Irradiation Variations and Applications

Food irradiation can be broadly divided into two application areas. One is the use of this technology as a critical control point in the final step of a comprehensive HACCP plan (Hazard Analysis Critical Control Points) to control microbial pathogens for food safety. The U.S. Food and Drug Administration and the U.S. Dept. of Agriculture’s Food Safety Inspection Service (FSIS) have allowed the use of irradiation for a variety of foods at defined doses and for different applications, according to Suresh Pillai of Texas A&M University, College Station, Texas (phone 979-845-2994). The other use of irradiation is for phytosanitary applications. In phytosanitary applications, the technology is used to kill insects and larvae to prevent movement of dangerous insects and pests across international boundaries. Later, we will describe a phytosanitary application in which insects are sterilized, but not killed.

Globally, irradiation has been used primarily for food safety. Spices have been the main commodity for which this technology has been widely used. It has been estimated that in the United States alone, approximately 80,000 metric tonnes (about 175 million pounds) of spices, 8,000 metric tonnes of ground beef, and 4,000 metric tonnes of produce (for phytosanitary purposes) are irradiated currently, acccording to Pillai.

Currently, irradiation is the only technology approved by the USDA’s Animal and Plant Health Inspection Service (APHIS) to treat imported guava, carambola, sweet lime, and manzano peppers from Mexico. Irradiated guavas, mangoes, papayas, purple sweet potatoes, rambutan, starfruit, and bananas are available and sold in the United States. These products are sold at retail stores and carry the “radura” symbol, showing they have been irradiated. Contrary to popular belief, these labels have not deterred purchases nor caused any reaction from consumers.

Electron Beam and X-Ray Technology
Electron beam (e-beam) technology is one of several ways to deliver electromagnetic energy to objects. The others are x-rays (created by e-beams hitting a metal target) and gamma rays, emitted by radioactive isotopes (cesium 167 or cobalt 60). Four commercial service facilities employing e-beam irradiation have opened in recent years, says Pillai. While many of these facilities cater to the medical device and packaging industries, some are being used for spice sterilization, pet food sterilization, and ground beef sterilization.

National Center for Electron Beam Research at Texas A & M University
The National Center for Electron Beam Research (www.ebeam.tamu.edu) serves as a venue for academic, government, and industry scientists to carry out strategic electronic pasteurization and sterilization research using e-beams and x-rays. In addition to serving research needs, the facility at Texas A&M University (TAMU) is also involved in commercial irradiation services. There are programs in vaccine development; pasteurization, sterilization and phytosanitary applications; fundamental biological responses; consumer, economic, and marketing studies; environmental treatment technologies; and material transformations. Vaccines against common foodborne pathogens such as salmonella are currently being commercialized.

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The e-beam and x-ray equipment is housed in a 16,000 sq-ft facility. The rated capacity of the irradiation facility is approximately 12,000 lb to 15,000 lb of product per hour. A dosimetry laboratory equipped with alanine and radio-chromic film dosimetry supports the activities of the center. (Dosimetry concerns measurement of the radiation dose received by a target.)

Two Texas A&M faculty, Elena Castell-Perez and Rosana Moreira of the Dept. of Biological Engineering, perform research on the application of radiation to fresh produce, pointing out that fresh produce, such as melons and berries, are susceptible to overdoses of radiation, so exposure of such products must be uniform, and may benefit from synergistic treatments that sensitize pathogens. Examples include the need to rotate packages of lettuce in an e-beam and the use of modified atmospheres or ozone to supplement radiation.

Sterilizing Fruit Flies on Papaya
Michael Kohn is president of Hawaii Export Company and of Pa’ina Hawaii (phone 808-834-0496) and is a new customer of GRAY*STAR, an irradiator equipment firm that has been described in past columns (Food Technology 57(3): 68-70; Food Technology 60(10): 73-75). The GRAY*STAR design, in contrast to other gamma irradiators, keeps the cobalt 60 source under water, reducing the need for expensive concrete shielding. Products to be irradiated are contained in a “diving bell,” which is lowered through the water to a location close to the source. The extent of exposure is determined by the strength of the source and the time during which the product is kept close to the source.

Hawaii Export is a papaya dealer that buys fruit from growers and ships to markets around the world. Tropical fruits, such as papaya, pose several challenges because of their fragility and because of pests they may carry. For the European market, genetically modified varieties were developed for resistance to virus, but then Europe acquired a preferance for non-GMO foods, and that market declined. For shipment to the mainland U.S., fruit needed to be heat treated to kill fruit flies, which is not good for the fruit quality.

Special packaging is needed for protection of the relatively fragile fruit. Refrigeration helps to preserve quality. Ethylene oxide was used as a sterilant, but it is toxic and explosive.

Research by the USDA showed that a low dose of gamma radiation would sterilize, but not kill fruit flies. This is adequate for shipment to California, where the release of sterile flies is used to help control the pest. The low dose does not affect fruit quality.

Kohn has established Pa’ina Hawaii as an irradiation service company to treat his papayas and other Hawaiian crops, such as sweet potatoes, other tropical fruits, and herbs. There are other heat treatment and irradiation facilities in Hawaii, but these are not open to individual growers, but rather are used captively by large growers, especially of sweet potatoes.

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Kohn said that he was investing about $4 million in his new facility, including the building. A major cost is the cobalt 60, which comes from MDS Nordion in Canada. The cobalt declines in strength about 5% per year and so lasts about 20 years. A few pencils (the metal rods of cobalt) are added each year to maintain a uniform source strength.

Clearly, irradiation is alive and in commercial use. Texas A&M University, with its National Center, is developing the basic information to apply e-beams and x-rays to a wide variety of materials. E-beams have the operating advantage of being capable, in principle, of being turned on and off as needed. They do operate in a high vacuum, which takes time to establish, and they have definite limitations on penetration depth, but for many applications, they can deliver a precise and adequate dose of energy. One issue could be supply of equipment, as the supplier for TAMU’s equipment is no longer manufacturing. Presumably, if there is enough demand, other firms will enter the market.

Meanwhile, GRAY*STAR and MDS Nordion offer gamma irradiators of unique designs. A large number of installations exist, most functioning as service centers, in which medical devices, packaging material (such as aseptic bags), and foods are treated, one after the other. Achieving high utilization is essential to the economics of irradiation because of the relatively high capital cost. Variable costs include energy, in the case of e-beams; addition of cobalt 60, in the case of gamma irradiators; and labor for material handling and dosimetry in both cases.

11th International Congress on Engineering and Foods (ICEF11)
The 11th International Congress on Engineering and Foods was held May 22-26, 2011, in Athens, Greece; it involved about 800 people from 60 countries, giving more than 1,100 presentations. Some of the more interesting results will appear here over time, as the proceedings are digested. The venue, organization, and logistics were excellently arranged by a hardworking group of Greek professors and student volunteers. Twenty-five “elders” in the field of food engineering, many of whom are IFT members, were honored with Life Achievement Awards. Four of those—George Saravacos, President of the Congress; Marc Karel of MIT and Rutgers; Henry Schwartzberg of the University of Massachusetts; and Walter Spiess of Karlsruhe, Germany and IuFoST—were further honored with special sessions describing their histories and presenting related research. Nonacademics are under-represented at these Congresses, but are most definitely welcome. (I was the only nonacademic elder.)

The next Congress will be in Quebec, June 14–18, 2015. Michele Marcotte, Agriculture and Agri-Food Canada ([email protected]) is the next Congress president. It is not too early for interested readers to plan on attending and, possibly, getting involved with preparations. There is a lot of work to be done, but being involved makes participation very satisfying.

J. Peter Clark,
Contributing Editor,
Consultant to the Process Industries,
Oak Park, Ill. 
[email protected]