| Chinese shrimp(Fenneropenaeus chinensis)is one kind of Chinese characteristic aquatic products.It is so popular around the world due to its tender,delicious and specially rich in nutrition.However,Chinese shrimp is susceptible to spoilage because of the existence of endogenous enzymes and microorganisms,which seriously affects its market value and economic value.Therefore,it is significant to develop effective fresh-keeping technologies to delay the spoilage and deterioration of Chinese shrimp and maintain its original flavor and quality.Chitosan-based coatings for preservation,a safety and effective technology,has already been used on aquatic products such as many kinds of fishes but a few species of shrimps including pacific white shrimp.The current research on chitosan-based coatings aquatic products mostly focus on evaluating the effect from the aspects of sensory,physicochemical,microbiological,however,the determination of the precise addition amount of each component and the mechanism of their preservation are less studied.In this study,Chinese shrimp was selected as subject,and chitosan,ε-polylysine(ε-PL)and carrageenan(CA)was selected as coating materials.The optimal compounding ratio of chitosan-ε-PL-CA composite coating solution has been determined by Box-Behnken experimental design,the effects on the preservation quality and shelf life of refrigerated Chinese shrimp were evaluated by sensory,physical,chemical and microbial indicators,and the preservation mechanisms of chitosan-ε-PL-CA composite coating were explored from the changes of flavor-related compounds,protein,fat,moisture distribution and spoilage bacteria.The present study could provide a theoretical basis for the application and development of chitosan-based composite coatings preservation technology for shrimp and other aquatic products.The results of our study are as follows:(1)The optimal compounding ratio of chitosan-ε-PL-CA composite coating solution was determined by Box-Behnken experimental design and the ratio was chitosan 1.82%,ε-PL 0.16%,CA 0.22%.(2)The effects of chitosan-ε-PL-CA composite coating on the preservation quality and shelf life of refrigerated Chinese shrimp were evaluated by sensory score,physical characteristics(color change,water lose,hardness,elasticity,chewiness),chemical characteristics [p H,total volatile basic nitrogen(TVB-N),K-value] and microbial indicators [total viable counts(TVC),psychrophilic bacteria].Samples treated with chitosan coating(CH group),chitosan and ε-polylysine coating(CH+ε-PL group),distilled water(control group)were set as controls.It is indicated that chitosan-ε-PL-CA composite coating(CH+ε-PL+CA group)can more effectively delay the deterioration of the sensory quality of Chinese shrimp than CH group and CH+ε-PL group.And compared with CH group and CH+ε-PL group,CH+ε-PL+CA group has higher levels of hardness,elasticity,chewiness,and lower levels of water lose,p H,TVB-N,K-value,TVC and psychrophilic bacteria counts.What’s more,compared with the control group,the CH and CH+ε-PL group prolonged the shelf life of Chinese shrimp by 1~3 days and 3~4 days,and the CH+ε-PL+CA group extended 5~7 days.(3)The effects and mechanism of chitosan-ε-PL-CA composite coating on flavor retention of refrigerated Chinese shrimp were studied by sensory evaluation of flavor,analysis of flavor-related volatile and non-volatile compounds.The results showed that the CH+ε-PL+CA group had the slowest decrease in flavor sensory score.HS-SPEM-GC-MS analysis indicated that a total of 43 volatile organic compounds were detected during the storage of refrigerated Chinese shrimp,including 11 aldehydes,11 alcohols,8 ketones,4 esters and 9 nitrogen or sulfur compounds.At the end of storage,40 kinds of volatile organic compounds were detcted in the control group,while CH group,CH+ε-PL group and CH+ε-PL+CA group were 31,26,19,respectively,and the CH+ε-PL+CA group reduces the generation or the relative content of odorous volatile organic compounds such as hexanal,2-ethyl-1-hexanol,2-nonanone and nitrogen or sulfur-containing compounds.Those could effectively reduce the production of unpleasant smells such as the fishy smell of Chinese shrimp.The electronic nose loading analysis showed that W1W(hydrogen sulfide)made a more contribution to the first main component,W2W(organic sulfide)and W5W(nitrogen oxides)made a more contribution to the second main component.Principal component analysis(PCA),Linear discriminant analysis(LDA)and radar chart analysis showed that the odor characteristics of the samples in the CH+ε-PL+CA group at the same storage time were the most close to those of fresh group,which was further confirmed that the chitosan-ε-PL-CA composite coating effectively maintained the inherent flavor characteristics of Chinese shrimp.In addition,chitosan-ε-PL-CA composite coating treatment inhibited the degradation of ATP-related compounds in Chinese shrimp,and delayed the decline in the umami substance IMP content and the off-flavor substances Hx R and Hx accumulation as well.Compared with CH+ε-PL+CA,the IMP content was 85.95% lower in the control group at the end of refrigerated storage,and the content of the Hx R and Hx in the control group was significantly higher(p<0.05)than that in the CH+ε-PL+CA group,which is one of the reasons why the chitosan-ε-PL-CA composite coating maintained the sensory and flavor quality of Chinese shrimp.(4)Based on the analysis of the main components of Chinese shrimp muscle,taking the protein composition,biochemical characteristics and electrophoresis,fat oxidation and moisture distribution change as the indicators of investigation,the effect of chitosan-ε-PL-CA composite coating on the changes in the main components of the muscles of refrigerated Chinese shrimp was studied.And the preservation mechanism of chitosan-ε-PL-CA composite coating treatment on the sensory,texture characteristics and flavor quality of Chinese shrimp were further explored and analyzed by the changes of muscle protein,fat and water distribution.The results showed that the chitosan-ε-PL-CA composite coating effectively slowed down the decrease of myofibrillar protein,sarcoplasmic protein,myostromin content and the increase of alkali-soluble protein.Biochemical characteristics had been analyzed and the result showed that the chitosan-ε-PL-CA composite coating could delay the decrease of salt-solubilities,SH contents,Ca2+-ATPase activities and the increase of surface hydrophobicity during refrigerated storage.Electrophoresis results also showed that the composite coating could delay protein degradation.At the same time,the chitosan-ε-PL-CA coating significantly(p<0.05)retarded the decline in fat content,the production of free fatty acids and the rise in TBA value during refrigerated storage,thereby maintaining the sensory,texture and flavor of Chinese shrimp.The water distribution analysis showed that the chitosan-ε-PL-CA composite coating effectively maintained the water distribution stability of Chinese shrimp,as a result,water loss was reduced,and the chemical reaction and microbial activity was delayed,which was one of the important reasons for maintaining the quality of refrigerated Chinese shrimp.(5)The effects of chitosan-ε-PL-CA composite coating on the bacterial phase and the spoliage bacteria of refrigerated Chinese shrimp were studied by 16 S r DNA sequence analysis and microbial counts,and the preservation mechanism of the chitosan-ε-PL-CA composite coating on Chinese shrimp was revealed from the point of microorganism.The results showed that Shewanella and Pseudomonas increased from 37.3% to 93.1% during the refrigerated storage period,which were the Specific Spoilage Organism(SSO)of Chinese shrimp.The chitosan-ε-PL-CA composite coating significantly(p<0.05)inhibited the growth of H2S-producing bacteria(including S.putrefaciens)and Pseudomonas in Chinese shrimp.H2S-producing bacteria in CH+ε-PL+CA group was 3.34 log10 CFU/g lower than the control group,and reductions of pseudomonas was 2.71 log10 CFU/g.At the end of storage,the counts of Shewanella and Pseudomonas were 65.4% in CH group,57.3% in CH+ε-PL group,50.1% in CH+ε-PL+CA group,respectively,which were significantly lower than 93.1% in control group.The results indicated that chitosan-based coatings,especially chitosan-ε-PL-CA composite coating could inhibite the growth of SSO in refrigerated Chinese shrimp,and better maintain the diversity of Chinese shrimp flora,reduced the accumulation of harmful substances due to SSO growth and metabolism,thereby maintained the quality of Chinese shrimp.This paper accurately determined the optimal compounding ratio of chitosan-ε-PL-CA composite coating solution,studied the benefit of the composite coating for quality retention and shelf-life prolongation of refrigerated Chinese shrimp,and illustrated the preservative mechanisms from the perspectives of flavor-related compounds,changes of muscle components and microorganisms. |
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