Hydrophobic Modification Of Chitosan And The Emulsifying Properties

Polysaccharides,proteins or peptides could be used as natural or natural resource emulsifier.However,there are still lack of the natural platform emulsifiers as like Span,Tween,or sucrose ester.That is because,the strong hydrophilicity of polysaccharides,the poor physichemical stability and water solubility of proteins or peptides have limited their applications.The cationic polysaccharide,chitosan(CS)possess great biocompatibility,biodegradability and renewability.As the major hydrophobic proteins,casein can offer peptides with more hydrophobic groups after enzymatic hydrolysis.By modifying CS with casein hydrophobic polypeptide(CHP),the amphiphilic properties of CS can be improved,and the shortcomings of poor stability and water solubility of peptide can be overcome.Meanwhile,the hydrophobic modifided products may some certain functional properties to the emulsions,such as antioxidant and antibacterial properties.In this paper,the hydrophilic polysaccharide,chitosan(CS)and the casein hydrophobic polypeptide(CHP)were employed to prepare CS-CHP emulsifiers with two green synthesis methods(dry Maillard reaction and anti-solvent procedure).The surface/interface properties,emulsification properties and other physicochemical properties or functional properties were investigated.The CS-CHP products present nice emulsifing ability and the emulsion stabilized mechanism was clarified.Moreover,emulsions stabilized with CS-CHP were employed for encapsulation,protection and release of curcumin.The detailed research and results are as follows:1.As the demand of natural source and wide range of HLB value platform emulsifiers,a seriers of CS-CHP conjugates were prepared and characterized;and their amphiphilicity,thermal stability,solubility and other functional properties as well as their structure-activity relationships were studied.Under the optimal conditions of dry-heat Maillard reaction,the conjugation between CHP(hydrolysis degree of 10%)and CS with different molecular weights and degree of deacetylation was acieved.A platform of conjugates was obtained with HLB value and degree of grafting(DG)ranging from 6 to 14 and 3.10%to 15.08%,respectively.Molecular weight,infrared spectroscopy and nuclear magnetic resonance spectroscopy of conjugates were characterized.Ultraviolet-visible spectrum and circular dichroism chromatography were used to analyzed the conformation of conjugates.These CS-CHP conjugates possess a structure-function relationship:with the increment of DG,the browning index,solution zeta potential was increased,and the HLB value decreased.The conjugates present better amphipathic activity,thermal stability and water solubility than CHP,CS and their non-covalent mixture.The conjugates can stablize emulsions in 120 days,while emulsions prepared with CHP,CS or their mixture creamed in 7 and 14 days,respectively.Emulsions prepared with conjugates also have great tolerance towards temperature and electrolyte.Rheological behavior and microscopic morphology characterization demostrate that they were elastic gel-like emulsions.The conjugates absorbed at the oil/water interface,forming thick film at the interface and network structure.Moreover,curcumin was encapsulated in the emulsion stabilized with CS-CHP conjugate.The encapsulation efficiency is close to 100%,and stable in 14 days.The stabilized encapsulation of curcumin was related to the emulsion stability.The emulsion stabilized with the conjugate released 60.39%of curcumin in the simulated gastrointestinal fluid in 6 h,which has a significant slow release effect compared to emulsions stabilized with CS and CS-CHP mixture,respectively.Futhermore,the antioxidant activity of conjugates may provid inhibition effect of curcumin degradation.2.Extended from the research on molecular emulsifier as the CS-CHP conjugate,CS-CHP nanoparticles that are expected to stabilize Pickering emulsion were synthesized through antisolvent procedure by altering the ratio of CS and CHP.The particle size and three phase contact angle was ranged from 1.17 to 9.14 nm and 12.5°to 87.4°,respectively.The isothermal titration calorimetry and zeta potential indicate that the interaction between CS and CHP was weak,and the exothermic bonding process was driven by hydrophobic interaction.The CS-CHP nanoparticles can significantly reduce surface/interfacial tension and absorb at oil/water interface quickly.The CS-CHP nanoparticles presented crystal structure,and the thermogravimetric results indicated that the thermal stability was elevated.The series of nanoparticles presented excellent antioxidant properties;and also exhibted significant inhibitory effect on E.coli and S.aureus.The cell toxicity of the nanoparticles was primary toxicity,without concentration dependency within 125-2000?g/m L.The nanoparticles have excellent emulsifing activity,the emulsion type can be switched from O/W to W/O by altering the oil fraction.It can also stabilize different oil to provide a broad spectrum emulsifying ability.The stability of the Pickering emulsion was realized by the combination of the viscoelastic soft barrier and intensity around the droplet provided by nanoparticles;for example,emulsion prepared with NPs3-1 was stable in 28 days.Moreover,the nanoparticles can provide the Pickering emulsions the p H and CO₂/N₂stimuli-responsive ability.With increased switching runs,emulsions still maintained relatively stable particle size and polydispersity.Furthermore,the emulsions stabilized by the two CS-CHP systems can both achieve efficient embedding of curcumin,but exhibit very different release behaviors.Pickering emulsion stabilized by CS-CHP nanoparticles can not only achieve efficient encapsulation and sustained release of curcumin,but also achieve targeted release of curcumin under different environmental stimuli.The curcumin encapsulated in Pickering emulsion released little in the simulated gastric fluid(SGF,p H 1.2),but gradually released in simulated intestinal fluid(SIF,p H 7.4),with broken emulsion.However,curcumin encapsulated in emulsion stabilized by the conjugates could be released in SGF and continue to be released in SIF.Therefore,chitosan-hydrophobic peptides are expected to become a class of functional platform emulsifier molecules through the regulation of molecular structure,and are expected to play roles in drug release.