| Shrimp head is the main by-product produced during headless shrimp and shrimpmeat process. For it is rich in protein, unsaturated fatty acids, amino acids, astaxanthin,vitamins and essential trace elements, it is the high quality raw materials for the productionof seafood flavoring. The main way to extract the shrimp head inclusions (autolysis) is tomake full use of endogenous enzymes activity of shrimp head, which had the advantagesof low cost, mild conditions, easy control, etc. But shrimp head inclusions often had badflavor such as bitter taste, fishy smell, etc, which affected the quality of the product.Adding flavourzyme while autolysis could reduce bitter taste, but it had little effect on theimprovement of fishy smell, off-flavor. At present, the main way to improve bad flavorwere physical method, chemical method and biological method. However, the method ofmicrobial fermentation had advantages of promoting protein hydrolysis, increasingfermentation aroma, inhibiting fishy smell, off-flavor, etc. It can solve the problem of badflavor of shrimp head enzymatic hydrolysate. In this thesis, the method of microbialfermentation was used to improve bad flavor of shrimp head enzymatic hydrolysate. Firstof all, its fermentation process were studied. Then, the changing rules of flavorcomponents and biochemical indicators of shrimp head enzymatic hydrolysate duringfermentation were explored. On these basis, the formation mechanism of flavor changingwas primarily research. The main contents and results are as follows:(1) The optimal strain combination was first chosen taking sensory evaluation andcontent of amino nitrogen as the indicator. The technology of fermenting shrimp headenzymatic hydrolysate was optimized by using orthogonal array design, and the effects offermentation temperature, fermentation time, inoculum size on the indicator wereinvestigated. Results showed that: Staphylococcus xylosus, Pediococcus pentosaceus andBacillus subtilis (ratio:1:1:1) was the optimal strain combination. The optimumtechnological conditions were as follows: fermentation temperature30℃, fermentationtime8h, inoculum size3%, respectively. With these conditions, fermentation broth hadremarkable shrimp flavor and the flavor was full. Besides, the content of amino nitrogenachieved0.453mg/100g under the fermentation condition.(2) The changes of main taste components of shrimp head enzymatic hydrolysateduring fermentation were investigated by chemical and instrument analysis. The results showed that: There were five major kinds of changing rules in the taste compounds ofshrimp head enzymatic hydrolysate during0-72fermentation:1) The content changednon-significantly or was below the threshold and the main compounds were inorganic ions,malic acid and AMP.2) The content decreased gradually and the main compounds wereglycogen and glucose.3) The content increased gradually and the main compounds wereacetic acid, citric acid and betaine.4) The content showed a trend of first increase thendecrease, the main compounds were nucleotide associated compounds, GMP, Hx, lacticacid, succinic acid. The content of all them were maximized at15h.5) The content showeda trend of first decrease, then increase and last decrease, and the main compounds wereamino acids and the umami amino acids. The content of amino acids first reduced at0-25h,then increased at25-48h, last reduced at48-72h and the content of the umami amino acidsfirst reduced at0-8h, then increased at48-72h, last reduced at48-72h.The main tastes of shrimp head enzymatic hydrolysate were nucleotide associatedcompounds, betaine, glucose and glycogen at8h. Compared with the fermentation at0h,the content of amino acids and the umami amino acids in shrimp head enzymatichydrolysate fermented at8h reduced by360mg/100mL,20mg/100mL, respectively, but thecontent of nucleotide associated compounds increased by13.11mg/100mL. Meanwhile,betaine, acetic acid, citric acid, lactic acid and succinic acid also increased at8h. Thecontent of AMP was most at8h, to a certain extent, it could inhibit bitter and enhance tastethough it was below the threshold. However, compared with the fermentation at15h, thecontent of Hx reduced by3.62mg/100mL, but the content of glycogen and glucoseincrease by1.34mg/100mL、666.5mg/100mL, respectively.(3) The changes of the volatile flavor compounds of shrimp head enzymatichydrolysate during fermentation were investigated by SPME-GC-MS. The results showedthat: During0-72h fermentation period, the number of Volatile compounds showed a trendof first increase then decrease, increased by16at0-25h, then decreased by11at25-72h.Volatile compounds mainly included aldehydes, alcohols, ketones and nitrogen compounds.The number of aldehydes increased from5kinds to7kinds at0-25h, then reduced to2kinds at72h, and the relative content of aldehydes reduced from20.45%to1.39%at0-72h. But the changes of straight chain aliphatic aldehydes, branched aldehyde,unsaturated aldehyde presented a different trend. The relative content of straight chainaliphatic aldehydes increased at0-8h, then decreased after8h. The relative content ofbranched chain aldehyde reduced at0-72h. The relative content of unsaturated aldehydehad been increasing at72h. The flavor of aldehydes gradualy changed from vanilla flavor,green grass and spicy smell to fish smell and the plants. The number of alcohols increased from6kinds to12kinds at0-25h, then reduced from12kinds to10kinds at25h-72h. Therelative content of alcohols increased from12.64%to26.12%at0-8h, then reduced to23.73%at8-15h, last increased to25.08%again at15-72h. However the changes of Longstraight chain alcohols, branched chain alcohol, unsaturated aldehyde presented a differenttrend. The relative content of Long straight chain alcohols increased at0-15h, then reducedafter15h. The relative content of branched chain alcohol increased at0-25h, then reducedafter25h. The relative contents of unsaturated alcohol changed non-significantly. Theflavor of alcohols gradualy changed from faint scent, wood and fat to rancid flavor and soilsmell. The number of ketones increased from3kinds to6kinds at0-72h. The relativecontent of ketones increased from8.16%to47.13%at0-72h. And the relative contentsof methyl ketone had also been increasing at0-72h. The flavor of ketones gradualychanges from the sweet fragrance of flowers and fruit flavor to benzoquinone, butter smells.The number of nitrogenous compounds increased from2kinds to5kinds at0-72h. Therelative content of nitrogenous compounds increased from10.35%to12.44%at0-25h,then reduced to4.4%after25h. The four compounds became the flavor subject of shrimphead enzymatic hydrolysate fermentation broth and other compounds also had a certaincontribution to fermentation broth.The main volatile flavor compounds of shrimp head enzymatic hydrolysate wereesters, straight chain aliphatic aldehydes with aroma, pyrazines,2,3-butanedione, dimethylsulfide, pelargonic acid at8h. When shrimp head enzymatic hydrolysate fermented at8h,the relative content of esters, straight chain aliphatic aldehydes with aroma ware most,5.6%,4.82%respectively. Compared with the fermentation at0h,2,3-butanedione andpyrazines began to appear at8h. Compared with other fermentation time, the relativecontent of branched chain alcohol with rancid, pungent smell was less, but dimethylsulfide and pelargonic acid only appeared at0h and8h, and the more fermentation time,the less furans and aromatic compounds contributed to the flavor.(4) The changes of pH, total acid, bad smell gas of nitrogen compositions, flavorrelated enzymes and microbial populations in shrimp head enzymatic hydrolysate during0-72h fermentation were measured by chemical and instrument analysis. The resultsshowed that: pH decreased firstly, then increased and tended to stable lastly and pH waslowest,5.52,at25h. The content of total acid increased firstly, then decreased and tendedto stable lastly and the content was most,0.528g/100mL, at25h. The content of bad smellgas of nitrogen compositions gradually increased at0-72h, and the content of ammonianitrogen, TVBN, putrescine, cadaverine respectively increased from57.5mg/100mL to698.5mg/100mL,28.59mg/100mL to203.51mg/100mL,10.85mg/100mL to25.60 mg/100mL,7.3mg/100mL to38.94mg/100mL. There were two major kinds of changingrules of flavor changing related enzymes.1) The activity showed a trend of rise, drop,again rise, again drop, and the main enzyme was protease. The activity of protease risedat0-8h.2) The activity showed a trend of rise, drop, again rise, and the main enzymeswere glutamate dehydrogenase and glutamate dehydrogenase. The activity of them bothrised at0-8h.3) The activity showed a trend of drop, rise, and the main enzymes wereornithine decarboxylase and lysine decarboxylase. The activity of them both dropped at0-8h.There were two major kinds of changing rules about bacteria during0-72hfermentation.1) Good bacteria showed a trend of first increase then decrease. The amountof Lactic acid bacteria was most,9.17log CFU/g, at25h. The amount of Staphylococcusxylosus was most,8.39log CFU/g, at8h.2) The amount of spoilage bacteria had beenincreasing at0-72h. The amount of Pseudomonas gradually increased from4.11log CFU/gto8.93log CFU/g at0-72h. The amount of Enterobacteriaceae gradually increased from4.73log CFU/g log CFU/g to8.17at0-72h.Staphylococcus xylosus, Pediococcus pentosaceus and Bacillus subtilis had theoptimal growth pH during0-8h fermentation. The activity of protease rose, but the activityof ornithine decarboxylase, lysine decarboxylase declined at0-8h. The content ofputrescine had no significant change at0-8h. Xylose staphylococcus had a majorcontribution to the flavor of shrimp head enzymatic hydrolysate. However, the content ofbiogenic amine, such as putrescine, increased rapidly and the speed of flavor deteriorationaggravated. So the content of putrescine at8h (11.5mg/100mL) could be an end pointjudgment of fermenting shrimp head enzymatic hydrolysate.The formation of the flavor of shrimp head enzymatic hydrolysate duringfermentation was a complicated process. The microbial metabolism and its secretion ofexogenous enzymes were the main motivation. And the hydrolysis of protein, peptide,amino acids and sugar added were the substrate for the formation of the flavor. Favorformated mainly by the way of the degradation of protein, amino acid metabolism, sugarand nucleotides degradation, etc. |
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