Effects Of Temperature Fluctuation On The Quality And Microbial Diversity Of Big-eye Tuna (Thunnus Obesus) During Logistic Process

Tuna is the generic name for the group of fish species belonging to the Scombroidae family,which mainly distributed in the Pacific Ocean,Atlantic Ocean and Indian Ocean.It is one of the most important marine fish resources in the world.It was enriched in sugar,protein,cholesterol,vitamin A,vitamin D,vitamin E,calcium?phosphorus,iron,sodium,potassium,magnesium,zinc,copper,etc.?,especially with n-3 polyunsaturated fatty acids?n-3 PUFA?such as eicosapentaenoic acid?EPA:C20:5n-3?and docosahexaenoic acid?DHA:C22:6n-3?.So,it is regarded as one of the world's three big fish nutrition by the International organization.Most of tuna are eaten as sashimi because of consumers'demand for fish color and nutrition value.Thus,it needs to be preserved in cryogenic temperatures such as lower than-55?.To guarantee its quality,temperature conditions are important.However,the quality of tuna is easily decreased at the periods of processing because of influence of geography,climate,seasons and other conditions.There are five chapters on big-eye tuna were carried on in this paper so as to detect the quality and improve the quality of big-eye tuna during logistic processes:Firstly,the experimental designed control group?CK?and five logistic processes?L1,L2.1,L2.2,L2.3,L2.4?to simulate the temperature fluctuation of big-eye tuna during logistics processes.Sensory,physicochemical?texture profile analysis?TPA?,metmyoglobin?metMb?,total volatile basic nitrogen?TVB-N?,histamine,pH value?and microbiological?total viable counts,TVC?parameters were measured during logistic processes.Low-field nuclear magnetic resonance?LF-NMR?and magnetic resonance imaging?MRI?were used to determine the water-distribution and water content of samples.The results showed that temperature variations during process could lead to a sharp degradation on quality,and the L2.3 showed the lowest quality.After 216 h?9d?,the sensory properties of L2.3 groups received a lower score?-2.00?than other groups,which marked as being unacceptable,metMb was increased to35.23%,TVB-N value was 19.14mg N/100g and pH value was 6.66,histamine and TVC were over the acceptance limits.The results of LF-NMR and MRI indicated that the water distribution was strongly affected by the range of temperature fluctuation.Especially the temperature changes in L2.3 led to the immobilized water(A₂₂)decreaed from 94.90%to 80.25%,and the free water increased from 1.73%to 12.96%.The content of immobilized water(A₂₂)showed a direct correlation with the physiochemical:MetMb%?R²=0.94?,hardness?R²=0.93?,chewiness?R²=0.92?,TVB-N?R²=0.87?and histamine?R²=0.87?.It suggested that LF-NMR technique can be used to one of the methods to determine the quality of fish and fish products.Secondly,near infrared spectroscopy?NIRS?and Low-field nuclear magnetic resonance?LF-NMR?were used to determine the quality changes of big-eye tuna with temperature fluctuation during logistic processes.Cold chain?CK?and chain scission?L1/L2?were simulated in this paper.Water holding capacity?WHC?,water activity?a_w?,SDS-PAGE and Scanning Electron Microscope?SEM?were measured during logistic process.The results showed that WHC of L2 samples were decreased more than 20%and the speed of protein degradation was increased with the rise of temperature at 216h?9d?.The curve of NIR were showed the certain changing trends,the absorbance value of NIR was in downward trend,and CK>0.4,L1?0.4,L2<0.4at 216h?9d?.Relaxation time T₂ of 3 kinds of water,bound water T_2bb was not any change;both the value and amplitude of immobilized water T₂₂2 was decreased while free water T₂₃3 and its amplitude was increased;relaxation time T₁ was not change significantly.The results of NMR were coincidence with that of other indexes and reflect the freshness change of big-eye tuna.It can be found that CK,L1 and L2 group samples respectively in 204 h?8.5d?,168 h?7d?and 168 h?7d?freshness was unacceptable with NIRS discrimination diagram,which corresponded to the changes of WHC,SDS-PAGE and SEM,Therefore,NIRS with LF-NMR could represent the quality change of big-eye tuna quality during circulation effectively.Thirdly,the effect of temperature fluctuation and freeze-thaw cycles on the the muscle tissue and functional properties of big-eye tuna were investigated.Big-eye tuna fillets was designed as three groups?F1,F2,F3?to each of 5 freeze-thaw cycles.Functional Properties,such as thawing loss,textural properties?Hardness,Springiness,Chewiness and Cohesiveness?,metmyoglobin,the contents of myofibrillar protein,total sulfhydryl,and the activity of Ca²⁺-ATPase,SDS-PAGE were carried out.LF-NMR was used to measure their properties.The results indicated that with the number of freeze-thaw cycles increased,thawing loss was increased and decreased from 4 times cycle.Hardness,Springiness and Chewiness were decreased significantly,while cohesiveness was increased.Metmyoglobin,the contents of myofibrillar protein,total sulfhydryl,and the activity of Ca²⁺-ATPase were decreased significantly.Moreover,LF-NMR could response protein denaturation effectively.The evidence of muscle SDS-PAGE changes could provide more intuitive information on mechanical damage in myofibrils.In contrast,with an increased number of freeze-thaw cycles,F1?-18?-2??and F2?-18?-2??caused significant loss in different molecular weight proteins from 4 times of repeated freeze-thaw cycles,but F3?-18⁴??sample from 3 times.Freeze-thawing cycles could damage the functional properties of muscle protein seriously,therefore,freeze-thaw cycles should be avoided,F1,F2 could not more than 4 times and F3 should not more than 3 times.Fourthly,the big-eye tuna was simulated in the cold chain?CK?and logistic process?L1/L2?with temperature fluctuation respectively,the relationship between the ability of oxidation reductase and color stability in the logistic process were researched.a*value,TBA,myoglobin oxidation state,myoglobin reductase,total phenol concentration?TPC?,ion antioxidant capacity?FRAP?characterization with visual analysis and SDS-PAGE.The results showed that the color of big-eye tuna was brown in the logistic process with the temperature change,a*value were 5.53?4.64and 2.43 at last,which is consistent with the result of visual analysis;oxygenation myoglobin was declined significantly with the increase of the content of myoglobin;the myoglobin of SDS-PAGE results proved that the increase of myoglobin protein content;oxidation ability in L2 were decreased significantly,fat oxidation was82.56%,TPC was decreased by 35%,FRAP were decreased by 16.15%,48.21%and64.78%respectively?P<0.05?.It concluded that samples in logistic groups had lower quality.The results of kinetic analysis in food quality equation showed linear changes in oxidation ability index?TPC?FRAP?,lipid oxidation?TBA?,myoglobin oxidation state and enzyme activity,the oxidizing capacity and color stability of big-eye tuna were decreased significantly after 168h?7d??sale at 2??.Finally,the big-eye tuna in cold chain?CK?and logistic process?L1?with the temperature fluctuation were simulated;the microbial diversity in the logistic process was studied.pH value,TVB-N,TVC and the psychrobacter with high-throughput sequencing analysis were measured respectively.The results showed that with the temperature fluctuation,CK group at-55?the microbial diversity was stable and after 216 h?9d?it increased slightly.pH value and TVB-N analysis showed that L1was lower than that in CK,TVC and the psychrobacter were that L1 was higher than that of CK group,then over the acceptance limit after 168 h?7d?.The dominant spoilage bacteria were determined according to results of the high-throughput sequencing analysis,the dominant spoilage bacteria in CK group were Brochothrix thermosphacta,Psychrobacter spp.,and that in L1 group were Brochothrix thermosphacta,Psychrobacter spp.,Pseudomonas spp.and Putrefaciens Shewanella respectively.