| As the most abundant protein in camel milk,casein micelles are closely related to the physicochemical properties and processing characteristics of camel milk.However,there is still a lack of research on the changes of protein composition and structure under different processing conditions.In the processing of camel milk,the processing method of cow milk is often used.Therefore,camel milk and cow milk were used as raw materials in this study.The composition and structure of camel milk and cow milk were determined by proteomics,SDS-PAGE,particle size,potential,transmission electron microscopy and small angle X-ray scattering.The protein composition and function of the two were compared and analyzed,as well as the difference in protein structure.At the same time,the effects of pH,calcium ion concentration and heat treatment intensity on the composition and structure of camel milk casein micelles were investigated,which provided a theoretical basis for the deep processing of camel milk.The specific results are as follows:1.Comparative analysis of composition and structure of camel milk and cow milk protein.Compared with cow milk,camel milk had 502 differentially up-regulated proteins and 513 down-regulated proteins.The results of bioinformatics analysis showed that the differentially expressed proteins were related to cellular anatomical entities,cellular processes,primary immunodeficiency,NF-κB signaling pathway and hematopoietic cell lineage.Camel milk contains a small amount ofκ-casein,and noβ-lactoglobulin was found.Compared with milk,camel milk had lower potential and larger particle size（p<0.05）.The results of fluorescence spectroscopy showed that the maximum fluorescence intensity of camel milk was higher than that of bovine milk in both endogenous and exogenous fluorescence spectra.The results of small angle X-ray showed that camel milk had the same inflection point at q≈0.03 nm-1as milk,and also had an inflection point at q≈0.08 nm-1.2.The effects of different pH（5.0-7.4）on the physicochemical properties and structure of camel casein micelles.With the increase of pH,the concentration of calcium and phosphorus in whey phase decreased significantly（p<0.05）.The dissociation degree of casein was the highest at pH 5.4,and the dissociation rate was slower at pH>5.8.When pH<6.2,the particle size of camel casein micelles increased significantly（p<0.05）,and the absolute value of Zeta potential decreased significantly（p<0.05）,but there was no significant change in particle size and potential between pH 6.2-5.8.With the increase of pH,the maximum fluorescence intensity of endogenous and exogenous fluorescence spectra increased sharply.At the same time,the results of transmission electron microscopy showed that casein micelles aggregated during acidification,and casein micelles expanded during alkalization.3.The effects of different calcium ion addition（0-40 m M）on the physicochemical properties and structure of camel casein micelles.With the continuous addition of calcium ions,the calcium concentration in the whey phase increased significantly（p<0.05）,and the phosphorus concentration decreased significantly（p<0.05）.Compared with the group without calcium ion,the average particle size of casein micelles increased with the increase of calcium ion concentration（p<0.05）.At the same time,the maximum fluorescence intensity of the endogenous and exogenous fluorescence spectra also showed an upward trend,but the maximum fluorescence intensity decreased when the calcium ion concentration was 40 m M.In addition,the results of transmission electron microscopy showed that the addition of calcium ions caused the surface structure of casein micelles to be rough,and small micelles of about 200 nm appeared when 40m M calcium ions were added.4.Effects of different heat treatment intensity on physicochemical properties and structure of camel casein micelles.In the range of 75°C-95°C,the concentration of calcium and phosphorus in whey phase increased with the increase of heat treatment intensity,and reached the highest at 85°C and 30 min.At 75°C,10 min,β-casein will undergo a certain degree of dissociation,and the degree of dissociation reaches the maximum at 95°C,30 min.With the increase of heat treatment intensity,the average particle size of casein micelles increased（p<0.05）,and the maximum fluorescence intensity of endogenous and exogenous fluorescence spectra increased gradually.At the same time,the results based on transmission electron microscopy showed that the increase of heat treatment strength led to the increase of micelle aggregation.In summary,there are differences in protein composition,function and protein structure between camel milk and cow milk.pH,calcium ion concentration and heat treatment intensity will affect the composition and structure of camel milk casein micelles,change the stability of camel milk casein micelles,and then affect the stability of camel milk. |
