Study On Histamine And Histamine-Producing Bacteria In Bigeye Tuna During Storage At Various Temperatures

Bigeye tuna (Thunnus obesus) was one kind of tuna with high economic value. Expect for the expection of corruptible like other fish, bigeye tuna has high histidine content, high level of histamine will be produced by histamine-producing bacteria at appropriate conditions, which may caused allergy and damaged human health. Nutritional components of different muscle parts from bigeye tuna were investigated and evaluated, the variation of the freshness and histamine content of Akami muscle during storage at various temperatures were studied, the main microorganism species who affect histamine production at various temperatures were researched; the isolation and identification of histamine-producing bacteria were carried out in the thesis. The research will provide theoretical basis for further processing and histamine control of tuna. The main results of the thesis were as follows: First, nutritional components of Akami, Chu-toro and O-toro muscle from bigeye tuna were investigated and evaluated. Results showed that chemical compositions expect for moisture contents were significant difference (P＜O.05) in three muscle parts, especially fat contents, ranging from0.16%in Akami,4.80%in Chu-toro to6.35%in O-toro. Essential amino acids content of three muscle parts were34.08%,24.90%and25.09%, EAAI were88.08,84.80and85.93, respectively. The polyunsaturated fatty acids contents were also abundant in three muscle parts, they were41.00%,28.12%and23.72%. Three muscle parts all contained abundant macro and micro elements. Consequently, Akami, Chu-toro and O-toro muscle from bigeye tuna were all contained high edibleness.To the question of temperature influence on the freshness and histamine content of bigeye tuna, the variation of the freshness and histamine content in Akami muscle during storage at0℃,4℃,15℃and25℃were studied. Results showed that the K values increase rates were2.6%/d,6.3%/d,18.6%/d and46.7%/d, and the total viable counts exceeded106CFU/g on14d,7d,36h and24h when Stored at0℃,4℃,15℃and25℃, respectively. It can be concluded that the shelf life of bigeye tuna was10d at0℃,5d at4℃,24h at15℃and12h at25℃according to the changes of all indicators. Histamine was only found after12d and5d at0℃and4℃, however histamine content increased rapidly and exceeded30mg/100g after48h and24h during storage at15℃and25℃, respectively. Therefore, low temperature storage is an effective method to slow down the rate of freshness decreasing, prolong the shelf life of bigeye tuna, and suppress the increase of histamine production.In order to analysis the microbial species which affect histamine production in bigeye tuna during stored at different temperatures, the thesis separated Enterobacteriaceae, Pseudomonas spp. and Vibrio spp. in bigeye tuna during stored at0℃,4℃,15℃and25℃, and the correlations between histamine contents and different microorganism counts were carried out. Results showed that all kinds of microorganism influenced mutually at25℃, histamine production was affected by combined action of all microorganism. At15℃storage condition, Vibrios spp. was proved to be the principal microorganism species who affected histamine production. At0℃and4℃storage conditions, the influence of low temperature to Pseudomonads spp. was weaker than other microorganism. Pseudomonads spp. count had high correlation index with histamine content indicated that Pseudomonads spp. was the main microorganism species which affected histamine production.To confirm the species of histamine-producing bacteria in bigeye tuna, a total of129bacteria were isolated from different selective media, and only28of them could produce histamine. PHPB (Prolific histamine-producing bacteria which produce>1000mg/kg histamine) were Morganella morganii, Klebsiella oxytoca and Enterobacter cloacae. Another bacteria such as Staphylococcus saprophyticus, Serratia grimesii, Enterobacter Agglomeranss, Proteus vulgaris, Citrobacter freudii, Proteus mirabilis, Pseudomonas putida, Vibrio alginolyticus, Hafnia alvei, Vibrio parahaemolyticus, Photobacterium damsela, Acinetobacter guillouiae and Plesiomonas shigelloides were weaker in the capatity of produce histamine. Histamine-producing bacteria mostly isolated from bigeye tuna Akami muscle stored at15℃and25℃. While stored at0℃and4℃, the relevance ratio of histamine-producing bacteria was far below than stored at25℃and15℃, and the histamine-producing bacteria selected with weak histamine produce capacity. This was also the reason why histamine content in Akami muscle increased with huge difference during storage at various temperatures.