| Large yellow croaker is tender in meat,delicious and rich in nutrients.It is one of the economic fish in our country and has a large market.The water content of large yellow croaker is high,and is easily corrupted due to the influence of microorganisms and enzymes during transportation and storage,resulting in a lot of economic losses.Therefore,it is particularly important to carry out research on the preservation technology of large yellow croaker.In recent years,coating preservation is a kind of preservation technology that has attracted much concern.It is a common research direction to mix natural polymers and natural bacteriostatic agents to prepare coating-preserving materials.However,with the prolongation of preservation time,natural bacteriostatic agents will gradually disappear,especially volatile natural bacteriostatic agents such as essential oils.Therefore,it is a method worth exploring to chemically combine natural polymers and natural bacteriostatic agents to prolong their bacteriostatic and fresh-keeping effect,and there are relatively few studies on this at present.In this study,chitosan with a molecular weight of 50 k Da was used as the raw material,and its water solubility was improved by carboxymethylation,and then it was reacted with perilla aldehyde,the main antibacterial component of perilla oil,to prepare a new antibacterial material carboxymethyl chitosan Schiff base.It was compounded with sodium alginate to obtain a new type of biological fresh-keeping coating.The stable bacteriostatic properties of Schiff base and the barrier properties of the coating were used to preserve the freshness of large yellow croaker in cold storage,and the physical and chemical indexes of large yellow croaker were determined.It provided reference for the development and utilization of biological fresh-keeping coating.The main findings were as follows:A novel,green,efficient and simple method for extracting perilla essential oil from perilla leaves was developed,namely microwave-ultrasonic assisted simultaneous distillation and extraction.The extraction conditions were optimized by single factor experiment and response surface analysis,and the optimal reaction conditions were determined as follows:the solid-liquid ratio was 1:6(g/m L),the microwave power was 933 W,the ultrasonic power was245 W,and the microwave-ultrasonic temperature was 50℃,microwave-ultrasonic time 4.5min.Under this condition,the extraction rate of perilla oil was 2.34%.The composition analysis and property research of the extracted perilla essential oil showed that its main components were perilla aldehyde(33.88%)and limonene,which had good anti-oxidative and bacteriostatic properties.The minimum bactericidal concentration for S.aureus,E.coli and S.putrefaciens were 10μL/m L.Using low molecular weight chitosan and perilla aldehyde as raw materials,perilla aldehyde carboxymethyl chitosan Schiff base was prepared by carboxymethylation and condensation reaction.The synthesis conditions were optimized by single factor experiment and response surface analysis,and the optimal conditions were determined as follows:the molar ratio of carboxymethyl chitosan and perillaldehyde was 1:7,the reaction time was 4.35h,the reaction temperature was 70℃,and the microwave power was 195 W,ultrasonic power165 W.Under this condition,the obtained Schiff base condensation rate was 74.76%.And in this way,the perilla essential oil carboxymethyl chitosan Schiff base was obtained,and its condensation rate was 69.82%.Used FTIR,~1H NMR,SEM,EA,TGA-DTG,DSC,XRD,water solubility,particle size potential,antibacterial experiments and other technical means.The structures and properties of chitosan,O-carboxymethyl chitosan,perilla aldehyde carboxymethyl chitosan Schiff base and perilla essential oil carboxymethyl chitosan Schiff base were characterized.The results showed that the carboxymethyl chitosan Schiff base had lower crystallinity,better water solubility,higher thermal stability,smaller particle size,more concentrated positive charges,and better bacteriostatic properties.Perilla essential oil carboxymethyl chitosan Schiff base showed the best antibacterial activity against S.aureus,E.coli,S.putrefaciens and P.fluorescens,and the minimum inhibitory concentrations were 2,1,1,1 mg/m L.The composite coatings of perilla essential oil carboxymethyl chitosan Schiff base with different concentrations and the composite coatings of different components were prepared for the antibacterial and fresh-keeping of refrigerated large yellow croaker.The results showed that the coating of large yellow croaker can effectively delay its quality deterioration.The fresh-keeping effect of the perilla oil carboxymethyl chitosan Schiff base composite coating in the late storage period was particularly outstanding.Compared with blank group,TVB-N was 40.2%lower,TVC was 22.8%lower,TBA was 20.8%lower.The addition of perilla oil or Schiff base to the coating film could better inhibit the growth of microorganisms,weaken the evaporation of water from the fish body,keep the fish body fresh,and prolong the shelf life by 2 to 3 days.In conclusion,the perilla oil carboxymethyl chitosan Schiff base composite coating was a new type of biological coating suitable for refrigeration and preservation of large yellow croaker,and it had great potential in the application field of aquatic product preservation. |
