Preparation Of PH-Responsive Cinnamaldehyde-Chitosan Schiff Base With Controlled Release Behavior And Its Application In Fruit And Vegetable Preservation

Fruits and vegetables are good sources of nutrients,which are indispensable in people's daily diet,but their quality is easily damaged after picking.How to effectively improve the preservation of fruits and vegetables and reduce the loss has become a major problem to be solved.Essential oils have good antiseptic and fresh-keeping effect on fruits and vegetables,but their shortcomings such as poor stability and volatility have become the main bottleneck restricting the efficient application of essential oils.In view of this,this study based on the law that the pH value gradually decreases due to the acidic changes in the headspace microenvironment of fruits and vegetables after harvest.Cinnamaldehyde-chitosan schiff base compouds(CS-Cin)were designed by using acid sensitive imine bond through mild schiff base reaction,which could sense pH signal changes and achieve dynamic regulation of antibacterial essential oils.The stability of the material,pH responsive controlled release performance and antibacterial performance had been studied,and after verifying the fresh-keeping effect on postharved fruits and vegetables,the pH responsive release performance was further optimized according to the practical application requirements.The main research results are as follows:CS-Cin was prepared by combining the active amino group of chitosan with the aldehyde group of cinnamaldehyde under mild reaction conditions.Fourier transform infrared spectroscopy and elemental analysis were used to determine whether the product was synthesized successfully.The effects of molecular weight,deacetylation degree of chitosan and reaction molar ratio of cinnamaldehyde/chitosan on the degree of substitution(DS)and pH responsiveness were investigated.Considering the utilization rate of cinnamaldehyde and economy,the optimal reaction conditions were determined as follows:the molecular weight of chitosan was 150 k Da,the deacetylation degree was 90%and the molar ratio of cinnamaldehyde/chitosan was 4:1,and the DS of the derivative was 0.26.The release of cinnamaldehyde from CS-Cin was different under different pH conditions,indicating that CS-Cin had the potential as a pH-responsive controlled-release material.The responsive release behavior and antibacterial effects of CS-Cin were further investigated.The results showed that in the pH buffer solution system,80%of cinnamaldehyde could be released from CS-Cin after being treated with pH 3.0 and pH 5.0 acid medium for 96h.In the acidic atmosphere system,the release rate of cinnamaldehyde increased with the increase of CO₂ concentration.When the volume fraction of CO₂ increases from 5%to 20%,the cumulative release of cinnamaldehyde could increase by 15%.This indicated that CS-Cin had good pH sensitivity,triggering the C=N bond fracture and releasing cinnamaldehyde.The inhibition rates of CS-Cin against on E.coli and S.aureus were 86.17%and 93.65%under slightly acidic conditions respectively,while only 9.21%and 13.04%under neutral conditions.Therefore,CS-Cin had a good pH-responsive antibacterial ability.And then,the application of CS-Cin in preservation of fruits and vegetables was investigated.Broccoli and strawberries belong to the"extremely high"and"high"respiratory intensity types respectively,and both have a high demand for preservation.The effects of CS-Cin on the postharvest preservation of two kinds of fruits and vegetables were investigated.The changes of physical and chemical indexes and microbial indexes of fruits and vegetables were measured regularly.The results showed that CS-Cin had good preservative and fresh-keeping ability,which could effectively inhibit the growth of spoile bacteria and delay the quality deterioration of fruits and vegetables.Due to the extremely high respiratory intensity of broccoli,the microenvironment of the packaging headspace was obviously acidified.The cinnamaldehyde release from the same amount of CS-Cin in broccoli packaging system increased by 20%compared to the strawberry system,which preliminarily confirmed that there was a certain correlation between the response release characteristics and the actual CO₂ and acid production of fruits and vegetables.Finally,the regulation of pH responsive controlled release behavior was optimized.The accumulation of CO₂ in the packaging environment of fruits and vegetables is closely related to their own respiratory metabolic characteristics.And a large amount of CO₂ is easy to increase the risk of corruption,so a more sensitive preservative is needed.Using?-polyglutamic acid with good water retention properties,porous chitosan was prepared via gas porogen,and then modified to obtain porous cinnamaldehyde chitosan schiff base(Porous CS-Cin).The DS of rivatives increased with the addition of the porogen,which confirmed that the changes of pore size and porosity of the porous structure could change the exposed active reaction sites and improve the utilization rate of cinnamaldehyde.Comparing the pH responsive release behavior of CS-Cin and Pourous CS-Cin,the results showed that Porous CS-Cin had better sensitivity to the change of pH signal.When the environmental pH value decreased from 7.0 to 6.0,Porous CS-Cin could respond quickly within 1 min.Finally,broccoli with high respiration rate was selected to verify the fresh-keeping effect of Porous CS-Cin.Porous CS-Cin prepared with 10%porogen addition had the advantages of both response sensitivity and release regulation due to its high degree of substitution,so it had the best preservative effect on broccoli.The cinnamaldehyde-chitosan schiff base derivative prepared in this study can effectively avoid the volatile and unstable application limitations of antibacterial essential oils,and has good pH sensitivity,realizing the release controllability of antibacterial essential oils.The release of antibacterial active substances is basically synchronized with the demand for inhibiting spoilage of fruits and vegetables,which provides a simple and promising strategy for active antibacterial materials.