Protective Mechanism And Microencapsulation Of Sinapic Acid-Chitosan Complex On Color Stability Of Black Rice Anthocyanins

Improved the stability of natural pigments and expanded the development and application of natural pigments are an important development direction of food industry due to the controversy between synthetic pigments and human health.Black Rice Anthocyanins（BRA）is a kind of natural polyphenolic pigment extracted from the seed coat of Black Rice（Oryza sativa L.）.It has many beneficial functions to human body.However,anthocyanins are easy to degrade and fade during food processing.Polysaccharides are often added to beverage systems and improve the quality of food,but their protective effect on anthocyanins is weak.Polysaccharides have been reported to improve the stability of anthocyanins by hydrogen bonding and van der Waals forces,but the development of novel auxiliary polysaccharides has not been promoted.Therefore,this paper collected 15 kinds of reported co-pigment small molecules,selected the best co-pigment small molecules based on quantum mechanical calculation,and grafted the co-pigment small molecules onto chitosan by Vc/H₂O₂free radical grafting method,nd the structure characterization and antioxidant activity evaluation were carried out.Then,the BRA simulation beverage system was constructed to explore its influence on the stability of BRA.Finally,molecular docking and molecular dynamics techniques were used to screen theoretically binding stable carrier proteins at the molecular level,which were used in combination with sinapic acid-chitosan covalent grafts as a nanoscale delivery system for BRA.（1）Synthesis,characterization and antioxidant activity of free radical-mediated sinapic acid-chitosan covalent grafts:Based on computational quantum chemistry,a small co-pigment molecule（sinapic acid）with strong binding effect on anthocyanin was screened.The covalent grafts of sinapic acid-chitosan were prepared by free radical mediation,and the results showed that:under the condition of weak acid or neutral,the grafting rate of sinapic acid-chitosan covalent graft copolymer increased gradually.UV-Vis spectra,FTIR spectra and ¹H-NMR spectra confirmed that the carboxyl group of sinapic acid reacted covalantly with the free amino group of chitosan to form the covalent graft copolymer.X-ray and SEM showed that the grafting of sinapic acid improved the crystal properties of the chitosan,and made the microstructure of the covalent grafts change obviously,which was like sticks.DPPH,ABTS free radical scavenging ability and iron ion reducing ability tests showed that the sinapic acid-chitosan covalent graft copolymer had strong antioxidant ability.（2）Graft chitosan with sinapic acid to improve the stability and mechanism of black rice anthocyanins:In the 7-day accelerated experiment,the presence of L-ascorbic acid accelerated the degradation and fading of BRA,and all polysaccharide added groups could improve the color stability of BRA.The sinapic acid mixed chitosan was superior to sinapic acid grafted chitosan,while sinapic acid grafted chitosan was superior to chitosan.Compared with chitosan,the grafting of sinapic acid improved the protective effect of chitosan on BRA,but the covalent binding limited the movement of sinapic acid,resulting in weaker protective effect on anthocyanins than the mixture group.The energy characterization of the system showed that the（Lennard-Jones）LJ potential energy between chitosan and anthocyanins was increased by the grafted sinapic acid,while the number of the non-solvent hydrogen bond was increased by the free erucic acid,and the total energy of the system was minimized.（3）Screening and interaction of the delivery of anthocyanin vectors based on cow milk protein:Milk protein can be used as a delivery carrier for anthocyanin.Among,the binding affinity ofαs1-casein andβ-casein with cyanidin-3-O-glucoside（C3G）was higher than that of whey proteins（β-lactoglobulin andα-lactoalbbumin）,and their binding affinity were-9.4 k J/mol,-8.4 k J/mol,-7.6 k J/mol and-6.6 k J/mol,respectively.Molecular dynamics simulation indicated that the binding of C3G toαs₁-CA andβ-CA caused their structures change to a more compact conformation showing decreases in Rg and SASA values compared withα-LA andβ-LG.Furthermore,the analysis of hydrogen-bonding number,RDF and interaction energy showed that the stability of interaction between C3G andβ-CA are better thanαs₁-CA,α-LA,andβ-LG.In summary,β-CA could be used as the most potential protein carrier of C3G to improve its stability and bioavailability.（4）Black rice anthocyanin microcapsules were prepared by sinapic acid-chitosan combined with casein:The bioavailability of BRA is very low,which can be improved by using microcapsule technology.BRA microcapsules were prepared by casein,chitosan-casein and sinapic acid-chitosan-casein,respectively.Compared with casein alone or the combination of chitosan and casein,the BRA microcapsules constructed by sinapic acid-chitosan and casein have smaller particle size,better dispersion and encapsulation efficiency.Among,the mass concentration ratio of sinapic acid-chitosan to casein was 25:1,the best BRA microcapsules were constructed with the particle size of 243.12±4.69 nm,PDI of 0.097±0.048,Zeta potential of 31.18±0.58 m V,and the encapsulation efficiency of 42.36±2.15%.The results of in vitro sustained-release study showed that the BRA microcapsule constructed by sinapic acid-chitosan and casein had excellent sustained-release effect,which was superior to other groups.The release model showed that the BRA released by microcapsules in the stomach belonged to the zero-order release model,while the release in the intestine belonged to the Higuchi release model.