Study On Effect Of Chitosan-based Coating On The Quality Of Refrigerated Grass Carp (Ctenopharyngodon Idella) Fillets

Freshwater fish are popular with consumers due to the delicious taste,nutritional values and low price.Considering the changes in consumption concepts with young generation,fresh fish products are gradually replacing the tradinional sale model in business.However,freshwater fish are easily vulnerable to spoilage under actions of microorganisms,endogenous enzymes and chemical reactions,which extremely limits their sales scope.Therefore,efficient presetvation technologies should be developed to prolong the postmortem shelf life of freshwater fish.At present,chitosan-based coatings have been widely concerned by researchers due to their advantages of abundant raw material,simple operation,low equipment requirement and desired preservation effect.However,the existing studies regarding chitosan coating mainly focus on marine fish and specific species such as rainbow trout,but its basic application research for freshwater fish is relatively lacking.Besides,the early studies mainly focused on the effect of chitosan-based coatings on the shelf life extention of aquatic products in terms of microbial enumeration,sensory scores and physicohemical analysis,without trying to understand the underlying mechansims of the protective effects.In this study,grass carp was selected as subject.This study investigaged the quality changes of grass carp fillets during refrigerated storage and the effects of chitosan coatings on the quality of fillets,developed the effective formulations with chitosan and active substances as antibacterial/antioxidant coatings,and explored the preservation mechanisms of chitosan-based coatings from the changes in flavor-related compounds,endogenous enzyme activities and bacterial flora.The present study is of great importance for promoting the application of chitosan-based coating preservation technology in freshwater fish products.The effects of chitosan coatings with different concnentrations?1%,2%and 2.5%?and preservative film wrapping?PE?on quality and shelf life extension of refrigerated grass carp fillets were first studied.Results showed that fillets treated with chitosan coatings has lower levels of total viable counts?TVC?,psychrophilic bacteria counts?TPC?,total volatile basic nitrogen?TVB-N?,water loss,histamine?HIS?,putrescine?PUT?and K-value,and higher levels of shear force and sensory score than those of control fillets during storage,and the preservation effect of chitosan coatings with high concnentrations??2%?was batter than that with 1%.Thiobarbituric acid reactive substance?TBARS?value of PE was close to the groups of 2%and 2.5%coatings,which were lower than that of the Control and 1%coating,indicating that high concnentrations??2%?chitosan coatings helped to better block oxygen,thus retarded lipid oxidation of fillets to some extent.According to evaluations of K-value,TVB-N,TVC and sensory score,the shelf life of control fillets was 7⁸ days,while the shelf life could be extended by 1³ days and 3⁵ days when the fillets were treated with 1%and 2/2.5%chitosan coatings,respectively.However,coated fillets still showed relatively poor sensory quality at the end of shelf life,indicating that the preservation effect of chitosan coating in antibacterial and antioxidation aspests needs to be further improved.To further improve the preservation effect,chitosan coatings enriched with active compounds were studied.First,three kinds of essential oils?extracted from clove,cinnamon and lemongrass?commonly used in coating preservation were selected as natural oxidation inhibitors,and the inhibitory effect of chitosan coating combined with essential oils on lipid oxidation of fillets was investigated.Results showed that the final values of peroxide value?POV?and TBARS in composite coated fillets reduced by 21⁴9%and 41⁹2%,respectively,compared to control fillets.In addition,chitosan-essential oil composite coatings maintained the activities of antioxidant enzymes,especially chitosan-clove essential oil coating,of which the final catalase?CAT?,glutathione peroxidase?GSH-px?and total superoxide dismutase?SOD?activities were increased by 117%,36%and 120%,respectively,compared with the control fillets.Further analysis showed that chitosan coating combined with clove essential oil also inhibited protein oxidation according to the results of sulfhydryl?SH?and carbonyl?CP?contents.Second,the microbial results of antibacterial composite coating showed the synergistic antibacterial effect between chitosan and GML.When GML concentration reached0.3%in coating solution,microbal counts in composite coated fillets were further reduced by1.1².3 log₁₀ cfu/g when compred to chitosan coated fillets during storage.Meanwhile,smaller changes of TVB-N,K-value and sensory score were observed when the fillets were treated with chitosan-0.3%GML coating.The effects of chitosan coatings combined with clove essential oil?CEO?and GML on flavor and texture retention of refrigerated grass carp fillets were investigated by determining flavor-related compounds,endonenous enzyme activities and protein degredation.Results showed that the trimethylamine?TMA?and hypoxanthine?Hx?contents of coated fillets were only 24⁵5%and 28³9%of the control fillets,while the inosine monophosphate?IMP?content of coated samples was significantly?P<0.05?increased by 0.78².10?mol/g compared to the control fillets after 15 days of storage.Besides,coating treatments also helped to reduce histidine accumulation and umami free amino acid?FAA?loss in fillets.It should be emphasized that better coating preservation effect was observed when the CEO concentration in coating solution was?0.5%.HS-SPEM-GC-MS indicated that 41 main volatile organic compounds?VOCs?in fillets were detected throughout the storage,mainly including 10aldehydes,9 ketones and 17 alcohols.The composite coatings sharply reduced the types and relative content of volatiles,such as hexanal,?E?-2-octenal,nonanal,1-octen-3-ol and 1-penten-3-ol with unpleasant smells.E-nose analysis further confirmed the benefits of chitosan coatings combined with CEO??0.5%?and GML for holding the odor composition of grass carp fillets.In addition,chitosan coating enriched with GML and CEO inhibited cathepsin B,B+L and calpain activities of fillets,with the most reductions of 57.5%,63.7%and 35.6%,respectively when compared to the corresponding values of control fillets,resulting in the significant alleviation of protein degradation and texture deterioration.After storage of 15 days,the shear forces of coated fillets were 28–53%higher than that of control samples.Microbial succession in grass carp fillets with a 2%chhitosan-0.3%GML-0.5%CEO coating during refrigerated storage were investigated.During storage,the microbial counts of coated group were significantly lower than that of control group,and the maximum reductions of Pseudomonas and H₂S-producing bacteria?including S.putrefaciens?between two groups exceeded 3 log₁₀ cfu/g.Meanwhile,the final values of TVB-N,TMA,K-value,PUT and HIS which were closely related to bacterial activity decreased by 35%,73%,38%,92%and 94%in coated groups,respectively,compred to the control group.High-throughput sequencing analysis indicated that the relative contents of Shewanellaceae,Pseudomonadaceae,Flavobacteriaceae and Moraxellaceae in spoiled fillets exceeded 93%,suggesting that they were the major spoilage floras.However,coating treatment helped to maintain microbial diversity and avoid the rapid growth of specific spoilage organisms of refrigerated grass carp,thus retarding their quality deterioration.This paper studied the benefit of chitosan coatings for prolonging the shelf life of refrigerated grass carp,developed two kinds of chitosan-based composite coating solutions using high speed homogenization and micro-emulsification methods with better antibacterial and antioxidation preservation effects,and illustrated the protective mechanisms of chitosan-based composite coatings from the aspects of flavor-related compounds,endogenous enzyme activity and microbial succession.