Ionizing radiation has been effective at killing pathogens in food-processing plants. Now it may have a new role in the post office.
Electron-beam systems, typically used to kill dangerous pathogens in food and sterilize medical products, have made the news lately as the U.S. Postal Service's technology choice to rid the mail of anthrax contamination. A proprietary electron-beam system from the SureBeam subsidiary of The Titan Corp., San Diego, will do the job.
The SureBeam system used to kill food bacteria is the same one used to destroy Anthrax except that the dosage level is much greater for mail sterilization than for food irradiation. For example, beam duration on ground beef is just a fraction of a second. It takes several seconds to a minute to sterilize mail, explains Wil Williams, Titan's vice president of corporate communications. The U.S. Postal Service will buy eight systems initially with the option to purchase 12 more.
Though electron beams are making headlines, X-rays and gamma radiation also still have a place in food packaging.
Targeting with electrons
Electron beam and X-ray systems operate in a similar fashion, explains Sean Austin, senior health physicist from the Radiation Safety Academy, Gaithersburg, Md. "Basically a high voltage is applied to a filament, usually tungsten, typically in a vacuum chamber. Electrons flow from the filament when an electric poten-
tial is applied and accelerate toward an anode. The higher the electric voltage applied, the faster the electrons accelerate," he explains. "In electron-beam applications, these electrons pass through and out of this vacuum directly into the material being irradiated. The electrons are the radiation that cause ionization in the material being treated or sterilized."
Electron-beam systems use an electron gun which deflects high-energy beams of electrons back and forth across a production line into the product. Think of an electron gun as basically a larger version of a television picture tube. Like a TV, electron-beam generators can simply be switched on and off. Still, some shielding is required to protect workers from radiation exposure.
Irradiation attacks the DNA in microbes. Regardless of whether the application is food or mail, when microbes are irradiated the energy from the rays transfers to the water and other molecules in the microbe, creating transient reactive chemicals that damage its DNA. Unless it can repair the damage, the microbe will die when it grows and tries to duplicate.
To date, the U.S. Dept. of Agriculture and the Food and Drug Administration have approved a variety of foods for irradiation including wheat flour, potatoes, pork, poultry, meat, herbs and spices, and many fruits and vegetables. Irradiation systems can be built directly into the processing line or set offsite at a service center. Generally, large cases of material to be treated are loaded in pallets and placed onto a conveyor system. They're then carried into an irradiation chamber, past the source, and back out to an unloading station. The dosage products receive (measured in Gray units, Gy) depends on the computerized conveyor's speed: The slower the conveyor the larger the dose and vice versa.
A long list of food producers use SureBeam technology from Titan — claimed to be the innovator of the nation's first electronic food-pasteurization system — including Cargill, Huisken Meats, Tyson Foods, Del Monte Foods, American Foodservice, United Food Group, and Omaha Steaks.
Besides killing bacteria in the mail, food products, and medical supplies, Titan is working with Hawaii Pride LLC to eliminate produce-damaging fruit flies. Hawaiian farmers, who've been denied access to the U.S. mainland fruit markets due to a federal fruit-fly quarantine, will now be able to ship exotic fruits to the States.
Other treatment methods including toxic chemicals, heat, or nuclear irradiation were considered or tried in the past but all proved harmful to the fruits' appearance, nutritional value, or the environment. Electron-beam technology, on the other hand, gives a postharvest quarantine disinfestation treatment that effectively rids the fruit of fruit flies and other plant pests without harming the environment, says U.S. Senator Daniel K. Inouye.
Recently, SureBeam Corp. patented a machine that lets a single electron beam treat food products of varying sizes and densities continuously and simultaneously. The new design, "has direct in-line application to ready-to-eat processors who want to process multiple products simultaneously, for example, an assortment of luncheon meats of differing weights or thicknesses," says Larry A. Oberkfell, president and CEO. SureBeam is reportedly the only company currently processing food for the retail market that uses both electron-beam and X-ray technology.
X-ray technology is an offshoot of electron beam and, as the newest, is still being developed, says the CDC. X-ray machines for food irradiation are more powerful than those used in hospitals. Here, a beam of electrons hits a metal target then creates a stream of X-rays out the other side to where the food is irradiated. Like electron-beam systems, X-ray units can be turned on or off with a flick of a switch. X-rays are able to pass through thicker food than electron beams, which can penetrate only to a depth of 3 cm, says the CDC. However, X-rays, like gamma rays, require heavy shielding for safety.
To the north, the Canadian company MDS Nordion, Ottawa, has developed a first: An X-ray irradiation system for processing meat, poultry, ready-to-eat products, fruits, and vegetables in their original packaging on intact pallets. This lets the irradiation process seamlessly integrate into a food-processing distribution network. "The principle advantage of the Palletron is that it can process packaged food on intact pallets with excellent dose uniformity," explains a company spokesperson. "Other radiation systems can process food products to good uniformity but not on intact pallets. Uniformity is important because it allows the highest pathogen reduction (determined by minimum dose in the stack) while not exceeding the limits of product tolerance to radiation (determined by maximum dose in the stack)." Dose uniformity is accomplished by dynamically controlling the geometry between the radiation field and the rotating pallet, and by the differential exposure rate of that rotating pallet to the radiation field.
Gamma rays go deep
Gamma rays can penetrate several feet deep into foods. When irradiating food or medical products, the radioactive source is pulled out of a water tank into a chamber surrounded by massive concrete walls. Products are brought into the chamber and exposed to the rays, and then the source goes back to its water tank.
"There are limits on the energy of the radiation that can be used for treating food," explains Austin. "All radiation has a limited range in matter, but because electrons are particles, they have a more limited range than an X-ray or gamma ray, which is a photon. This must be taken into account during design so the range is deep enough to deposit sufficient radiation energy to destroy organisms of interest."
So, how much is enough? Consider a single chest X-ray that has a dose of approximately a half of a milliGray. According to the CDC, the level needed to kill salmonella in fresh chicken is approximately 4.5 kGy, about 7 million times more irradiation than a single chest X-ray. Ultimately, dosage varies depending on food type and intention. Low doses (up to 1 kGy) control insects in grains, inhibit sprouting in white potatoes, and control Trichina parasites in pork. Medium doses (1 to10 kGy) control bacterial pathogens in meat, poultry, and fish. While high doses (greater than 10 kGy) kill microorganisms and insects in spices.