Research On Preservation Technology And Bacterial Diversity Of Thunnus Obesus During Chilled Storage

Tuna is one of the most important pelagic edible fish,which is rich in nutritional value and economic value.Tuna is extremely perishable and has a short shelf-life.This thesis mainly foucused on those problems mentioned above.Initialy,we investigated the bacterial diversity of tuna during chilled storage by phenotypic and polymerase chain reaction denaturing gradient gel electrophoresis(PCR-DGGE)analysis.In order to identify the specific spoilage organisms of tuna,the spoilage potential of the predominant bacteria isolated from spoiled tuna was evaluated.Based on those,the effect of electrolyzed water combined with natural preservative on the quality changes of tuna during chilled storage was investigated.And then,the influenc of the fresh-keeping techniqule on the spoilage potential of dominant spoilage bacteria was carried out.Finally,we explored the best ratio of atmosphere packaging and electrolyed water combined with biological preservation for tuna preservation.The specific research contents and results are as follows:1.The quality changes of tuna during chilled storage were studied by sensory assessment,colorimetric parameters,texture,total volatile basic nitrogen(TVB-N),K value and aerobic bacterial count,psychrotrophic bacterial count.Phenotypic analysis in combination with polymerase chain reaction-denaturing gradient gel electrophoresis(PCR-DGGE)was used to investigate bacterial diversity of tuna during chilled storage.The results showed that tuna had an obvious sensory deterioration during the chilled storage,as well as L*value,a*value and chewiness,hardness.After 4 days of storage,the total bacterial counts rose to 4.66 lgCFU/g and the psychrotrophic bacterial counts increased more quickly than total bacterial counts.At the end of storage,TVB-N value and K value rose to 25.00 mg/100 g and 23.00% respectively.The bacterial community structure of tuna,to some extent,had a similarity in the storage period.The initial microbial flora was dominated by Hydrogenophaga sp.,Acinetobacter spp.,Pseudomonas spp.,Pseudochrobactrum saccharolyticum,Bosea eneae,Sphingomonas spp.and Brevundimonas olei.The bacterial diversity of tuna decreased along with storage time,and Rhodoplanes oryzae,Acinetobacter johnsonii,Pseudomonas spp.,Pseudochrobactrum saccharolyticum became the predominant bacteria after 6 days of storage.2.The spoilage potential of Pseudomonas spp.,Acinetobacter spp.and Brochothrix thermosphacta selected from chilled raw Thunnus obesus at the end of shelf life were taken as the research objects,and spoilage bacterial counts,sensory quality,total volatile basic nitrogen,biogenic amine contents,the growth kinetic parameters of spoilage bacteria and the yield factor of metabolic products,YTVBN/CFU,were assayed on sterile Thunnus obesus inoculated with spoilage bacteria during chilled storage.It was found that the model equation of modified Gompertz,Baranyi and Roberts,and Logisitic could describe the dynamic microbial growth of Pseudomonas spp.,Acinetobacter spp.and Brochothrix thermosphacta in cold temperature thunnus obesus better,respectively.The lag phase of the 3 bacterial strains were 10.85 h,83.93 h and 79.11 h,respectively,the relative maximum growth rate were 0.04192 h-1,0.18287 h-1 and 0.11111 h-1,respectively.The shelf life of inoculated tuna was 96 h,108 h and 105 h,respectively.The postinoculation TVBNs of sterile Thunnus obesus inoculated with Pseudomonas spp.,Acinetobacter spp.and Brochothrix thermosphacta was 27.32mgN/100 g,22.80 mg N/100 g and 24.85 mg N/100 g,respectively.Studying on the changes of biogenic amines contents of Thunnus obesus during chilled storage were found that the ability of Pseudomonas spp.of producing biogenic amines was stronger than those of Acinetobacter spp.and Brochothrix thermosphacta.The biogenic amine index of Pseudomonas spp.inoculated tuna was 155.84 at the 96 th storage hour,and it was higher than the threshold value 100.This study also show that the YTVBN/CFU values of the 3 strains were 7.36×108mg TVBN/CFU,1.85 mg TVBN/CFU and 5.07 mg TVBN/CFU,respectively,indicating that Pseudomonas spp.played the most important role in spoilage of chilled raw Thunnus obesus,followed by Brochothrix thermosphacta and Acinetobacter spp.3.In order to extend the shelf life of tuna under chilled storage,reponse surface method was used in this experiment to form a complex preservation method with electrolyzed water,clove extract and ginger extract.Chilled tuna were treated with optimized comlex preservation method.Redness value,textural analysis,total volatile basic nitrogen contents,K value and aerobic bacterial count,psychrotrophic bacterial count,bacterial diversity were determined to verify the effects of preservation.The result showed that the best combination of complex preservation was soaking the tuna meat with electrolyzed water which the concentration of NaCl solution was 0.18% united with bio-preservative which was made up of clove 2.09% and ginger 1.64%.And the shelf life of chilled tuna treated with complex preservation method was extended to 5~6 days stored under 4?.4.The effect of complex preservation method acquired from previous research of spoilage potential of Pseudomonas spp.and Brochothrix thermosphacta selected from chilled raw Thunnus obesus at the end of shelf life were taken as the research objects,and spoilage bacterial counts,sensory quality,total volatile basic nitrogen,the growth kinetic parameters of spoilage bacteria and the yield factor of metabolic products,YTVBN/CFU,were assayed on sterile Thunnus obesus inoculated with spoilage bacteria during chilled storage.It was found that with the complex preservation treatment the lag phase of Pseudomonas spp.was prolonged 3.19 times,the relative maximum growth rate of Brochothrix thermosphacta decrecased by 55.83%.The shelf life of chilled tuna treated with complex preservation method was 192 h and 144 h,respectively.The postinoculation TVBNs of Thunnus obesus inoculated with Pseudomonas spp.and Brochothrix thermosphacta was 24.26 mgN/100 g and 23.78 mgN/100 g,respectively.The complex preservation method could reduce the YTVBN/CFU value of Pseudomonas spp.and Brochothrix thermosphacta by 39.30% and 38.74%,respectively.The bio-preservative composed by clove and ginger had a strong inhibition function against the bactetria,the minimum inhibitory of Pseudomonas spp.and Brochothrix thermosphacta was 9% and 6%,respectively.5.In order to investigate the effects of five kinds of packaging methods including group CK(air packaging),group CK2(air packaging + complex preservative),group MAP1(5%O2,40%CO2,55%N2 + complex preservative),group MAP2(5%O2,60%CO2,35%N2 + complex preservative),and group MAP3(15%O2,60%CO2,25%N2 + complex preservative)on preservation of chilled tuna,the sensory evaluation,physical-chemical indexes,and microbiological analysis of chilled tuna during storage at 4? were measure.Results showed that group MAP2 was much better on keeping the quality of chilled tuna than other treatments.The growth of microorganisms were controlled,and stability of color(redness)were also maintained.Therefore,better quality and extended the shelf life of tuna was obtained,and the shelf life of chilled tuna was extented to 6~7 days tored under 4?.