| Egg yolk powder is the ideal substitute for fresh eggs because it is a protein and lipid-rich powder made from fresh egg yolk.It not only retains the properties of high biological activity and excellent processing of egg yolk,but also reduces the costs of transportation,thus effectively extending the shelf life.An important factor in its processing characteristics is the solubility and emulsifying properties of egg yolk powder.However,the structure and composition of yolk powder are complex,especially the poor solubility of its yolk granules will increase the inconvenience in the processing of yolk powder,as well as affect the emulsification stability,and ultimately limit its market application in the field of solid beverage,etc.In this paper,ultrasonic pretreatment and enzymatic hydrolysis of subtilisin were used to modify egg yolk powder.Study the effects of enzymatic hydrolysis on the composition,structure and properties of egg yolk powder,and explore the mechanism of its solubility improvement.The effects of ultrasonic pretreatment combined with enzymatic hydrolysis on the emulsification stability of yolk powder were further explained and analyzed,and the improvement mechanism of emulsification stability was explored.The research results of this thesis provide a theoretical basis for the egg yolk powder products in the field of instant preparation and emulsification processing.The main research contents and results were as follows:(1)In this study,papain,trypsin,and subtilisin were used to hydrolyze egg yolk,the effects of enzymatic hydrolysis on the solubility of egg yolk powder was investigated,and the mechanism of the solubility improvement of powder was studied through the characterization of structural properties.The results showed that subtilisin treatment significantly improved the WD,powder solubility and stability coefficient of egg yolk powder(P < 0.05)by 41.67%,98.16%,and 66.21%,respectively,while its solubility did not change significantly after the treatment with papain and trypsin.The degree of hydrolysis is related to the active site of the enzyme.The subtilisin acts mainly on the side chain of the hydrophobic amino acid residue,and the degree of hydrolysis was higher after treatment.Additionally,the results of SDS-PAGE,particle size distribution and SEM proved that the molecular weight of subtilisin-treated powder was lower than powder after the other treatments.Moreover,surface hydrophobicity and zeta-potential results indicated that molecular hydrophobicity and the amount of surface charge increased.LF-NMR results showed that enzymatic hydrolysis changed the content of bound water inthe powder,resulting in the changes of its ability to combine with water.(2)In this study,the effects of different ultrasonic power and time pretreatment combined with subtilisin hydrolysis on the solubility of egg yolk powder were studied.The mechanism of improving the solubility of egg yolk powder by ultrasonic pretreatment combined with enzymolysis was analyzed.The solubility of egg yolk powder was significantly improved by ultrasonic pretreatment(300 W,150 W,60 W)combined with enzymatic hydrolysis.The best solubility of egg yolk powder(P < 0.05)was 60W20 min.Compared with the control group,natural egg yolk powder increased by128.65% and 211.80%,respectively,which was 24.33% and 41.10% higher than that of the enzymatic control group.Ultrasonic pretreatment promotes the enzymatic hydrolysis reaction,changes the conformation of egg yolk protein and promotes the production of soluble aggregates.The results of SDS-PAGE and zeta potential indicate that ultrasonic pretreatment did not cause changes in protein composition and an increase in the amount of surface charge.Ultrasonic pretreatment mainly changed the secondary structure of egg yolk powder protein.After treatment,the apparently increased protein random coil structure,enhanced molecular flexibility,exposure to active sites buried inside the protein,acceleration and enzyme action can be demonstrated in the FTIR spectrum.The degree of hydrolysis of egg yolk increased first and then decreased with the increase of ultrasonic power and time.The results of particle size analysis and SEM showed that the particle size of the egg yolk powder in the ultrasonic treatment group decreased slightly with the increase of ultrasonic time and power,which aggravated the generation of soluble aggregates of small particles and promoted the dissolution of egg yolk powder.(3)The morphology,structural characteristics and internal force mode of egg yolk granules were studied.The mechanism of ultrasonic pretreatment combined with enzymatic hydrolysis on the structure and emulsion stability of granules was analyzed.The results of SEM showed that the aggregation state of granules was affected by ultrasonic combined enzymatic modification.The three-phase contact angle of granules was reduced from 106.3° to 93.4° after enzymatic hydrolysis,which significantly reduced the hydrophobicity of the granules.The zeta potential showed that the surface charge amount after modification was slightly decreased(P < 0.05).The results of SDS-PAGE showed that the molecular weight of the protein of the yolk granules decreased after enzymatic hydrolysis.The FTIR spectra and particle size changes under the influence of protein perturbations indicate that the internal hydrogen bonding of the granules isenhanced after enzymatic hydrolysis,and the main internal forces of the modified granules are hydrophobic interactions and hydrogen bonding.The emulsion stability of the granule-stabilized emulsion was studied by changing the protein concentration of granules.The results showed that the ESI and thermal stability were significantly improved after ultrasonic combined with enzymatic hydrolysis.When the protein concentration was 0.40%,the milking index decreased by 40.00%,the thermal stability increased by 81.82%,and the ESI increased by 115.43%(P < 0.05).And the emulsion stability of modified granules was significant increased with the increasing of the protein concentration,the protein concentration was increased from 0.2% to 1.0%,and the ESI value of enzyme-hydrolyzed granule emulsion was increased by 135.27%(P < 0.05).(4)The effect of ultrasonic pretreatment combined with enzymatic hydrolysis on the emulsion stability of egg yolk plasma stable emulsion and the effect and mechanism of adding cationic guar gum and oleic acid on the emulsion stability of modified plasma emulsion were studied.The emulsion stability of the plasma-stable emulsion was significantly improved by ultrasonic pretreatment combined with enzymatic hydrolysis compared to the untreated plasma.The absolute value of the emulsion zeta potential increased,the initial water-oil interfacial tension decreased significantly,the emulsion precipitation rate decreased 7.32%,the ESI increased by nearly 141.33%,and the protein content adsorbed to the interface was lower(P < 0.05).Meanwhile,the results of emulsion precipitation and ESI showed that the addition of cationic guar alone and the addition of cationic guar in the presence of 1% oleic acid improved the emulsion stability of the slurry emulsion to some extent.Compared with the enzymatic hydrolysis egg yolk emulsion,ESI value increased by 90.21% and 88.03%,respectively,and it increased and then decreased with the increase of cationic guar concentration.The results of the centrifugal stability coefficient indicate that the emulsion stability in the presence of oleic acid is higher than that of the cationic guar gum series alone.The absolute value of the emulsion zeta potential increases with the increase of the concentration of cationic guar gum.The results of fluorescence microscopy and particle size analysis showed that the particle size of the stabilized emulsion of egg yolk plasma gradually decreased with the increase of cationic guar concentration.The concentration of unadsorbed protein increased first and then decreased with the increase of cationic guar concentration. |
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