An Overview of Ionizing Irradiation Technology for Pathogen Control in Fishery Feed Products for Marine Mammals
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 feedfish.
Because feedfish 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 feedfish 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 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 fresh water 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 feed 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 feed 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
1. Bergy's Manual of Systematic Bacteriology, Vol. 2. Eds. Sneath et al. 1986. Williams
and Wilkins pp. 1246-1247.
2. Dierauf LA. (Ed). 1990. CRC Handbook of Marine Mammal Medicine: Health, Disease, and
Rehabilitation. CRC Press, Boca Raton, FL.
3. Giddings BB. 1984. Radiation processing of fishery products. Food Processing. 38(4):61-97.
4. Grodner RM, LS Andrews. 1991. Irradiation In: Microbiology of Marine Food Products, Chapter
17, pp.429-440. D.R. Ward and C. Hackney, Eds. Van Nostrand Reinhold, New York.
5. Howard EB. 1983. Pathobiology of Marine Mammal Diseases, Vol. I and Vol. II. CRC.
6. Josephson ES, MS Peterson, Eds. 1983. Preservation of Foods by Ionizing Radiation. Volumes
I and II. CRC Press. Boca Raton, Florida.
7. Henkel J. 1998. Irradiation: a safe measure for safer food. FDA Consumer, May-June 1998.
Publication No. (FDA) 98-2320.
8. International Consultative Group on Food Irradiation (ICGFI--FAO/WHO/IAEA), ICGFI Fact Sheet
Series No. 1-14. 1991. Vienna, Austria.
9. Kilgen MB, MT Hemard, D Luke, S Rabalais, D Duet. 1998. Collaborative Evaluation of Commercial
Irradiation Processing of Gulf Coast Oysters for Vibrio vulnificus Control. 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, MT Hemard. 1996. Evaluation of commercial irradiation and other processing methods for
Vibrio vulnificus control in Louisiana oysters. Proceedings of the 19th and 20th Annual Conferences of the Tropical and
Subtropical Seafood Science and Technology Society of the Americas. pp. 300-310.
11. National Fisheries Institute news release #99-41, June 25, 1999.
12. Osterholm MT, ME Potter. 1997. Irradiation pasteurization of solid foods: taking food safety to the
next level, Emerging Infectious Diseases 3 4: 575-577.
13. Thayar DW, Josephson ES, Brynjolfsson A, Giddings GG. 1996. Radiation pasteurization of food. Ames, IA:
Council for Agricultural Science and Technology: Issue Paper No. 7.