Study On Preparation Of Mixed Standard Substance And Microcosmic Characteristics And Functional Properties Of Casein From Goat Milk

Goat milk(GM)is favored by more and more consumers as a kind of new milk source,and its deep processing and comprehensive utilization have become a research hotspot in recent years.Each of milk sources has its unique advantages in milk composition and content,and the changes of milk composition and interaction between milk components play a key role in dairy quality and milk thermostability.Rapid detection of milk composition by Milk Analyzer Method has been in advantage,the key lies in the preparation and calibration of mixed standard.The GM composition and structure are different from cow's milk.In this study,the mixed standards were designed for rapid detection of GM compositions by hollow fiber ultrafiltration technology,and the uniformity and stability were discussed.The results showed that the contents of protein,fat and lactose of this standard substance were independent of each other(P > 0.05).Adopting the quasi level design method and the method of formulation with no emulsifier was the optimal solution.Compared with cow's milk standards in the prior art,this standard was more accurate and suitable for the calibration of instrument to rapidly detect GM compositions.The uniformity and stability of the mixed standard substance were of better,it could be stored for 15 days at 4 degrees centigrade.And the uniformity and stability of the in-units and inter-units could meet the national requirements for the preparation of standard substance.The thermal stability and functional properties of goat casein(CN)directly determines the quality of GM protein.The heat treatment is a very important part in the production process of GM,and will also reduce the stability of GM at the same time ensuring the quality and safety of GM,which has a significant effect on the nutritional quality of GM.The main reason leading to the reduction of the stability is the interaction between CN and beta lactoglobulin in whey protein generating complex aggregates.Changes in micelle structure and functional properties of CN from GM in different lactation and heat processing were studied,in order to lay a theoretical foundation for the comprehensive and further development of deep processing of dairy products containing CN and the whole GM industry.The isoelectric point of CN was determined by the method of centrifugal precipitation.The molecular weight and purity of CN were determined by sodium dodecyl sulfate-polyacrylamide gel electropheresis(SDS-PAGE).The results showed that the isoelectric point of CN from Laoshan dairy GM was 4.10,the molecular weight was about 20 ~ 40 kDa.The CN had higher purity and could be divided into ?s1-CN,?s2-CN,?-CN,?-CN,and ?-CN had the highest contents.In the process of heat treatments of GM,the aggregates between ?-lactoglobulin and CN were formed through the polymerization.The results of the effects of lactation,homogenization and heat treatments on micelle characteristics of goat CN showed that thermal denaturation(Td)temperature gradually increased as the extension of lactation,the changes of heat enthalpy were irrelevant to lactation.With the extension of lactation,the average particle size of the micelle and turbidity significantly decreased(P < 0.05).The Zeta potential was not significantly affected by lactation(P > 0.05).After homogenization and heat treatments of GM,Td temperature and heat enthalpy of CN were gradually decreased,the average particle size of the micelle and turbidity significantly increased(P < 0.05).The change trend of turbidity was consistent with average particle size.The effect of homogenization on Zeta potential of CN was not significant(P > 0.05),heat treatments could lead to significant changes of Zeta potential of CN(P < 0.05).Scanning electron microscope(SEM)indicated that the micelle surface structures of CN from fresh GM and homogenized GM were more regularly spherical.The micelle surface structures of CN from heat treated GM were destroyed,the shape was similar to flake.Compared with homogenized GM and heat treated GM,fresh GM had serious reunion.The analysis results of circular dichroism spectra showed that the secondary structure of goat CN mainly included ?-pleated sheet and random coil.The content changes of the secondary structure were not significantly affected by homogenization and heat treatments(P > 0.05),which was consistent with the results of Td temperature.The functional properties of goat CN affected by lactation,homogenization and heat treatments were investigated.The results showed that emulsifying activity index(EAI),foam stability(FS),water holding capacity(WHC),oil holding capacity(OHC)and solubility were significantly affected by lactation(P < 0.05).And emulsion stability(ES)and foam capacity(FC)were not significantly affected by lactation(P > 0.05).With the extension of lactation,there was a positive correlation between the FC and FS of CN solution.After homogenization and heat treatments of GM,EAI and solubility of CN were significantly increased(P < 0.05).ES of CN was not significant(P > 0.05).FC,FS,WHC and OHC of CN were significantly decreased(P < 0.05).Compared with homogenized GM,heat treated GM could lead to significant increase of FC,WHC and OHC of CN(P < 0.05),and decrease of FS(P < 0.05).