| Frigate tuna (Auxis thazard) is a small kind of tuna, one of the major tuna fishery species. Frigate tuna is highly perishable and has high levels of free histidine in muscle, which is prone to decarboxylation under the action of histamine-producing bacteria and generates a large number of histamine. It brings safty risks to the consumption of frigate tuna. In this paper, the nutritional components of back muscles from frigate tuna were analyzed, the change of the freshness, histamine content and histamine-producing microorganism species at various temperatures were studied, the isolation and identification of microorganism capable of producing histamine were carried out. The main findings were as follows:Firstly, the nutritional components of back muscles from frigate tuna were analyzed and evaluated. The results showed that the content of moisture, ash, crude protein and crude fat were72.40%,1.53%,24.65%and1.85%, respectively.23kinds of fatty acids were detected in this experiment. Amongst them, palmitic acid (C16:0) had the highest content of24.26%. Polyunsaturated fatty acids accounted for37.93%of total fatty acids. At the same time, DHA+EPA accounted for31.22%of total fatty acids. Among17kinds of amino acids detected in back muscles of frigate tuna, Glu contained the highest content of11.01%. According to the AAS and CS, the proportion of essential amino acids in line with FAO/WHO standards, the content of the essential amino acids and essential amino acid index were32.51%and0.89,respectively. Thus, the back muscle of frigate tuna had a good quality of protein. However, the content of histidine in muscle of frigate tuna was up to7.89%, which provided the conditions for a large number of histamine production.Secondly, the change of the freshness and histamine content during storage at0℃,4℃,15℃,2℃were studied. The results showed that with the extension of storage time, TVB-N, K value, microbial and other indicators were rising constantly, while sensory quality kept deteriorating. There was a good linear relationship between the sensory indicators and the K value (R2=0.8988). The shelf life of frigate tuna during storage at0℃,4℃,15℃and25℃was15days,10days,2days and1days, respectively. At0℃, frigate tuna could hardly produce histamine during storage, while at4℃, histamine content of frigate tuna reached38.92mg/100g on the14th day, exceeding the national standard(30mg/100g). However, at15℃and25℃, histamine content increased rapidly and reached89.09mg/100g and186.72mg/100g after storage of3days and2days, respectively. Therefore, in order to ensure food safety and processing applicability, frigate tuna should be immediately frozen and kept cold storage after catch.Thirdly, the change of Enterobacteriaceae, Pseudomonads spp. and Vibrio spp. at0℃,4℃,15℃and25℃during storage were studied. The results showed that at0℃and4℃, most of microorganisms were inhibited, of which Vibrio spp. showed no growth during the entire storage, and Enterobacteriaceae grew slowly, while Pseudomonads spp. which grew well at low temperatures and became dominant bacteria at0℃and4℃. At15℃and25℃, the microorganisms grew rapidly, the correlation between Enterobacteriaceae, Pseudomonas spp., Vibrio spp. and histamine content was high, which Enterobacteriaceae was proved to be the principal microorganism species which affected histamine production.Finally, the isolated microbial strains which had ability to produce histamine were identified. The results showed that145isolates were separated, of which29isolates had shown the ability to produce histamine under suitable conditions. The strongly histamine-producing isolates (≥1000mg/L) like Klebsiella pneumoniae, Proteus vulgaris, Enterobacter aerogenes, Pseudomonas fluorescens were identified, and most of them came from15℃and25℃. Besides, the weak histamine-producing isolates (<1000mg/L) were identified as Providencia rettgeri, Enterobacter cloacae, Pseudomonas putida, Vibrio alginolyticus, Photobacterium phosphoreum, Acinetobacter lwoffi and Proteus sp. etc. Histamine formers obtained from frigate tuna at room temperature (15℃and25℃) was significantly more than those obtained at low temperature (0℃and4℃). Thus, storage at low temperature inactivated histamine-producing bacterias. |
