Bacterial Dynamics During Fermentation Of Shrimp Paste And Its Effect On Flavor Formation

Traditional fermented shrimp paste is one typical kind of fermented aquatic condiment,which is produced by multispecies solid-state fermentation,taking fresh Euphausia pacifica as main materials.Its quality and characteristic flavor are largely determined by microbial composition and metabolic activities.At present,the correlation between the bacterial ecosystems and flavors are still not understood completely.In this paper,traditional fermented shrimp paste was used as the material to identify and analyze the composition of bacteria and volatile flavor components to explore the correlation between bacteria and aroma components,and then isolate dominant microorganisms.It provides useful insights for determining the main related bacteria in flavor formation during fermentation,and provides a theoretical basis for selecting high-efficiency species as proppant candidates.Main results of this thesis were summarized as follows:(1)Using high-throughput sequencing technology to analyze the bacterial composition of shrimp paste,it was found that Salimicrobium,Lentibacillus,Lactobacillus and Tetragenococcus are the main dominant genus with the highest proportion of 96.73%.These 4 genera exist in the whole fermentation stage,indicating that the microorganisms of these three genera are the main functional microorganisms in the shrimp paste fermentation process.The systematic action of a variety of bacteria may be the main reason for giving the shrimp paste a rich and unique aroma and taste.The volatile components in the shrimp paste samples of each fermentation stage were detected by SPME-GC-MS,and 77 substances were detected,including alcohols,ketones,aldehydes,nitrogenous compounds,sulfur compounds,hydrocarbons,acids,esters and other compounds.(2)The temporal correlation between bacteria and flavor compounds was explored using O2 PLS method.It was found that the change of volatile compounds during the fermentation of shrimp paste had a strong correlation with the bacterial composition.Among them,Lactobacillus,Tetragenococcus and Salimicrobium contribute the most to the changes in the content of various volatile compounds.They have a high positive correlation with various characteristic aroma components in mature shrimp paste,and have a higher negative correlation with compounds producing unpleasant odor,It is indicated that the three fungi play an important role in the changes of volatile compounds,and they are important genus that affect the quality of shrimp paste.(3)The traditional culture method was used to culture and isolate the strains in the different fermentation stages of the traditional shrimp paste.A plurality of strains including 12 strains of Tetragenococcus halophilus,4 strains of Bacillus subtilis,2strains of Corynebacterium casei,Lactobacillus fermentum,Lactobacillus casei and Lactobacillus plantarum,and 2 strains of Serratia nematodiphila were isolated.The fermentation performance and safety of the isolated strains were evaluated.By comparison and screening,T.halophilus,L.plantarum,L.casei,B.subtilis,C.case and L.fermentum with good acid and protein hydrolysis ability under high salt concentration were obtained.(4)The electronic nose was used to test the samples of the various stages of shrimp paste inoculated with different strains.Principal component analysis found that the fermentation cycle of different strains was divided into three stages,but the corresponding time of each stage was different due to different strains,which may be due to the difference in growth rate and metabolic activity of different strains.A total of 35 volatile compounds were identified by SPME-GC-MS.Six kinds of bacteria were beneficial to reduce the content of trimethylamine.T.halophilus,B.subtilis and L.fermentum were beneficial to the increase of 3-methyl-pyrazine content,and T.halophilus and L.casei promoted the increase of acetic acid content.Principal component analysis revealed that these compounds may be produced or reduced in large quantities due to metabolic activities during the fermentation of shrimp paste,indicating that the dominant species in the six shrimp pastes have a significant effect on the flavor of shrimp paste.