| The largest producer and consumer of eggs is China.Fresh eggs are a major consumer product.The consumption of fresh eggs is over 90%,However,the quality of eggs would be changed significantly during their storage,such as egg white thinning and pH increasing.Based on Haugh unit,weight loss rate,egg white pH value and other indicators,the research compared the atmosphere preservation conditions of eggs,and detected the main changes of the qualities,which provided a technical support and theoretical basis for fresh-keeping eggs.The main findings were as follows:Based on the measurement of the texture analysis and the quality indexes such as Haugh unit and weight loss rate,the research studied the variations of egg quality under different storage temperatures and air conditioning packaging conditions by using scanning electron microscopy.The results showed that after their storage for 35 days at 25℃,the air cell diameter of eggs increased to 26.07±0.96 mm,the weight loss rate increased to 5.043±0.393%,and the HU fell to the B level.The pH of the egg whites rose rapidly and remained at a high level(8.69,9.55,9.40 for 0,7,and 35 days,respectively).The apparent viscosity of the egg white natural gel characteristics decreased to 0.014 Pa·s.The elastic modulus(G’)and the viscous modulus(G")were G’<G"at 14 days.And the egg white was thinned.At the same time,the scanning electron microscope showed the egg white gel network fibers were gradually destroyed.Compared with the 25℃ group,the air cell diameter of 50%CO2,7-11%O2,39-43%N2 modified atmosphere packaging(25℃)groupand 4℃ group were much smaller(P<0.05).Also,the weight loss rate was lower(0.5-0.7%,1.6%,respectively).HU were AA grade and pH values changed little(7.3-7.5,8.95,respectively).The apparent viscosity decreased slightly(0.042,0.048 Pa·s).Likewise,G’>G" at 35 days,it was gelatinous.Therefore,the results showed that packaging conditions of 50%CO2,7-11%O2,39-43%N2 and storage at 25℃ were more effective on keeping fresh eggs.Besides,the natural gel properties and the pH value of egg white were important to affect the quality of eggs.By means of SDS-PAGE,HPLC,and LC-MS/MS,the research mainly focused on the potential mechanism of egg white natural gel properties.The results showed that α-subunit and β-subunit of ovomucin degraded gradually after the storage at 25℃ for 35 days,the content of ovomucin decreased rapidly from 2.931 to 0.341 mg/g.Meanwhile,N-configuration of ovalbumin changed to S-configuration rapidly and the content of S-ovalbumin increased sharply from 14.867%to 94.345%.The content of lysozyme increased significantly from 1.488 to 4.851 mg/mL,while the lysozyme activity decreased significantly from 30166.7 to 15933.3 U/mL.In order to study the peptide changes of egg white,the egg white was precipitated and the supernatant was passed through 10 kDa and 3 kDa ultrafiltration membrane.The contents of the pepetide in both fractions of<10 kDa and<3 kDa increased gradually(P<0.05).The increase in peptide content demonstrated protein degradation.The protein in the fractions of 10-3 kDa was gallin(ovodefensin,-7 kDa),which gradually degraded and completely disappeared after 28 days.The peptide fragments with significantly increased content in the fractions of 3 kDa belong to Ovotransferrin,Ovomucin,Ovomucoid,Ovoinhibitor,respectively.These four proteins were gradually degraded into peptides of<3 kDa,in which the protease inhibitors(Ovomucoid and Ovoinhibitor)were inactivated,which accelerated protein degradation.The mechanism of the pH value changes of egg white was studied by ICP-OES,ICP-MS and other methods to determine the functional groups of egg white proteins.The results showed that the content of surface SH and free NH2 of egg white increased rapidly at room temperature.S-S was changed to SH and combined with H+ to increase the pH value of egg white.Free NH2 was alkaline,and its content increased,which might cause the pH value of egg white to rise.In addition,the content of Ca,Na,P,Fe,Zn and Cu in the fractions of<10 kDa egg white increased gradually(P<0.01),lead to raise the pH of the egg white.Moreover,the pH value of egg white and SH,S-S,free NH2,8 kinds of inorganic salt ions in the fractions of<10 kDa were used for multiple regression analysis,the multiple regression equation:pH=4.435+1.241SH+0.100NH2-0.090SS+0.082Fe+0.058Zn.T-test analysis showed that the pH value of egg white had a causal relationship with the changes of SH,NH2,S-S,Fe and Zn.In summary,the study screened out a method of keeping fresh eggs based on the index of egg quality change,and revealed the mechanism of the natural gel structure destruction of egg white and the increase of pH value of egg white,which could be used to guide the practical production. |
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