| Silver carp(Hypophthalmichthys molitrix)is one of the famous four major Chinese carps and the annual yield is on the top three of cultured freshwater fish production in China.Along with the high-speed development of freshwater fish processing industry,the fish fillets,the primary processing products of silver carp,enter innumerable homes.Fish products are perishable during the process of cold chain transportation and sales because of rich in water and nutrients.Therefore,the changes of freshness indictors of silver carp during refrigerated storage were evaluated.Meanwhile,the microbial indicators were non-linear fitted through shelf-life primary model and the secondary model to establish shelf-life prediction model.The changes of silver carp microflora during storage at different temperatures were analyzed and the specific spoilage bacteria were separated and identified.The above experimental researches offered a theoretical basis for improving silver carp muscle quality and developing targeted inhibition.The main results obtained in this study were as follows:1.Total volatile basic nitrogen(TVB-N)value,pH,trimethylamine(TMA)value,2-thiobarbituricacid(TBA)value,K value,the total viable counts and sensory evaluation were investigated to evaluate the quality change of silver carp fillets during chilled storage and partial freezing storage.The results showed the TVB-N value,TMA value and the total viable counts of silver carp fillets stored at 4? and partial freezing at-2? increased with the extension of storage time,pH value increased after an initial decrease and the score of sensory evaluation showed a decreasing trend.Taking the change tendencies of all indicators into consideration,the shelf life was 6 days at chilled storage(4?)and 18 days at partial freezing storage(-2?).It could be concluded that partial freezing temperatures extended the shelf life obviously.2.In order to develop the shelf life predictive models of silver carp during cold storage,the changes of total viable counts(TVC),pseudomonas and H2S-producing bacteria in silver carp fillets at different temperatures were investigated,separately.Microbial growth was non-liner fitted by different growth dynamic models and the most suitable model was chosen to establish predictive model of silver carp.The first-order chemical reaction kinetics model,the modified Gompertz model and Baranyi and Roberts model were used as the primary dynamics to describe relationship between microbiological growth and the time.Belehradek equation(square root model)was used as secondary models to describle the influence of temperature on microbiological growth.The results showed that the modificatory Gompertz equation described the dynamic change of microbial growth more accurately,the secondary model established by Belehradek equation could predict shelf life exactly and the relative-error was within the scope of±20%.3.The changes of silver carp microflora during storage at different temperatures were analyzed by 16S rRNA sequence analysis and the specific spoilage organisms were separated and identified using the VITEK 2 automated microbial identification system.The results showed that the composition of fresh silver carp bacterial flora was similar,however,the microflora gradually became unitary with the extension of storage time.Pseudomonas spp.represented the highest proportion of 78.3%,64.2%and 43.6%at the end of shelf life at-2,0 and 4?.Secondly,Aeromonas spp.represented proportion of 15.4%,20.6%,22.3%and Shewanella spp.represented 6.3%,11.4%,18.1%respectively.The proportion of Pseudomonas spp.increased with the decrease of storage temperature.The sequences,amplified by polymerase chain reaction(PCR),were compared by the National Center for Biotechnology Information(NCBI)database,and the automated microbial identification system was utilized for all measurements.These results identified Pseudomonas fluorescens to be the dominant spoilage,followed by Aeromonas sobira and Shewanella putrefaciens4.The growth of spoilage bacteria and the freshness indexes including sensory evaluation,TVB-N,TMA,electric conductivity for sterile silver carp tissue blocks inoculated Pseudomonas fluorescens,Aeromonas sobira and Shewanella putrefaciens stored at 4? were analyzed,respectively.The yield factor for production of metabolite and the cell concentration determined at the time of off-odour detection were used as quantitative measurements of microbial spoilage ability.Results showed the freshness of sterile silver carp tissue blocks inoculated spoilage bacteria deteriorated obviously than the sterile control group.Spoilage ability of Shewanella putrefaciens was strongest,and YTMA/CFU of Pseudomonas fluorescens was lower than Shewanella putrefaciens and Aeromonas sobira.Three kinds of spoilage bacteria had certain spoilage abilities,however it was not significant until it reached high counts. |
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