Nonthermal food processing technologies, such as X-ray and electron-beam irradiation, may be used to eliminate pathogens in raw foods, pasteurize delicate food products such as fresh produce, and perform phytosanitary treatment.
The world’s population is expected to reach about 9 billion by 2024. This presents a daunting challenge for scientists and engineers who must develop the technologies that will ensure adequate water, food, clothing, shelter, educational opportunities, and jobs for the growing population. One hurdle that every country should view of strategic importance is providing nutritious, safe, and sustainable supplies of food.
In many parts of the world, the middle class is expanding. This growing middle class expects high-quality foods — free of additives, microbial pathogens, pesticides, and other chemicals. In response, the food industry is adopting advanced food processing technologies and establishing globalized supply chains and distribution networks.
The amount of fresh fruits and vegetables shipped internationally indicates that today’s consumers want minimally processed foods that are produced in an environmentally sustainable fashion. And, they are willing to pay a premium for these types of foods.
In developed countries, consumers are becoming more sophisticated in terms of their food needs and expectations, which range from the yearlong availability of fresh fruits to foods that are minimally processed and free of pesticides. In less-developed parts of the world, consumers are also willing to pay for good-quality fresh foods, as well as foods that can be stored for prolonged periods at room temperature.
To meet these demands, the food industry is investing in nonthermal processing research and development (R&D) for pathogen elimination and shelf-life extension. One such technology, food irradiation, is applicable to a wide variety of foods. It is one of the most extensively researched food processing technologies. Food irradiation has won universal endorsements by the World Health Organization (WHO), the United Nations’ Food and Agricultural Organization (FAO), the U.S. Food and Drug Administration (FDA), the U.S. Dept. of Agriculture (USDA), and the international food standards setting organization Codex Alimentarius.
Unfortunately, food irradiation is also one of the least understood food processing technologies. This article discusses the science, technology, and current applications of two methods of food irradiation — electron-beam (eBeam) and X-ray technology.
Food irradiation technologies exploit the part of the electromagnetic spectrum that encompasses wavelengths shorter than 10–10 m. Radiation from this region of the electromagnetic spectrum is called ionizing radiation, because it can ionize materials and molecules that it encounters (Figure 1). Gamma radiation, X-ray, and eBeam radiation are examples of ionizing radiation. Radio waves, microwaves, and ultraviolet (UV) radiation are examples of nonionizing radiation.
Food irradiation has traditionally employed gamma radiation. Gamma radiation is generated by photons emitted from radioactive isotopes such as cobalt-60 and cesium-137. Even though the energy range of these photons is narrow — 1.17 mega-electronvolt (MeV) to 1.33 MeV — these photons have no mass and therefore have great penetrating abilities.
Cobalt-60 and cesium-137 are radioactive, which poses significant challenges in acquiring, transporting, storing, and safeguarding them. Because of the potential for theft of the radioactive material, there is a strong push by the International Atomic Energy Agency (IAEA), the U.S. National Nuclear Security Administration (NNSA), and the U.S. Defense Threat Reduction Agency (DTRA) to limit the commercial use of these radioactive materials because alternative technologies are available (1).
Although eBeam and X-ray irradiation technologies employ ionizing radiation, the radiation is not produced by radioactive materials. Rather, it is generated by specialized equipment called industrial electron accelerators (2). These accelerators are switch-on/switch-off technologies that can be turned off when not in use. In contrast, radio-active sources, such as cobalt-60 and cesium-137, cannot be switched off and generate gamma radiation continuously because they are undergoing natural radioactive decay. The ability to switch the radiation source on and off has major implications in terms of operating costs, worker safety, and the carbon footprint of an eBeam or X-ray food processing facility.
Industrial eBeam and X-ray processing
Electron beam and X-ray technologies can be used to treat food and food ingredients to eliminate microbial pathogens (i.e., pasteurization), or at higher doses to sterilize food ingredients. They can also be used at very low doses for phytosanitary treatment, which eliminates insects and pests on agricultural products to prevent their accidental introduction into other areas.
Each irradiation treatment process is calibrated in terms of absorbed dose — i.e., the amount of energy absorbed per unit mass. The standard unit of absorbed dose is the kilogray (kGy). The minimum dose and the maximum dose that are delivered to a particular food product are based on the specific application — whether it is phytosanitary treatment, pasteurization, or sterilization — and regularity limits that may be applicable.
Beam energy, measured in units of electronvolt (eV), is the penetration power of the electrons or X-rays and depends on the specifications of the accelerator (2). Note that the actual penetration depth of electrons or X-rays also depends on the density of the target food item. Table 1 shows the dose rates of gamma, X-ray, and eBeam irradiation at different beam energy levels, as measured at the National Center for Electron Beam Research at Texas A&M Univ. (3).
|Table 1. The beam energy and corresponding dose rate of eBeam, gamma, and X-ray radiation (3).|
|Source||Energy, MeV||Dose Rate, Gy/sec|
|Gamma||1.59 (from lanthanum-140)||~0.06–0.12|
The beam energy of commercially available eBeam accelerators ranges from 80 keV to 10 MeV. The U.S. Nuclear Regulatory Commission (NRC) has set an upper limit of 10 MeV for eBeam processing and 7.5 MeV for X-ray processing used on foods. Outside the U.S., the limit for eBeam processing is also 10 MeV, but for X-ray processing, the maximum allowable energy is 5 MeV.
The photons in X-rays are similar to gamma irradiation in that they have very high penetration capabilities. However, X-ray processing is significantly faster than cobalt-60-based gamma processing (and eBeam processing is faster than X-ray processing).
Food processing applications of eBeam technology can be broadly divided into low-energy (<1 MeV), medium-energy (1–8 MeV), and high-energy (8–10 MeV) applications. Current low-energy applications include the inline sterilization of packaging materials and the inline disinfestation/sterilization of seed surfaces. Medium-energy applications include phytosanitary treatment of packaged fruits and vegetables. High-energy applications include pasteurization of packaged meats, spices, seafood, and food ingredients.
There is a direct relationship between an accelerator’s electron beam current and the speed at which materials can be processed. Today, industrial-scale accelerators with an electron beam current ranging from a few milliamps (mA) to hundreds of mA are available. The corresponding beam power ranges from a few kilowatts (kW) to as high as 700 kW.
The radiation dose that is delivered to a product during irradiation is controlled by the amount of time the product is held under the beam. Food irradiation technology is tunable — meaning that the eBeam or X-ray dosage can be increased or decreased by varying the speed of the conveyor system for different applications. Computerized logic controllers control the speed of a conveyor so that the product on the conveyor passes under the beam for a specified amount of time. This processing is performed within specially designed radiation-shielded chambers or areas. Resources are available to determine the type of shielding that would be required based on the energy of the eBeam or X-ray that is used (4).
In the U.S., the use of eBeam and other ionizing radiation technologies is regulated by different government agencies depending on the product that is being processed. For example, the FDA regulates the use of this technology for sterilizing medical devices, pharmaceuticals, and food processing in general. The FDA has established maximum doses that can be applied to different types of foods, spices, and seasonings (Table 2) (5). The USDA Food Safety and Inspection Service (FSIS) oversees the application of this technology to fresh and frozen meat and poultry products, and the USDA Animal and Plant Health Inspection Service (APHIS) oversees its use for phytosanitary treatment of fresh fruits and vegetables.
|Table 2. The FDA has set maximum allowable dosages for food irradiation applications in the U.S. (5).|
|Food or Food Ingredient||Application||Maximum Allowable Dose, kGy|
|White potatoes||Sprouting inhibition||0.15|
|Fresh, nonheated processed pork||Pathogen control||0.3–1.0|
|Wheat flour||Mold control||0.5|
|Fresh produce||Insect disinfestation||1.0|
|Fresh produce||Growth and maturation inhibition||1.0|
|Fresh or frozen uncooked poultry products||Pathogen control||3.0|
|Fresh shell eggs||Pathogen control||3.0|
|Fresh iceberg lettuce and fresh spinach||Pathogen control||4.0|
|Refrigerated, uncooked meat products (sheep, cattle, swine, and goat)||Pathogen control||4.5|
|Fresh or frozen molluscan shellfish||Pathogen control||5.5|
|Frozen, uncooked meat products (sheep, cattle, swine, and goat)||Pathogen control||7.0|
|Seeds for sprouting||Pathogen control|
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