| Because of its high protein content,high water content and near neutral muscle pH,freshwater fish are easy to decay in storage,transportation,processing and sale.This is mainly due to the presence of microorganisms,fish skin,gill and intestinal bacteria growing in fish reproduction,resulting in deterioration of the fish.With three species of freshwater fish samples(Crucian carp,Bream carp and Tilapia carp)in Guangzhou area as the research objects,explore the law of quality change of freshwater fish during cold storage.Bacterial succession of three important storage sites(freshness,end point and decay point)of freshwater fish was revealed by means of high throughput sequencing,which identify the low temperature characteristic spoilage organisms of freshwater fish.The aim was to provide theoretical basis for targeted preservation of fresh water fish during cold storage.The main research methods and results are as follows:1.Three popular freshwater fish samples(Crucian carp,Bream carp and Tilapia carp)were collected in Guangzhou.Through the measurements of pH and TVB-N,K value,microbial.counts and QIM analysis,the quality changes of fresh water fish at 4 ? during cold storage were expounded.The result shows that pH and TVB-N,K value,prolong the microbial count and QI scores freshwater fish under refrigerated conditions increased significantly with the increase of storage time(p<0.05).Among them,TVB-N,K value,microbial counts and sensory analysis showed a significant correlation(p<0.01),pH and sensory analysis showed a significant correlation(p<0.05);from the evaluation of the indicators,of three kinds of freshwater fish,tilapia carp corruption was the fastest,followed by bream carp and crucian carp;crucian carp kept good quality within 8 days,while tilapia carp and bream carp within 4 days,and all indexes of three fishes showed decay when they were stored for 12 days;2.The amplification was combined with high-throughput sequencing technology,aiming to assess and understand the microbial communities in depth of freshwater fish during cold storage.A total of 574,357 high quality sequences obtained after quality filtering of raw reads were clustered into 24354 OTUs at 97%identity threshold.The number of OTU fish at fresh point was higher than that at middle point and spoilage point;in the classification level,25 gates,64 classes,111 orders and 379 genera were measured.The five main gates Proteobacteria is Firmicutes,Fusobacterium,Actinobacteria and Firmicutes.Among them;379 genera belong to 101 Proteobacteria,41 Firmicutes,32 Clostridium,15 actinomycetes and 7 bacteriodetes;the dominant bacteria of freshwater fish at fresh point were Pseudomonas,Pseudomonas,Cetobacterium,Chryseobacterium and Akkermansia;dominant bacteria of freshness end point included Pseudomonas,Cetobacterium and Lactococcus;the dominant bacteria at spoilage point were Pseudomonas,Carnobacterium and Lactococcus;therefore,during the whole storage,Pseudomonas and Akkermansia were the dominant genera but Pseudomonas increased,and Akkermansia showed the opposite trend.In addition,with the proportion of Cetobacterium,Cetobacterium,Chryseobacterium and Lactococcus increasing gradually as time goes on,Pseudomonas and Akkermansia were identified as characteristics of spoilage bacteria of chilling freshwater fish;3.With three species of freshwater fish in Guangzhou as experimental objects,Screenning corruption strain from them.All the colonies were round,flat,smooth and lustrous.Most colonies were opaque and could be divided into milk white,white and yellow.The results of Gram staining showed that there were 16 strains of G'and 4 strains of G+ in 20 strains of cold tolerance and the shapes of the bacteria were divided into three kinds:rod-shaped,globular and club shaped.By morphological observation,physiological and biochemical analysis and 16S rDNA sequencing,20 strains of cold resistance were identified.It turned out that 20 strains of cold resistance were mainly Pseudomonas fluorescens,Pseudomonas,Shewanella spp.,psychrotrophic bacillus.And Pseudomonas accounted for the highest proportion,reaching 90%;4.Determining pH,TVB-N,growth curve and protein,chitinase of three bacterial strain by simulated corruption experiment,The results showed that TVB-N,chitinase and the growth curve in the refrigeration process,prolong significantly with storage time increased,while pH protein changed little,protein concentration decreased with the extension of cold storage time.The increasing trend in the process of cold storage of Pseudomonas's TVB-N value and chitinase is greater than two other bacterial strain,and the growth curve is the opposite trend;The descend trend of Pseudomonas's protein concentration is greater than two other bacterial strain.Combined with the sensory evaluation,Pseudomonas's spoilage ability was significantly higher than that of two other bacterial strain,Pseudomona's can be determined as the dominant spoilage bacteria. |
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