radiation-sterilized chicken as 35 percent of their total diet during multigenerational studies. The 46-kGy dose used for sterilization in these studies far exceeded the doses used to pasteurize products such as ground beef, for which the Food and Drug Administration has set a maximum of 4.5 kGy (7 kGy for frozen beef).
Research and development have continued, and today, accelerated electrons and gamma and x-ray photons are used both in the treatment of patients and to sterilize many therapeutic products. Many people are unaware that radiation is used to sterilize or treat many of the products that they use in their own homes, such as baby-bottle nipples, personal-hygiene products, cosmetics, bandages, polymerized flooring materials, Teflon-coated skillets, and insulation on electrical wire. Most spices are contaminated with 1 million or more bacteria per gram, so many commercial facilities irradiate spices. Unfortunately, irradiated foods are in limited supply in the United States, although our astronauts have been eating steaks sterilized with 45 kGy of gamma radiation since 1960.
The radiation applied to food is much more limited than that used in radiotherapy. Only two isotopic sources of gamma rays have been approved for use — cobalt-60 and cesium-137. Electron energies are limited to a maximal acceleration of 10 MeV, and x-rays generated by the electron bombardment of a metal such as tungsten are limited to 5 MeV. None of these types of radiation are capable of generating radioactivity. The choice of the most appropriate form of technology is largely dependent on the product. Electrons with a maximal energy of 10 MeV penetrate to a depth of only 4.5 cm in water or equivalent, limiting their use to thin packages or to products with very low density; however, the required dose of radiation is delivered extremely quickly. The generation of x-rays is not very efficient, since only 6 to 12 percent of the electron energy is converted to x-rays; the remainder generates heat, which must be removed before the target melts.
The absorption of electrons or of photons produces the same effect, ionization. When a gamma or x-ray photon is absorbed, an electron is released, causing ionization. Water is the principal target for the radiation, because it is the largest component of most foods and microorganisms. Normally, approximately 70 percent of the radiation-induced ionization will occur in cellular water, and the target organisms will be inactivated because of secondary reactions, not because of a direct effect on the bacterial DNA. The same sequence occurs in frozen products, but the ice structure limits the migration of the free radicals that are generated by the ionization; therefore, a higher dose of radiation is required for frozen foods. There is much greater potential to produce adverse sensorial effects in fresh products than in frozen products.
The doses of radiation that are required to inactivate 99.9 percent of a contaminating population of a few important foodborne pathogens in meat and poultry are listed in the Table. The dose required to inactivate 99.9 percent of Escherichia coli O157:H7 in ground beef increases from approximately 0.90 kGy at 5°C to 1.35 kGy at –5°C. Food irradiation may offer the only reliable method of controlling foodborne pathogens in ground meat or poultry without cooking. Unfortunately, a high proportion of the poultry we bring into our homes or commercial kitchens remains contaminated with one or more of the pathogens listed in the Table. Cooking will kill most of these pathogens, but the problems associated with the cross-contamination of other foods remain. Some restaurants are now using irradiated poultry to prevent such contamination, and the public would benefit from greater implementation of this method of ensuring the safety of foods.
Table. Dose of Radiation Required to Inactivate 99.9 Percent of Foodborne Pathogens in Meat or Poultry with Electron, Gamma, or X-Ray Ionizing Radiation at 5°C.
Dr. Thayer reports having received consulting or lecture fees from CFC Logistics, Master Foods, and Zero Mountain.
Source Information
From Lower Gwynedd, Pa.(Donald W. Thayer, Ph.D.)