Study On The Properties And Association Behavior Of Citral-Hydroxypropyl-β-Cyclodextrin Micro-Emulsion Based On Supramolecular Self-Assembly

Citral,as a monoterpene naturally present in herbs and citrus fruits,has a fresh lemon flavor and was widely used in food,beverage and daily chemical fields.However,its aroma was unstable and volatile,resulting in unpleasant taste and limited application.As a cyclic oligosaccharide,cyclodextrin has its own unique’outer hydrophilic,inner hydrophobic’structure to protect unstable hydrophobic molecules.Based on this,citral was took as the research object,selected hydroxypropyl-β-cyclodextrin（HP-β-CD）as the inclusion wall material,and four kinds of Tween surfactants with different molecular structures as emulsifiers to prepare citral microemulsion.The optimal emulsion system was screened by particle size,polydispersity index（PDI）,encapsulation rate and loading capacity.Then,the microstructure,interaction force type,thermal stability and aroma release performance of the emulsion were further analyzed by various characterization methods.Finally,the mechanism of synergistic stabilization of citral by cyclodextrin-surfactant interaction in’ternary system’was analyzed by molecular docking and molecular dynamics（MD）simulation.This topic has deepened our understanding of the interaction between cyclodextrins and aroma compounds to a certain extent,which will help us to fully understand their formation and stabilization mechanisms.The main conclusions are as follows:（1）The emulsion was prepared by the inclusion of citral with HP-β-CD by saturated water solution method.Under the optical microscope,the emulsion showed large droplet shape and uneven dispersion.Four kinds of Tween-type surfactants（Tween-20,Tween-40,Tween-60 and Tween-80）with different molecular structures were used to compound with cyclodextrin.The emulsion stability,embedding rate and loading capacity,storage time and storage temperature were used as evaluation indexes to screen out the best emulsifier type.It is concluded that compared with the absence of surfactant,the citral emulsion with surfactant can observe dense inclusion complex droplets under high magnification of microscope.The embedding rate and loading capacity of four citral emulsions were calculated by UV photometry and thermogravimetric method.The embedding rate and loading capacity of the citral emulsion compounded with Tween-80 reached the maximum,and the embedding rate was 87.25%,the loading capacity was 10.70%.The microemulsion characteristics of low particle size and high stable state could be obtained when stored at room temperature.The solution was always light yellow and transparent,the particle size was stable around 12nm,and the PDI was about 0.120.（2）A series of steady-state characterizations of citral emulsions under the combination of four surfactants were carried out to explore the effect of the interaction between cyclodextrin and surfactant on improving the thermal stability of citral guest molecules,and its aroma sustained release properties.It was concluded that under the observation of scanning electron microscopy（SEM）,under the preparation of the first three surfactants,the morphology of the blank state without the addition of the guest molecule citral was smooth and flat at a scale of50μm.In contrast,the microemulsion powder prepared by Tween-80 showed obvious’honeycomb sponge’-like special structure.After the inclusion complex was formed,dense,smooth and full spherical inclusions were attached to the surface of the wall material.Under the observation of transmission electron microscopy（TEM）,the outer ring interface film was clearly visible.Furthermore,the inclusion reaction was confirmed by Fourier transform infrared spectroscopy（FTIR）and X-ray diffraction（XRD）on the interaction of functional groups between host and guest and the change of crystal form.Thermogravimetric（TG）analysis showed that the thermal stability of citral after embedding was significantly improved.The results of gas chromatography-mass spectrometry（GC-MS）combined with electronic nose analysis showed that the main aroma components of citral microemulsion,neral（E-）and geranial（Z-）,were embedded in citral microemulsion.The aroma profile of citral microemulsion was almost overlapped,indicating that the aroma of the aroma substances in the inclusion complex was almost unaffected with the prolongation of time,which confirming that it had good sustained release characteristics.（3）Molecular docking and molecular dynamics（MD）simulation were used to explore the molecular scale inclusion mechanism of citral’ternary’system embedded in three-dimensional network structure formed by self-assembly of cyclodextrin/Tween-80 complex.It is concluded that citral can bind to the inner or outer wall groups of cyclodextrin through weak interaction by predicting the molecular docking model of’ternary system’.In the simulation experiment,the root mean square deviation（RMSD）of the guest molecule Tween-80 and cyclodextrin in the binding process is in the dynamic trajectory of the spatial conformation in 0-50 ns,and the spatial conformation tended to the best steady state in the last 10 ns.The molecular mechanics/Poisson Boltzmann surface area（MM/PBSA）method was used to calculate the contribution of various interaction forces to the binding free energy during the formation of the inclusion complex.It is concluded that theΔG_Bind values were always less than zero in the whole process,indicating that the inclusion process is a thermodynamic spontaneous exothermic process.Based on the binding free energy obtained in the last 10 ns,△G_Bind=-19.15±3.25 kcal/mol was obtained.The inclusion complex conformation was the most stable.The van der Waals interaction force was the main driving force,followed by hydrogen bonding force,electrostatic force,polar solvent and non-polar solvent force.