Study On Construction And Mechanism Of Zein Based Pickering Emulsion Gel

Emulsion gel is a type of common food system,combining the advantages of both emulsion and gel,which have drawn growing interest for the wide applications in food area.The stability of emulsion during gelation is a key factor to prepare emulsion gel.Pickering emulsion stabilized by particles is regarded as a desirable candidate to prepare emulsion gel due to its high stability.Zein,an edible corn protein,is a sub-product of the food and ethanol industry,which can easily self-assemble into nanoparticles（ZNP）.However,single zein exhibits poor emulsifying properties because of its strong hydrophobicity,which limits its wide application.Therefore,in this research,glycyrrhizic acid（GA）was firstly combined with ZNP to form glycyrrhizic acid-zein composite nanoparticles（ZGNP）via electrostatic interaction.Stable Pickering emulsion and high internal phase Pickering emulsion（HIPPE）were successfully prepared using ZGNP as stabilizers.The mechanism underlying the influence of interface behaviors of ZGNP on the emulsion stability was investigated.Furthermore,Pickering emulsion gel was developed via ionic bridging and cationic chitosan（CS）bridging,respectively.By investigating the influence of ionic strength and CS concentrations on the network structure and rheological properties of the emulsion gel,the mechanism of Pickering emulsion gelation induced by ion bridging and polymer bridging was clarified.Using these Pickering emulsion gels as bio-inks for 3D printing,the relationship between rheological properties and printing quality was clarified.This research aimed to provide theoretical support for the behavior moderation and functional design of zein-based Pickering emulsion gel.The main research results are as follows:（1）ZNP was fabricated by antisolvent method and then complexed with GA to form ZGNP via hydrogen bonding and electrostatic interaction.The DLS results showed the zeta-potential of the nanoparticles changed from positive to negative after the complexation with GA.The absolute value increased and the average size decreased with the increasing addition of GA.ZGNP_1:1 showed the best emulsifying capacity and its mean diameter and zeta-potential is 190.97±4.14 nm and-23.47±0.58 m V,respectively.Using ZGNP as emulsifiers,HIPPE with oil phase volume fraction of up to 75%was successfully prepared.The CLSM images indicated that ZGNP could absorb into the oil-water interface to form a thick layer,thereby preventing coalescence.As the ZGNP concentration increased,the HIPPE showed better stability and smaller droplet size.When the concentration of ZGNP is 2.5%,the droplet size of HIPPE is 5.31±0.45μm,and this HIPPE could stay stable throughout 20 days of storage.The rheological results indicated the HIPPE exhibited shear-thinning behavior,and high viscoelasticity,and the apparent viscosity increased with the increasing concentration of ZGNP.（2）Based on ZGNP-stabilized Pickering emulsion,gel-like HIPPE was prepared by ionic bridging.The optical microscopy images showed the oil droplets come closer together and some aggregation occurred after the addition of magnesium cations.The mean droplet diameter of HIPPE gel containing 90 m M magnesium cations increased to 20.53±0.59μm.The addition of magnesium cations induced the electrostatic screening and salt bridge formation,which strengthened the interactions between ZGNP-coated oil droplets and ZGNP network in the aqueous phase,contributing to the formation of stable emulsion gel network.Their rheology and stability characteristics enabled them to be used to as edible bio-inks for3D printing applications.The bridging HIPPE gel with more cations showed higher elastic modulus and better 3D printing behavior.（3）Pickering emulsions gel（PEG）was fabricated by physically crosslinking anionic ZGNP-coated oil droplets with cationic CS.Stable PEG could be formed when the volume fraction of the oil phase exceeded 60%.However,considerably larger droplets were observed in the PEG containing 75%oil,which had a negative impact on their gel strength.Cryo-SEM images showed that CS could form electrostatic bridges between neighboring oil droplets,as well as a network structure in the surrounding aqueous phase.At relatively low chitosan concentration（less than 1.5%）,the oil droplets were not fully coated with CS and some droplet coalescence occurred,which reduced their stability and mechanical strength.When the CS concentration was sufficiently high（2.0%～3.0%）,a polysaccharide coating was formed around the oil droplets that protected them from coalescence and promoted droplet-droplet adhesion,thereby leading to the formation of plastic-like soft solids with good stability and mechanical strength.These soft materials exhibited physicochemical attributes that made them highly suitable as edible inks for 3D printing applications The polymer-crosslinked PEG could 3D-print selected shapes with high resolution and shape fidelity.By centrifugation,the droplets were squeezed together much more strongly,leading to the concentration of oil droplets.High internal phase PEG with 74%oil fraction was prepared when the centrifugation force was 6000 g.As centrifugation force increased,the gel strength enhanced.However,ultra-centrifugation force（over 8000 g）led to more coalescence in PEG and its exceeded mechanical strength caused squeezing difficulty,thus reducing its 3D printing quality.