51A-37

S. H. KIM¹, R. J. Price², D. S. Chang, and H. An. (1) OSU-Seafood Laboratory, Oregon State University, 2001 Marine Dr., Astoria, OR 97103, (2) Food Science & Technology, University of California, Davis, CA 95616

JUSTIFICATION: Scombroid fish, i.e., tuna and mackerel, as well as non-scombroid, mahi-mahi, have been implicated in the outbreaks of scombroid poisoning due to their high levels of free histidine in muscle. In our previous study, M. morganii was isolated and identified as the most prolific and predominant histamine former in fish during decomposition. Since the optimum condition for histamine formation of this strain is closely related to pH and salt content in fish muscle, its presence and proliferation are of great concern for seafood safety.

OBJECTIVES: The objective was to characterize histamine formation and proliferation of M. morganii in various fish species under various storage conditions.

METHODS: M. morganii isolated from temperature-abused albacore was inoculated at 10⁵ cells/g into the fish muscles, i.e., albacore, mackerel, mahi-mahi, and salmon. Fish muscles were incubated at -30, -20, 4, 15, 25, and 37°C. During storage, aerobic plate counts and histamine were determined by the FDA and AOAC methods, respectively.

RESULTS: Histamine formation in fish muscles followed the pattern of bacterial growth at elevated temperatures. The temperature of 25°C was optimal for histamine formation. Mackerel was identified as the most susceptible fish species for histamine formation followed by albacore and mahi-mahi. Although salmon was least susceptible to histamine formation, it supported growth of M. morganii, and histamine accumulated up to 200 ppm, well above the FDA guideline, 50 ppm. The growth of M. morganii was inhibited in all the fish species at 4°C, but the histamine levels increased slightly. During frozen storage, counts of M. morganii in all the fish species decreased to 10⁴ CFU/g in 1 mo, and no histamine was found.

SIGNIFICANCE: Mackerel, albacore, and mahi-mahi were shown as good substrates for decarboxylation reaction by M. morganii at elevated temperatures. Proliferation of this strain in all fish tested including salmon induced the hazardous levels of histamine production.