The association of histamine producing bacteria with tuna fish salad formulations containing textured soy protein

This research was conducted to investigate the effect of storage time and temperature on histamine production by histidine decarboxylase positive bacteria in tuna fish salad extended with textured soy protein (TSP). Tuna salad mixtures containing 0, 15, and 30% TSP by weight were inoculated by a mixed culture of Proteus morganii 110SC2; Hafnia alvei, T8; Proteus morganii, im; Citrobacter freundii, T₃, and Escherichia coli, 58. The tuna salad samples were incubated at 8°C, 24°C, and 37°C. Chemical and microbial evaluations were conducted over a storage period of 6, 12, 24, and 48 hours. Histamine concentration was determined fluorometrically at an excitation wavelength of 360 nm and an emission of 450 nm. Histi dine analysis was performed using an amino acid analyzer. A differential plating-medium was used for quantitative detection of histamine-producing bacteria. The addition of soy protein resulted in a significant increase in growth rates and histamine production. The temperature of storage significantly affected the bacterial growth and histamine formation. Significant differences (P < 0.05) were also noted in histamine levels due to the levels of TSP and between the storage periods. Toxic levels of histamine were noted in samples extended with 15% and 30% TSP at 24°C and 37°C. The highest level observed was 149 mg/100 g in samples extended with 15% TSP at 37°C. Addition of TSP to tuna salad resulted in a higher initial pH and provided optimum conditions for bacterial activities. The lower temperature limit for toxic level of histamine production with the mixed culture used in this study was found to be 8°C.