Heat Resistance of Histamine-producing Bacteria in Irradiated Tuna Loins

Introduction: Consumption of foods high in biogenic amines leads to an illness known as histamine or scrombroid poisoning and it is commonly associated with consumption of fish. Morganella morganii is one of the most prolific histamine formers in certain species of fish including sardines, mackerel and tuna. Enterobacter aerogenes is the main histamine-producing organism in Korean fish.  M. morganii has been shown to produce histamine when present in sufficient numbers (≥ 10⁶ CFU/ml) with histidine available as a substrate.

Purpose: To determine the thermal parameters of the five highest histamine-producing bacteria, M. morganii, Raoultella planticola, Hafnia alvei, E. aerogenes, and Photobacterium damselae in pre-irradiated tuna loins.

Methods: Three-strain composite for each organism was used. Tuna-samples (5 g/sample) were submerged into one bacterial composite at a time and held on ice for 30 min to allow the bacteria to adhere to the tuna loin. The final inoculum level on samples was ~ 1x10⁸ CFU/g.  After inoculation, samples were aseptically transferred to sterile plastic pouches, vacuum sealed, and heat treated to as many as four different temperatures for four predetermined heating times per temperature.  After heating, samples were cooled down in ice slurry for ~30-40 s. To enumerate the recovered cells, each pouch was diluted using 0.1% peptone water (PW) and stomached for 2 min; further decimal dilutions were made in 9.9 ml PW before spiral plating onto tryptic soy agar (TSA). The plates were incubated for 48 h at 30 °C before enumeration. D-values were calculated using the GInaFiT (Geeraerd and Van Impe Inactivation model Fitting Tool) xla program.

Results: M. morganii was the most heat resistant histamine-producing organism tested in tuna loins (D_54^oC= 8.1 ± 1.7 min, D_56^oC = 2.7 ± 1.06 min), followed by E. aerogenes (D_54^oC = 5.87 ± 0.78 min, D_56^oC = 0.81 ± 0.08 min), H. alvei (D_54^oC = 2.6 ± 0.48 min, D_56^oC = 0.9 ± 0.34 min), and R. planticola (D_54^oC = 2.26 ± 0.24 min, D_56^oC = 0.52 ± 0.18 min).

Significance :  The reported D-values for M. morganii, the most heat resistant histamine-producing organism tested, will help the tuna industry better understand the impact of pre-cooking temperatures on kill rates for the five histamine-forming bacteria evaluated in the study.