An Overview of Ionizing Irradiation Technology for Pathogen Control in Fishery Feed Products for Marine Mammals
American Association of Zoo Veterinarians Conference 2000

Marilyn B. Kilgen, PhD

Department of Biological Sciences, Nicholls State University, Thibodaux, LA, USA


Bacterial infections are the main cause of disease and death in marine mammals in captivity. The most common include respiratory infections, wound infections that may become systemic, and primary septicemias.2,5 Sources of pathogens or opportunistic pathogens are the animals themselves (carriers), the natural aquatic environment, sewage pollution introduced into the aquatic environment, and contaminated feeder fish.

Because feeder fish that could be contaminated with potential pathogens or opportunistic pathogens are introduced into the aquatic environment of the marine mammals, the food safety technology of ionizing irradiation would be very useful in greatly reducing or eliminating potential pathogens from the feeder fish in bulk frozen packaging.

Ionizing irradiation, like pasteurizing or canning, is a processing technology used to preserve foods and kill pathogens and parasites that cause food-borne illness. There are two basic types of ionizing irradiation used for food processing: 1) electromagnetic waves as gamma ray emission from radioisotopes (Cobalt-60) or X-rays generated by linear accelerators (at energies of ≤5 MeV), and 2) subatomic particles generated by electron accelerators (electrons at or below an energy level of ≤10 MeV).6,8

Gamma ray emission from Co60 is the main ionizing irradiation used for commercial applications because it is extremely penetrating and very effective for pre-packaged bulk food products that are fresh, refrigerated, or frozen. The disadvantage is that it is generated by a radioactive element (Co60) and must be carefully contained and regulated as it cannot be turned off. Electrons and X-rays are generated by a linear accelerator and have the advantage of being able to be turned off when not in use. However, production of X-rays has not been practical for industrial use, and electrons generated by a linear accelerator are not very penetrating. The density or thickness of the product to be processed by accelerated electrons (beta particles) must be carefully measured and characterized for effectiveness. For these reasons, the most common and effective source used for commercial applications has been gamma irradiation from Co60 rods.6,8,13

The irradiation used for food processing is called “ionizing” because it is capable of removing electrons from molecules and atoms to convert them into “ions.” Ionizing irradiation can also displace electrons to produce “free radicals” which are very reactive and cause many chemical reactions which impart the beneficial effects of destroying bacteria, yeast, molds, parasites, and insects or their eggs or larvae. They can also slow down ripening or maturation of certain fruits and vegetables by causing biochemical reactions in physiologic processes of plant tissues. Ionizing irradiation when applied in the wrong doses to the wrong food products can also cause the possible disadvantageous side-effects of “off” flavors.6,8,12

The primary goal of irradiation processing is to greatly reduce or eliminate any spoilage or pathogenic microorganisms that may be present in foods without inducing sensory or organoleptic changes in the product.

Aquatic or fishery products are an important and basic source of protein but have a relatively short shelf-life unless frozen onboard or very shortly after harvesting. A review by Shewan and Hobbs (1967) of the microflora of fresh marine fish caught in the north and mid-Atlantic indicated that the predominate natural bacterial flora (60%) on gills, intestines, and slime of freshly caught fish and shellfish are the Pseudomonas and Achromobacter sp. Isolates of Corynebacterium, Flavobacterium, and Micrococcus comprised 20% of the natural flora isolates. Frazier (1967) found that the intestines of both marine and freshwater fish contain mainly Achromobacter, Pseudomonas, Flavobacterium, Vibrio, Bacillus, Clostridium, and Escherichia. The psychrotrophs Pseudomonas, Achromobacter, and Flavobacterium are typically associated with fish spoilage and impart slimy, smelly characteristics.4

Some potential pathogens of marine mammals that can be associated with the natural aquatic environment of fishery products include Erysipelothrix, the vibrios, Pseudomonas, Aeromonas, corynebacteria, Klebsiella, Enterobacter, Serratia, Hafnia, staphylococci, alpha hemolytic streptococci, Clostridia, and Nocardia.1,2,5

Processed seafoods may contain, in addition to their normal environmental flora, pathogenic bacteria from sources introduced into the marine environment by waterfowl populations or other animal fecal non-point sources. Additionally, harvesting, processing, and handling can also be a source of potential or opportunistic pathogens in the feeder fish products. These could include Salmonella, Erysipelothrix, beta hemolytic streptococci, staphylococci, Clostridia, Proteus, Edwardsiella, corynebacteria, enterotoxigenic Escherichia coli, and Pasteurella species.

A comprehensive review of the pathobiology of diseases of marine mammals is found in the CRC Pathobiology of Marine Mammal Diseases and the CRC Handbook of Marine Mammal Medicine.2,5

For fishery products that are frozen on board or shortly after harvest, gamma irradiation processing would offer the best technology for elimination of potential pathogens naturally present in the fishery products or introduced by processing and packaging. Although frozen food products must receive a higher dose of radiation because microorganisms are protected by deep temperatures and require higher doses in the deep-frozen state as compared to iced or refrigerated temperatures, irradiation of frozen products generally produces no detectable adverse sensory changes in the product. Doses effective for certain fishery products (fresh fatty fish do not respond as well to irradiation) range from medium doses of 1–3 kGy for fresh fish to 7 kGy for frozen seafoods.3,4,6,9,10 In 1998, the International Consultative Group on Food Irradiation (ICGFI) compiled a comprehensive review of literature of approximately 200 references on the microbiologic effectiveness of irradiation of seafood products for human consumption.

This processing technology has been approved for more than 40 different foods in about 37 countries for human consumption. Most recently, it has been approved for meat products in the United States.7 The National Fisheries Institute currently has a petition before FDA to allow irradiation processing of live and processed molluscan shellfish products.11 Some of the national and international agencies that have approved the safety of this technology are the World Health Organization (WHO), FAO, the International Atomic Energy Agency (IAEA), the Joint Expert Committee on Food Irradiation (JECFI), the U.S. Food and Drug Administration (FDA), the U.S. Department of Agriculture (USDA), the American Medical Association (AMA), the American Veterinary Medical Association (AVMA), and the Codex Alimentarius.

It is recommended that the effectiveness of ionizing irradiation processing technology be evaluated for significant marine mammal pathogens in common feeder fish, which include herring, smelt, and squid. The use of this “cold pasteurization” to treat bulk packaged fresh and frozen marine mammal fish food products would not require FDA approval, and would greatly reduce or eliminate the potential pathogens found on the fish food products without adversely affecting their sensory qualities.

Literature Cited

1.  Sneath PHA, et al., eds. Bergey’s Manual of Systematic Bacteriology. Vol 2. Williams and Wilkins; 1986:1246–1247.

2.  Dierauf LA, ed. CRC Handbook of Marine Mammal Medicine: Health, Disease, and Rehabilitation. Boca Raton, FL: CRC Press; 1990.

3.  Giddings BB. Radiation processing of fishery products. Food Processing. 1984;38:61–97.

4.  Grodner RM, Andrews LS. Irradiation. In: Ward DR, Hackney C, eds. Microbiology of Marine Food Products. New York, NY: Van Nostrand Reinhold; 1991: chapter 17, 429–440.

5.  Howard EB, ed. Pathobiology of Marine Mammal Diseases. Vol I and II. Boca Raton, FL: CRC Press; 1983.

6.  Josephson ES, Peterson MS, eds. Preservation of Foods by Ionizing Radiation. Vol I and II. Boca Raton, FL: CRC Press; 1983.

7.  Henkel J. Irradiation: a safe measure for safer food. FDA Consum. May–June 1998;32:12–17.

8.  ICGFI Fact Sheet. Vienna, Austria: International Consultative Group on Food Irradiation (ICGFI—FAO/WHO/IAEA), 1991:1–14.

9.  Kilgen MB, Hemard MT, Luke D, Rabalais S, Duet D. Collaborative evaluation of commercial irradiation processing of Gulf Coast oysters for Vibrio vulnificus control. In: Proceedings of the III Meeting of the FAO/IAEA/PAHO on Irradiation as a Public Health Intervention Measure to Control Foodborne Diseases in Latin America and the Caribbean. Havana, Cuba: November 15–21, 1998. (U.S. Representative).

10.  Kilgen MB, Hemard MT. Evaluation of commercial irradiation and other processing methods for Vibrio vulnificus control in Louisiana oysters. In: Proceedings of the 19th and 20th Annual Conferences of the Tropical and Subtropical Seafood Science and Technology Society of the Americas. 300–310.

11.  National Fisheries Institute news release. June 25, 1999:99–41.

12.  Osterholm MT, Potter ME. Irradiation pasteurization of solid foods: taking food safety to the next level. Emerg Infect Dis. 1997;3:575–577.

13.  Thayar DW, Josephson ES, Brynjolfsson A, Giddings GG, eds. Radiation Pasteurization of Food. Ames, IA: Council for Agricultural Science and Technology; 1996: Issue Paper No. 7.


Speaker Information
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Marilyn B. Kilgen, PhD
Department of Biological Sciences
Nicholls State University
Thibodaux, LA, USA

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