Pros and Cons of Commercial Irradiation of Fresh Iceberg Lettuce and Fresh Spinach: A Literature Review

Posted on October 1, 2008 by Bill Marler


Source of Article:

This is the first part of a multi-part series on the Pros and Cons of Commercial Irradiation of Fresh Iceberg Lettuce and Fresh Spinach.  Given the recent outbreaks, hopefully this is timely.

On August 22, 2008, FDA published a final rule for the safe use of ionizing radiation (also termed irradiation, irradiation pasteurization, cold pasteurization) of fresh iceberg lettuce and fresh spinach for control of foodborne pathogens, and extension of shelf-life. A few weeks later, the US Government Accountability Office (GAO) released a report entitled, “Improvements Needed in FDA Oversight of Fresh Produce.” This report states that FDA’s intervention efforts for reducing the risk of contamination during the processing of fresh-cut produce have been limited. Interestingly, the GAO reviewers only briefly mention irradiation, and brought little context to the implications of introducing irradiation as a potential control (“kill”) step during produce processing.

Currently, a serious outbreak of E. coli O157:H7, possibly linked to iceberg lettuce, is unfolding in Michigan and other parts of the United States. Since 1995, the FDA has documented at least 22 other E. coli O157:H7 outbreaks traced to leafy greens likely contaminated before retail distribution, including a number of outbreaks involving fresh iceberg lettuce and spinach. Clearly, there is a need for improved methods to prevent contamination of produce before it reaches the consumer.

Most food safety experts would agree that there is no silver bullet (defined by Webster’s dictionary as “a magical weapon ; especially : one that instantly solves a long-standing problem”) to guarantee protection of any food from contamination. The use of comprehensive “farm-to-table” approaches is well accepted as the best way to combat the complex problems in food safety.

Where does irradiation of food fit into this evolving continuum including the new rule in the United States for lettuce and spinach?

Irradiation is probably the most studied, and the most controversial, food processing method in history. Several years ago, two renowned food safety leaders, Drs. Robert Tauxe (2001) and Michael Osterholm (2004), published elegant summaries describing the role of irradiation in food safety and protecting the public health. They did not promote irradiation as a silver bullet, but their commentaries suggested the process is one tool in the toolbox, and may be a silver lining (defined as “a hopeful side of an otherwise desperate or unhappy situation”) in the burgeoning problem of foodborne disease.

To better understand the implications of FDA’s new rule, I hit the books with the goal to examine the “pros and cons,” (perhaps more appropriately described as “advantages and limitations”) of using irradiation as a control step during fresh lettuce and spinach processing. The following is the first in a series summarizing the findings.

Part I. Historical Perspective and Definitions

Irradiation as a processing method for food is not a new technique. Indeed, research into using ionizing radiation to improve food quality and shelf-life began in the late 1800’s. In 1905, scientists received the first patents for application of ionizing radiation as a food preservation process to kill bacteria. In the 1940’s, the term “irradiation” was first used in the literature, but some have since questioned using this language to describe the technology. Molins (2001a), an expert in the field of radiation, characterized the term as: “a most unfortunate occurrence because it brought a direct and conceptually misleading association of a food processing technique with the nuclear establishment.” He suggested use of the word “irradiation” was inappropriate because “it does not describe the actual process of applying ionizing radiation in ways that would set it apart from other processes used in the food industry. Thus, microwaves and infrared light – both of which generate heat – are also forms of radiation, and their use in cooking, heating foods in a microwave oven, or simply keeping the food warm under infrared light – as is customary in many restaurants – could just as properly be termed “food irradiation.”

Fifty years ago, the FDA defined food irradiation as a “food additive” in the Food, Drug, and Cosmetic Act. Tauxe (2001) made this comment on the classification in his review paper: “By an historical quirk, the use of irradiation on food was formally approved as though it were something added to food, rather than a process to which the food is subjected.”

Regardless of the potential pros and cons of food irradiation, poor terminology is a disservice to the scientific community, industry, and the public; furthermore, the “mystery” surrounding food irradiation has potentially lead to unnecessary controversy and miscommunication. Before beginning this review into the potential advantages and limitation of food irradiation, it seems critical to review some definitions and basic chemistry behind the process.

How does food irradiation work?

Food irradiation is based on the principle of using energy to ionize a material, in this case food. Ionizing irradiation treatment involves chemical reactions with microbes, but these reactions are not dissimilar to chemical reactions induced by cooking, canning, curing, drying, freezing, or other food processing techniques. There are pros and cons to every food processing technique. In food irradiation, high speed particles or rays are harnessed by a machine. The particles used for this purpose are common in nature, and part of the energy that comes from the sun. These particles are focused in the process to penetrate the food, and result in the creation of free radicals that damage the DNA of organisms, especially microbial contaminants at the doses used for food. Depending on the organism and irradiation dose, this process is capable of enhancing food safety and quality of the food (the nature of this process as it applies to food safety and comparison with other food processing methods such as cooking, and effects on food quality such as nutrients, are described in subsequent parts of this review).

There are 3 sources of ionizing radiation approved in the context of food processing:

1. Gamma rays
2. X-rays
Electronic beams (E-beams)

Only gamma rays require the use of radioactive material (Cobalt 60), but the levels required are too low for creation of “radioactivity” in the food or packaging. Thus, the food or packaging are not radioactive. In contrast, X-ray and electronic beam applications do not involve the use of radioactive material. For example, with E-beam technology, electricity is the source for generating electrons that damage DNA of certain microbes that contaminate food. Photons are generated by gamma and x-ray technology, and these methods provide deeper penetration into the food compared with E-beam, but the difference in penetration is not significant in the context of fresh lettuce and spinach. None of these methods for food irradiation create neutrons, the particles associated with nuclear technologies.

The “dose” applied to the food is an important consideration in understanding the chemistry of food irradiation. There are three general categories for irradiation dose in food processing. The dose of ionizing radiation is measured in units called gray.

1. Low (< 1kGy) is used mostly to kill insects that infest foods
2. Medium (1-10 kGy) is used primarily to reduce pathogens and prolong shelf-life of foods
3. High (>10 kGy) is used to reduce organisms resistant to low-medium doses, or to sterilize food

FDA currently permits food irradiation in the “medium” dose range to control pathogens (primarily bacteria and parasites) for the following foods:

• Fresh, non-heated processed pork
• Fresh or frozen, uncooked poultry products
• Refrigerated and frozen, uncooked meat products
• Fresh shell eggs
• Seeds for sprouting
• Fresh or frozen molluscan shellfish
• Fresh iceberg lettuce and spinach

The susceptibility of organisms to different doses of irradiation varies based on the biology of the organism. Damage is greatest in more complex organisms that may be a problem in food. The required dose to reduce or eliminate pests, pathogens, or spoilage organisms is generally in decreasing order as follows:

Insects < parasites < molds/yeasts < vegetative (non-spore forming) bacteria < spore forming bacteria < viruses < prions

Implications for the Lettuce and Spinach Industry

The new FDA rule for food safety and quality in fresh lettuce and spinach allows a maximum dosage of 4 kGy, which has been shown to be effective at reducing or eliminating the major pathogens linked to produce outbreaks (for example, E. coli O157:H7 and other STECs, Salmonella).

In Part II of this review, the pros and cons (advantages and limitations) of commercial irradiation of fresh iceberg lettuce and spinach relating to microbial contamination and food safety will be explored. Part III examines the food quality and food security considerations. Part IV concludes with an overview of the cost-benefit considerations that both industry and consumers must face in deciding how food irradiation fits into the big picture of prevention and control of foodborne illness.


Anonymous. Questions and answers about final rule on irradiation of fresh iceberg lettuce and fresh spinach, U.S. Food and Drug Administration, August 21, 2008. Available at:

Anonymous. Improvements needed in FDA oversight of fresh produce, U.S. Government Accountability Office, September 2008. Available at:

Cleland, M. R. 2006. Advances in gamma ray, electron beam, and X-ray technologies for food irradiation. In: Food Irradiation Research and Technology. Sommers, C. H. and X. Fan (ed). Blackwell Publishing, Ames, Iowa.

Josephson, E. S. 1983. An historical review of food irradiation. J Food Safety 5:161-89.

Molins, R. A. 2001a. Introduction. In: Food Irradiation: Principles and Applications. R. A., Molins (ed). John Wiley & Sons, Inc., New York, New York.

Molins, R. A, Y. Motarjemi, F. K. Kaferstein. 2001. Irradiation: a critical control point in ensuring the microbiological safety of raw foods. Food Control 12:347-56.

Murano, E. A. 1995. Microbiology of irradiated foods. In: Food Irradiation: A sourcebook. E. A. Murano (ed). Iowa State University Press, Ames, Iowa.

Niemira, B.A. and X. Fan. Low-dose irradiation of fresh and fresh-cut produce: safety, sensory, and shelf life. In: Food Irradiation Research and Technology. Sommers, C. H. and X. Fan (eds). Blackwell Publishing, Ames, Iowa.

Olson, D. G. 1995. Irradiation Processing. In: Food Irradiation: A sourcebook. E. A. Murano (ed). Iowa State University Press, Ames, Iowa.

Osterholm, M. T., and A. P. Norgan. 2004. The role of irradiation in food safety. N Engl J Med 350:1898-901.

Osterholm, M. T., and M. E. Potter. 1997. Irradiation pasteurization of solid foods: taking food safety to the next level. Emerg Infect Dis 3:575-7.

Paquett, K. E. Irradiation of prepackaged food: evolution of the Food and Drug Administrations's Regulation of the packaging materials. 2004. In: Irradiation of Food and Packaging. Komolprasert, V. and Morehouse, K. M. (ed). American Chemical Society, Oxford University Press, Washington, DC.

Shea, K. M. 2000. Technical report: irradiation of food. Pediatrics 106:1505-1510.

Tauxe, R. V. 2001. Food safety and irradiation: protecting the public from foodborne infections. Emerg Infect Dis 7:516-21.



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