Richard G. Hunter
Ph.D.
Food Technology Service, Inc.
Mulberry, FL
USA
Introduction
Transmission of food borne illness, primarily Salmonella serotype Enteritidis (SE) by shell eggs has been raised as a concern by public health officials over the past several years. The prevalence of SE in raw shell eggs has been estimated at 1 in 20,000 (Florida Department of Agriculture, 2003). Most recently, the Centers for Disease Control and Prevention associated an outbreak of food borne illness in South Carolina prisons that occurred in 2001 with Salmonella Enteritidis from improperly prepared hard-boiled eggs. Concern over this issue has led to industry and government-sponsored initiatives to improve egg safety and there is evidence that these efforts are making progress in decreasing the prevalence of SE.
As part of a comprehensive egg safety plan, the U.S. Food and Drug Administration recommends that pasteurized eggs be substituted for raw shell eggs in foods that are not cooked to at least 155º F for 15 seconds. A few firms in the U.S. use hot-water baths or hot-air systems to achieve a 5-log reduction in Salmonella levels in shell eggs that are marketed as pasteurized (Mermelstein, 2001).
Irradiation
The irradiation process can also be used to reduce Salmonella levels in shell eggs. Irradiation passes energy through products to break the chemical bonds in the DNA of organisms living in the product. This energy may be obtained directly from radioactive elements such as Cobalt 60 or generated from machines such as linear accelerators or x-ray machines. Each of the three methods of producing the energy has advantages and disadvantages from an industrial standpoint. However, the results of the process are the same no matter what source of energy is used for the irradiation.
Irradiation has been used to sterilize medical and consumer goods for several decades and there are currently about 40 irradiation facilities in the U.S. treating such products. Due to legislative and regulatory uncertainty about how to classify irradiation in the 1950’s, the process is technically regulated as a food additive. This is despite the fact that nothing is added or retained through the process. Because of this classification, irradiation is allowed only for foods items that are specifically approved by the FDA.
The earliest approvals for food irradiation date to the mid-1980s when pork, spices, seasonings and herbs were authorized. Irradiation was approved to reduce pathogen levels in fresh and frozen poultry and ground beef in 1992 and 1997 respectively.
Consumer interest in irradiated foods has increased significantly since terrorist incidents in late 2001, in part due to widespread and favorable publicity associated with the irradiation of mail to destroy anthrax (DeRuiter and Dwyer, 2002). Because of the ability of irradiation to prevent food borne illness, food irradiation is supported by the CDC, the American Medical Association, the American Dietetic Association, the Association of State and Territorial Health Officials and the World Health Organization.
The FDA-approval process for food irradiation involves the submittal of a petition demonstrating the safety and efficacy of the process for the specific food. In the case of shell eggs, the petition was submitted by Dr. Edward Josephson of the University of Rhode Island. The FDA conducted a detailed review of the merits of irradiation for use on shell eggs over the next several years. FDA approved the use of irradiation for shell eggs in July 2001 and allowed a maximum dose of 3.0 Kgry.
The dose of irradiation required to eliminate 90% of the organisms or one decimal log is termed the D-value. To achieve a 5-log reduction in pathogens of public health significance (a “pasteurizing dose for shell eggs) requires 5 times the irradiation dose needed for a 1-log reduction. The D-value differs by organism, product and product state (temperature, oxygen content, etc) and generally relates to the relative amount of DNA in the organism. For example, destruction of insect larvae in fruits requires a very small irradiation dose while bacteria require a higher dose. Viruses require a still higher dose and prions are highly resistant to irradiation because they lack DNA. Reported D value ranges for Salmonella Enteritidis include 0.25 to 0.50 (Murano,1995) and 0.56 to 0.77 Kgry (Thayer et al, 1990). Based on these values, irradiation alone may not reliably achieve a 5-log reduction of Salmonella Enteritidis within the FDA maximum dose.
Technical aspects of shell egg irradiation
Food Technology Service, Inc. (FTSI) operates a gamma-irradiator in central Florida. The process uses large carriers that move product past a Cobalt-60 energy source within a heavily shielded irradiation chamber. The dose is controlled by the length of time the carrier remains in the irradiation chamber and is dependent on the mass of the contents of the carrier. Each carrier consists of two, 7-foot high levels and accommodates standard 40 by 48 inch pallets. The irradiation chamber holds 9 carriers and the process operates on a continuous batch basis in which a carrier of non-irradiated product displaces a carrier that has completed the irradiation process at timed intervals.
Inquiries by potential customers regarding irradiated eggs have increased during the past six months. The majority of these inquiries are from nursing homes that have been advised not to serve poached or soft-boiled eggs to residents. The residents are dissatisfied because they can no longer have their eggs cooked in the desired manner and the nursing homes are seeking means to accommodate the residents. Because of the relatively small volume of eggs involved and the diverse locations of the nursing homes, FTSI has not begun production of shell eggs for commercial sale.
FTSI has irradiated shell eggs on a trial basis and achieved good dose uniformity in palletized bulk-pack boxes holding 15 dozen cartons. Production scale runs using thirty cases of eggs per pallet could produce dose uniformities of approximately 2.0 to 2.8 Kgry. Configurations using twelve cases per pallet could achieve dose uniformities of approximately 2.5 to 2.9 Kgry. There were no changes to the taste or odor of the raw or cooked eggs. However, qualitative changes to the egg structure discussed below made these dose limits impractical. Such changes were minimized when the maximum dose did not exceed 1.0 Kgry and, within this limitation, production scale runs resulted in dose uniformities of approximately 0.6 Kgry to 1.0 Kgry. None of the control eggs or the irradiated eggs tested positive for Salmonella Enteritidis so efficacy cannot be confirmed but can only be inferred from the reported D-values and the achieved dose. Production scale runs using a maximum dose of 1.0 Kgry would add less than ten cents to the price of each dozen eggs for the cost of irradiation.
The eggs irradiated by FTSI exhibited qualitative changes to the egg white and yolk strength that have been previously reported by Moon and Song (2000) and others. These changes appear to increase in a relatively linear fashion with increasing dose. From a practical standpoint, the egg white is less viscous and resembles that found in older eggs. With the exception of fried eggs, which may spread further in the pan, this is not objectionable in most uses even at doses approaching 3.0 Kgry. Unfortunately, at doses above 1.0 Kgry, the yolk becomes exceedingly fragile and it is difficult to break the egg while leaving the yolk intact.
Virtually all processes intended to preserve food involve trade-offs between qualitative changes to the food and achieving the desired state of preservation. Irradiation is most useful when such changes are minimal at doses that achieve good control of the pathogens of concern. For example, fresh and frozen poultry maintain excellent quality while achieving significant reductions in Salmonella, Listeria and Campylobacter at irradiation doses of 3.0 Kgry or less. Similarly, effective control of E. coli is achieved in ground beef at doses of 3.0 Kgry or less although ground beef containing higher-fact content requires measures to control oxidation of fat. These include the use of modified atmosphere packaging, anti-oxidants and frozen temperature but these or similar measures are not applicable to shell eggs.
Education regarding the benefits of irradiated foods is important to allow consumers to weigh the safety advantages of irradiated foods versus the increased cost of the item. This would be especially important in the case of irradiated shell eggs which, unlike most irradiated foods, exhibit significant changes to the raw food texture. It is likely the best potential market for such eggs is large-scale users seeking an additional barrier to disease transmission in addition to cooking or heating.
Conclusion
Irradiation of shell eggs at a maximum dose of 3.0 Kgry is approved by the FDA and there is demand for these products. Production scale runs of shell eggs have shown reduced viscosity of the egg white and increased fragility of the yolk at the upper range of the approved dose. These changes can be minimized by using a maximum dose of 1.0 Kgry although this significantly reduces the logarithmic reduction of Salmonella Enteritidis. No matter what dose is used, irradiation should be combined with other methods such as proper cooking, in order to maximize the potential for Salmonella Enteritidis transmission.
References
- Centers for Disease Control and Prevention. 2002. Outbreaks of Salmonella Serotype Enteritidis Infection Association with Easting Shell Eggs – United States, 1999-2001. MMWR. 51. pp 1149-1152.
- DuRuiter, F.E. and J. Dwyer. 2002. Consumer acceptance of irradiated foods: dawn of a new era? Food Sci. Tech. 2 pp 47-58.
- Florida Department of Agriculture and Consumer Services. Salmonella Information Page. http://doacs.state.fl.us/ai/salmonel.htm
- Mermelstein, N.H. 2001. Pasteurization of shell eggs. FoodTechnology. 55 pp 72-74.
- Murano, E.A. 1995. Microbiology of Irradiated Foods. in Food Irradiation: A Sourcebook. Iowa State University Press. Ames, IA. pp. 48.
- S. Moon and K.B. Song. 2000. Effect of γ-Irradiation on the Molecular Properties of Egg White Proteins. Food Sci. Biotechnol. 9 pp. 239-242.
- Thayer, D.W, Boyd, G., Muller, W.S., Lipson, C.A., Hayne, W.C. and S.H. Baer. 1990. Radiation resistance of Salmonella. Jour. Ind. Micro. 5 pp. 383-390.
From Proceedings of the “Midwest Poultry Federation Convention”, St. Paul, Minnesota, U.S.A.
