Study On The Quality And Microorganims Change During The Fermentation Of Large Yellow Croake

Cultured large yellow croaker is tender in texture, has sweet taste and plain nature with higher level of fat which influences the flavor, storage and processing. In the recent years, with increasing output of cultured large yellow croaker, developing processing method for large yellow croaker has become one of effective methods in the sound development of the industry. The "Sanbao" traditional fermentation method populated in Eastern Zhejiang district could not only contribute products with good flavor, but also increase the added-value of large yellow croaker. However, the salt content added was50%of the wet content which would be harmful for long term consumption. The thesis aimed on investigating in microorganisms population and physicochemical property changes during the fermentation process of large yellow croaker pickled at different sanities (15%,20%,25%) and temperatures (15℃,22.5℃,30℃), and volatile compounds extracted from large yellow croaker pickled for20days. What’s more, the difference of volatile compounds was analyzed based on the comparison with control group (pickled with50%salt content).Nutritional components of the optimal group and control group were further analyzed. Lastly, the predominant microorganisms were selected and then analyzed using Restriction fragment length polymorphism (RFLP).The main research contents and results were as follows:The effect of different fermentation conditions on qualities of pickled large yellow croaker. TVB-N value increased with higher salt content and lower temperature, while TBA values did not show much difference. Lactic acid bacteria and staphylococci were the predominant microorganisms during the fermentation. The amount of lactic acid bacteria and staphylococci increased significantly at the beginning of the fermentation, and it lowered for increased salt content and decreased water content. The growth of enterobacteriaceae was inhibited with the action of salt.Volatiles compounds extracted from pickled large yellow croaker were analyzed by HS-SPME,78compounds were identified. At15℃, the relative content of esters, aldehyde and ketone differed significant among the three group. At22.5℃, the content of alcohols, aldehydes and acids increased with higher salt content. Compared with samples fermented at15℃, large yellow croakers at22.5℃were detected with lower content of hydrocarbons and higher sulfur-containing compounds. Also sulfur-containing compounds were detected. At30℃, nitrogen containing compounds were the most abundant. The lower hydrocarbons, higher sulfur and nitrogen containing compounds were detected with the decrease of salt content.Combined the result with Hierarchical cluster analysis, Principal component analyze and ROAV analyze method. The control group and products pickled at22.5℃with25%salt content had similar flavor, the main flavor contributors were dimethyl disulfide,3-Methylbutanal and ethyl acetate. The croaker pickled at15℃with25%and15%salt content with aldehydes, esters and alcohols as main contributors, had the totally different flavor from those pickled at22.5℃. But trimethylamine was also the key flavor compounds in large croaker pickled at15℃with15%salt content.The radar chart drawn with10kinds of flavor index showed that relative similar flavor were found for croaker picked with25%salt content at22.5℃and control group. Products with25%salt content at15℃had the characteristic fruity and grass smell. These two groups had better flavor and could be selected for optimal groups.Evaluate nutritional components in the optimal and control group. Histamine detected from control group, croaker picked at15℃and22.5℃with25%salt content were91.67mg/kg,2.92mg/kg, and41.50mg/kg, respectively. There are17kinds of amino acids in all the samples. Glutamic acid, aspartic acid and leucine were the most abundant. Amino acid index indicated that the fresh large yellow croaker was fine protein source, while the pickled large croakers were high-class protein source. Fatty acid were mainly saturated fatty acid, unsaturated fatty acid relative content in fresh fish, control group, croaker with25%salt content at15℃and22.5℃were44.00%,38.21%,48.58%and52.96%, respectively. 275isolates were selected and purified on MRS and MSA medium. Through RFLP analyze and16S rDNA sequence,259isolates were identified as staphylococci,4isolates as lactic acid bacteria, and12isolates as Proteus. The dominant species in the control group were S. xylosus, S. saprophylococcus, and S. nepalensis. Large croaker pickled with25%salt content at15℃was predominated by S. xylosus, S. vitulinus, S. saprophyticus and S. nepalensis. S. xylosus, S. saprophyticus and S. vitulinus were the most prevailing cultivable species recovered from large yellow croaker pickled with25%salt content at22.5℃. In the early fermentation period, the popular enterobacteriaceae was Pseudomonas putida in control group, Pseudomonas putida and Proteus penneri in large croaker pickled with25%salt content at15℃, Pseudomonas putida and Proteus vulgaris in yellow croaker pickled with25%salt content at22.5℃.