Using Irradiation To Make Food Safer For — страница 2

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ripen in storage, due to the change of starches into glucose, and become mushy. Irradiation disables the macromolecules responsible for this change. Precooking the potatoes has the same effect but is not practical (Satin 13). One of the concerns about irradiation is the formation of free radicals. These free radicals, or electrically charged particles, are formed during the process of irradiation. Free radicals are slightly unstable and try to find another compatible free radical to link to, forming a stable radiolytic compound. This happens faster in moist than in dry foods. This is because the free radicals are more free to move in liquids than in solids. These radiolytic compounds and free radicals may sound scary to many people when they are taken out of context. However,

these compounds are very common, and they are formed during everyday events such as metabolism and other simple biochemical reactions (Satin 18). Gamma radiation can be used to irradiate food. This requires an unstable isotope which slowly decays and emits gamma radiation. One such isotope is Uranium-238. Although it is not used in irradiation, it is commonly used as an example to show how gamma radiation is emitted. Uranium-238 is very unstable and can hardly hold itself together. Eventually, a particle made of protons and neutrons breaks free from the atom forming a new atom, thorium-234. The process continues, changing into protactinium-234, and eventually into lead-206. Occasionally during the remainder of the decay, non-particle radiation is released in the form of gamma (y)

radiation. Gamma radiation is the form of nuclear radiation that is used in food irradiation. Gamma rays (as previously mentioned) are used in one type of irradition plant, the gamma ray type facilities. The most common radioactive substance used in this process is cobalt-60, but cesium-137 is also used. Pellets of the cobalt-60 are stored in stainless steel cylinders called pencils. Each pencil is about 17.75 inches long and one half inch in diameter (Murano 11-12). The pencils are transported to the facility in a lead cast to prevent contamination of people or other things during transfer. The cobalt-60 pencils are held on a source rack. Since most products must be exposed to the gamma rays for several hours, a conveyer moves the food past the source rack, stops, and then moves

again. The cobalt-60 emits gamma rays continuously in all directions. A conveyer loops all the way around the sourcerack to take advantage of the gamma rays being emitted in all directions and to maximize efficiency. A standard gamma ray facility contains about one million curies (Murano 11-12). The curie is “a unit of radioactivity equal to 3.7*1010 disintegrations per second” (Webster’s “curie”). When new, each pencil contains about six-thousand to thirteen-thousand curies. Electron beam facilities are the second type of irradiation plants. These plants do not use atomic radiation, but rather, an electron beam generator and require extensive electrical components and heat exchangers to cool them. The plants usually use an electron beam of five to ten million electron

volts (Murano 15-16). However, a five megavolt beam from both sides of the food can only penetrate one and one half inches. The throughput is determined by the number of watts. Since the beam is directed at the product and not in all directions, the electron beam is more efficient. A ten kilowatt beam generator is as powerful as about one-million curies from a cobalt-60 source (Murano 14-15). The electron accelerators produce from five to ten million electron volts and ten to fifty kilowatts (Murano 15-16). The electron beam efficiency can be increased by aiming the beam at a metal target, usually tungsten or tantalum. This produces Bremsstrahlung x-rays. This type of x-ray can penetrate as well as gamma rays, but two-hundred kilowatts would be required to achieve necessary

throughput. Irradiation requires different levels of exposure for different tasks. The International System’s unit of radiation is the Gray (Gy). One Gray is equal to one joule of energy absorbed by one kilogram of food. A high dosage needed to sterilize food, as is done during canning, requires more than ten kiloGrays. A medium dosage, which can “pasteurize” food, is one to ten kiloGrays. A low dosage, which simply prevents ripening and kills insects or larger pathogens, is less then one kiloGray (Murano 5-6). Irradiation plants must be licensed and are strictly inspected on a regular basis. The plants are completely automated; there is little room for human intervention. People are not exposed to the radiation on a normal basis. Sophisticated computer controls and