Quality Control And Effect Of Temperature Fluctuation On Color And Texture Of Litopenaeus Vannamei During Cold Chain Circulation

Litopenaeus vannamei tops the list of cultured shrimps worldwide in terms of cultivation and demand.It is rich in nutrition,delicious in taste and delicate in meat,which is deeply loved by consumers.L.vannamei is difficult to survive after leaving the water.Because of its high moisture content,high enzyme activity and high vulnerability,it is easy to deteriorate.At present,it is mainly stored and circulated through freezing for a long time.However,due to the relative lack of cold chain equipment,backward logistics technology and management,temperature fluctuations are prone to occur in the production,transportation,storage and processing with frozen L.vannamei,resulting in changes in its color,texture and flavor,and the nutritional value is greatly reduced.Therefore,it is particularly important to explore the effect of temperature fluctuation on the quality of frozen L.vannamei and its regulation.At present,the research on temperature fluctuation of L.vannamei at home and abroad mainly focuses on the quality change during the freeze-thaw cycle and logistics stages,and most of the fluctuation process is mainly switching in different ambient temperatures,which do not fit the actual situation of cold chain logistics.This study took L.vannamei as the object of study.Firstly,the quality changes of frozen L.vannamei under different temperature fluctuations（-18℃～-12℃,-18℃～-6℃and-18℃～0℃）were investigated,and the main apparent indicators of quality deterioration were determined by partial least squares method.Secondly,the internal causes of the deterioration of color and texture were further investigated.Finally,the regulatory effects of different packaging methods（ordinary packaging,vacuum packaging and modified atmosphere packaging）on lipid and protein oxidation of frozen L.vannamei were discussed.This study aims to provide theoretical and technical support for the quality preservation of frozen aquatic products in the cold chain circulation.The main research contents and results are as follows:（1）Effect of temperature fluctuations on the apparent quality characteristics of frozen L.vannamei.The temperature fluctuations of frozen L.vannamei during storage were designed into three groups（-18℃～-12℃,-18℃～-6℃and-18℃～0℃）.Samples with fluctuation of 0,1,3,5 and 7 times were taken for the determination of relevant indicators.The results showed that the sensory scores,L*values,and W values of the whole shrimp and shrimp color of the three groups of L.vannamei showed a downward trend during temperature fluctuations.After seven fluctuations,the whole shrimp a*value increased by20.99%,29.01%,and 32.06%,respectively,and the shrimp a*value increased by 20.38%,27.31%,and 34.62%in the-18℃～-12℃,-18℃～-6℃and-18℃～0℃groups,with the largest increase in samples from the-18℃～0℃group.The muscle tissue score,texture characteristics（hardness,cohesion,chewability and shear force）and water content of the samples in each group decreased with the increase of the number of waves.The ice crystals gradually become larger and irregular,and the muscle tissue showed large holes,resulting in a decrease in integrity.The results of PLS-R model showed that the color,texture and water holding capacity of frozen L.vannamei in each group deteriorated significantly during temperature fluctuation,and the greater the range and frequency of temperature fluctuation,the greater the degree of deterioration.The results of（PLS-R）model showed that the quality of frozen L.vannamei changed significantly after fluctuating for 7,5,and 3times in the-18℃～-12℃,-18℃～-6℃and-18℃～0℃groups,respectively（P<0.05）,with the most significant changes in the b*values,W values,cohesion,chewability and water content indicators of the whole shrimp.The color and texture characteristics of frozen L.vannamei in each group deteriorated after temperature fluctuations.（2）The causes of the degradation in color and texture characteristics of frozen L.vannamei during the temperature fluctuations.With the increase of temperature fluctuations,the polyphenol oxidase（PPO）activity and TBARS value of samples increased continuously in the-18℃～-12℃,-18℃～-6℃and-18℃～0℃groups.After seven fluctuations,the PPO activity in the shrimp shell increased by 83.33%,100%and116.67%respectively.The TBARS value in the-18℃～0℃group was 1.46 and 1.23 times that in the-18℃～-12℃and-18℃～-6℃groups.There was no significant change in astaxanthin content in each group（P>0.05）.During the temperature fluctuations,the carbonyl content in the-18℃～-12℃,-18℃～-6℃and-18℃～0℃groups increased significantly from 0.80 nmol/mg to 1.51 nmol/mg,2.12 nmol/mg and 2.33 nmol/mg（P<0.05）,and the sulfhydryl content decreased by 20.07%,30.44%and 35.32%respectively.α-helix andβ-fold gradually changed toβ-turned and irregularly curled structures.The decrease of FI_maxin the 18℃～0℃group was the largest and significantly lower than that in the other two groups.SDS-PAGE diagram showed the partial oxidative degeneration of myofibrillar protein during the temperature fluctuations.Correlation analysis revealed that the correlation coefficients of color indicators（L*,a*,b*and W values）with PPO activity and TBARS values were high,and the catalytic action of PPO and lipid oxidation during temperature fluctuations were the main causes of color changes in L.vannamei.Texture characteristics（hardness,cohesiveness,chewiness and shear force）have strong correlation with carbonyl content,sulfhydryl content and FI_maxof myofibrillar protein.Oxidative denaturation of protein played an important role in texture deterioration of L.vannamei.（3）Regulation of packaging methods on the oxidation of lipid and protein of frozen L.vannamei.The frozen L.vannamei were separately packaged in polypropylene bags,and then processed by ordinary packaging machine,vacuum packaging machine and modified atmosphere packaging machine,and then fluctuated at-18℃～0℃.The samples after fluctuation for 0,1 and 7 times were taken to determine the relevant indicators.The results showed that,compared with the modified atmosphere packaging group,the hardness of L.vannamei in the ordinary and vacuum packaging groups decreased by12.26%and 5.41%after seven temperature fluctuations,respectively.The thawing loss increased by 1.80%and 4.66%,and the TBARS value increased by 135.51%and 64.29%.The modified atmosphere packaging group has maintained a high myofibrillary protein solubility,sulfhydryl content and FI_max.The TBARS value and LPS enzyme activity of the vacuum packaging group were lower than those of the ordinary packaging group,which could inhibit the lipid oxidation of L.vannamei in the process of temperature fluctuations to a certain extent,but its muscle tissue was affected by external forces,and the protein structure changed,resulting in serious protein denaturation.According to the comprehensive PCA analysis,the modified atmosphere packaging is a regulation method that can effectively inhibit the oxidation of lipid and protein and maintain the better color and texture characteristics of L.vannamei.