| High Internal Phase Emulsions(HIPEs)refer to emulsions with an internal phase volume fraction of more than 74%,which can simulate the solid-like texture and taste of fat,with excellent storage stability and encapsulation delivery ability.Poultry egg resources is rich in China,and egg yolk is an important part of poultry eggs.Egg Yolk Granules(EYGs)are water-insoluble components of egg yolk with high protein,low cholesterol,low fat and good thermal stability,and have good emulsification potential as new natural pickering solid particles.At present,there are few systematic studies on EYGs in HIPEs systems.The mechanism of EYGs stabilizing HIPEs has not been elucidated,especially the processing methods affect the formation of HIPEs.It limited the application of EYGs in emulsified foods.In this paper,we investigated the mechanism of stabilization of HIPEs by EYGs through the regulation of p H and ionic strength.We further constructed the EYGschitosan(CS)complex system,and investigated the complex behaviour of EYGs and CS,the dependence of emulsification properties on the ratio of EYGs/CS and p H value.Then We analyzed the microstructure and physicochemical properties of the complex system.Finally,the constructed EYGs-CS composite system was used to prepare HIPEs,and the prepared HIPEs had stronger environmental resistance.The main research contents and results are as follows.(1)Study on the preparation of EYGs-based HIPEs and their formation mechanism.The microstructure and physicochemical properties of EYGs at different ionic strengths and different p H as well as the variation pattern of the properties of their prepared HIPEs were investigated.The structure and properties of EYGs and the intrinsic connection between them and the HIPEs they form were elucidated.The results showed that EYGs exhibited good stabilization ability of HIPEs(85% in oil phase).With increasing ionic strength or p H,the dense calcium-phosphorus bridge structure of EYGs was disrupted and gradually cleaved into soluble phosvitin and LDLs micelles.These micelles could be more easily and effectively adsorbed on the oil-water interface of the emulsion,resulting in smaller particle size and more stable emulsions.Thus,at Na Cl > 0.2 M or at 0.3 M Na Cl with p H>5.0,HIPEs stabilized by EYGs exhibit a semi-solid gel-like appearance with some self-supporting properties.The rheological results showed that an increase in ionic strength or p H also improved the viscosity and elasticity of the HIPEs.The HIPEs also showed excellent storage stability and centrifugal stability due to the formation of the internal threedimensional network structure,but the freeze-thaw stability and thermal stability were poor.(2)The construction of EYGs-CS composite system and its emulsification stability behavior.We studied the effects of EYGs/CS ratio and p H on the physicochemical properties and emulsifying properties of EYGs-CS composite system.The dependence of EYGs/CS ratio and emulsifying properties on EYGs/CS ratio and p H value were explored.The results showed that EYGs and CS(ratio 2:1)could form electrostatic complexes with the highest aggregation degree at p H 5.0,and the aggregation degree of the complex system increased with the improvement of CS ratio.The results of multiple light scattering indicated that under the condition of EYGs/CS ratio of 1:2 or p H 5.0,the composite system exhibited poor stability due to strong flocculation.In addition,the contact angle and surface tension results showed that the samples with EYGs/CS ratio of 2:1 exhibited a contact angle closer to 90° and a lower oil-water interfacial tension for the different EYGs/CS ratios.On the basis of the EYGs/CS ratio of 2:1,it was also found that the acidic p H conditions(p H3.0,4.0 and 5.0)had a contact angle closer to 90° and a lower oil-water interfacial tension than the neutral p H conditions(p H 6.0,7.0 and 8.0),resulting in a stronger emulsification potential.The results of emulsification activity,emulsion stability,emulsion capacity and emulsion index further indicated that the composite system exhibit stronger emulsification behavior at a 2:1 ratio of EYGs/CS or at acidic p H conditions.(3)Structural characterization and rheological properties of EYGs-CS composites.The effects of EYGs/CS ratio and p H on the structure and rheological properties of EYGsCS composite system were investigated to explore the intrinsic relationship between the structure and emulsification properties of EYGs-CS composite system.The results showed that the EYGs-CS composite system formed aggregates with higher cross-linking degree with the increase of CS ratio at p H 5.5,and exhibited a more stretched structure at the EYGs/CS ratio of 2:1.When the ratio of EYGs/CS was 2:1,at p H 3.0 and 4.0,EYGs and CS formed a structurally stretched complex system driven by electrostatic repulsion and steric hindrance.At p H 5.0 and 6.0,EYGs banded negatively charged,CS was positively charged.They cross-linked into complex condensates driven by electrostatic attraction and hydrogen bonds.At p H 7.0 and 8.0,CS has poor solubility,so the EYGs-CS composite system presents a relatively dense structure.The stretched structure of EYGs-CS composite system could expose hydrophobic groups on the surface.Thereby,the adsorption properties at the oil-water interface were enhanced.In addition,the formation of the EYGs-CS composite system at p H 5.0 led to loosening EYGs structure,which caused a decrease in the thermal denaturation temperature of the composite system(96.7 °C-86.5 °C)and an increase in the mobility of the bound water of the system.Meanwhile,the EYGs-CS composite system exhibited strong viscoelastic behavior at EYGs/CS ratio of 1:2 or p H 6.0.(4)Preparation and stability study of EYGs-CS HIPEs.The effects of p H and ionic strength on the stabilization of HIPEs by EYGs-CS composite system were studied to analyze the differences between EYGs-CS composite system and single EYGs-stabilized HIPEs.The results showed that with the decrease of p H,the EYGs-CS composite structure was more stretched and could be adsorbed more effectively at the oil-water interface.With the increase of ionic strength,the electrostatic interaction between EYGs and CS was gradually shielded.EYGs and CS could adsorb synergistically at the oil-water interface.Therefore,the EYGs-CS composite system could form a dense interfacial film on the oilwater interface to stabilize HIPEs under acidic p H(p H 3.0,4.0 and 5.0)or high salt(0.2 M,0.3 M,0.5 M)with natural p H.However,under the conditions of neutral p H(p H 6.0,7.0and 8.0)or low salt(0,0.1 M)with natural p H,the HIPEs could not be sufficiently adsorbed on the oil-water interface to stabilize HIPEs.The rheological and moisture distribution results showed that the HIPEs prepared under acidic p H or high salt conditions had more uniform moisture distribution,stronger viscoelastic behavior,and exhibited stronger stability.The HIPEs stabilized by the EYGs-CS composite system had a denser interfacial film than the EYGs-based HIPEs,thus exhibiting stronger thermal stability and freeze-thaw stability,which broaden the application range of EYGs in high-oil emulsion food systems. |
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