Preparation And Application Of Polyphenol-chitosan Conjugates

Chitosan is considered a versatile biopolymer with promising applications.Chitosan has received great attention in the fields of food and nutraceutical because it is non-toxic and has good antimicrobial activity and biocompatibility.Chitosan also has good flocculability,thus it can be used to clarify fruit juice and improve the visual quality of fruit juice.However,chitosan is insoluble in water and it is not a good antioxidant due to the lack of H-atom donors.In this study,chitosan was combined with two kinds of natural polyphenolic,tannins acid and quercetin,respectively,by free radical grafting to produce water-soluble tannic acid-chitosan conjugate(TA-CS)and quercetin-chitosan conjugate(Q-CS)for enhancing the antioxidant properties of chitosan.Moreover,their structures were characterized,and their biological activities were studied.First,by adjusting the molar ratios between polyphenols and chitosan repeat units to optimize the preparation conditions,the optimal graft ratios of TA-CS and Q-CS measured by Folin–Ciocalteu method were 267.1 mg TAE/g TA-CS and 13.9 mg QE/g Q-CS,respectively.The grafting reaction of TA-CS or Q-CS mainly occurred on the amino and C₆hydroxyl groups of chitosan,which was confirmed by FTIR,UV-vis,and ¹H NMR.The crystallinity and thermal stability of TA-CS and Q-CS were slightly lower than those of chitosan.Then,the antioxidant properties of TA-CS,Q-CS,and their substrates were determined by free radical scavenging,reactive oxygen species scavenging,reducing power,and metal ion chelation assayes.The results showed that the antioxidant activity of TA-CS was stronger than that of Q-CS,and the antioxidant activity of CS was the weakest.These results indicated that the antioxidant activity of chitosan could be effectively enhanced by grafting with polyphenols.After that the antibacterial properties of TA-CS,Q-CS,and chitosan were studied.The results showed that the conjugates had better antimicrobial activity at lower p H.For Escherichia coli,Staphylococcus aureus,Micrococcus luteus,Bacillus subtilis,and Proteus vulgaris,TA-CS and Q-CS had the strongest inhibitory effects on Bacillus subtilis,and their minimum inhibitory concentrations were 32?g/m L and 64?g/m L,respectively.Finally,the clarification conditions of TA-CS and chitosan in kiwifruit juice were optimized by single factor analysis and response surface methodology.The results showed that the optimal conditions were the TA-CS concentration of 600?g/m L,p H=3.5,and the heating temperature of 70?;the transmittance of kiwifruit juice was 99.3%after clarification.The remaining percentage of vitamin C in the kiwifruit juice clarified with TA-CS was 97.0%,which was significantly higher than that of chitosan treatment,indicating that TA-CS could be used as an effective clarifying agent for fruit juice.