Study On Thermodenaturation,Microscopically And Functional Properties Of Whey Protein From Goat Milk

As one of the main proteins in goat milk,whey protein has good biological titer and processing properties.But it is sensitive to processing temperature.Thus,the effect of heat treatment on its microscopic and functional properties needs to be systematically considered.In this paper,goat milk was used.The whey protein was separated from goat milk after processing with homogenization and different heat treatments.Then the changes of whey protein components were identified by SDS-PAGE.The changes of secondary structure,particle size,Zeta potential and surface structure of whey protein from raw milk,homogeneous milk and different heat-treated milk were analyzed.The surface hydrophobicity,emulsifying property,foaming property,water holding capacity and oil holding capacity of heat-treated milk and lactations were analyzed.The results of SDS-PAGE and scanning analysis showed that there were 6 kinds of proteins in whey,and the relative molecular weight of the protein was as follows:lactoferrin(80.0kDa),serum albumin(67.3 kDa),LgG-H(55.2 kDa),?-casein(28.1kDa),?-lactoglobulin(18.4kDa),?-lactalbumin(13.6kDa).The relative molecular weight of protein was close to whey protein from cattle milk.The content of ?-lactoglobulin and ?-lactalbumin in whey phase was the highest,accounting for about 70% of the total whey protein.The effect of heat treatment on the relative percentage content of each component in whey was significant(P < 0.05),especially the interaction between ?-lactoglobulin and ?-casein was greatly affected by heat treatment.The effect of thermodenaturation and the microstructure properties of whey protein were detemined.With the increase of heat treatment intensity,the denaturation degree increased.There was the highest(about 78%)of denaturation degree for whey protein treated by UHT due to the highest temperature.Although the temperature of pasteurization is low,because the heating time is up to 30 mins,about 50% of whey protein was thermally denatured.The secondary structure of whey protein was significantly affected by heat treatment(P< 0.05).The particle size of whey protein was significantly reduced by homogenization.The size of G2,G3,G4 and G5 increased with the increase of heat treatment.The average particle size of G4 after treatment was the largest,reaching 301.00 nm.With the increase of heat treatment intensity,the particle size of whey protein increased first and then decreased,which was related to the aggregation of whey protein.The effects of homogenization and heat treatment on Zeta potential were different,and homogenization decreased the stability of whey protein in the system.The surface structure of whey protein was observed by SEM.The surface of whey protein in raw milk and homogeneous milk presented regular globular structure,but there was slight agglomeration in raw milk.The distribution of whey protein in homogeneous milk was more uniform,the size of it was smaller,and the arrangement was regular.The surface structure of whey protein was obviously changed after different heat treatment,the globular structure was destroyed after heat treatment,the shape was irregular,the surface structure was close to flake seriously,and the molecular aggregation was observed.The turbidity of whey protein solution was significantly affected by heat treatment(P < 0.05).Homogenization significantly decreased the turbidity.The turbidity increased after treatment with Baxa and UHT.The turbidity of different thermal production processes was significantly different,among which G2 had the least influence on it and G4 had the greatest influence on it.The effects of heat treatment on functional properties of whey protein were studied.The hydrophobicity of whey protein was significantly increased at 65?/30 min and 85?/15 s,and decreased significantly after heat treatment at 125?/4 s and 135?/4 s.The effects on the EAI and ES were significant(P < 0.05).Compared with raw milk,the EAI and ES of whey protein decreased after homogenization,and increased significantly after different heat treatment(P < 0.05).After treatment at 65?/30 min,the EAI was the highest and ES was the lowest.The effects on foaming properties were significant(P < 0.05).FC of G4 and G5 increased first and then decreased,FS decreased gradually,and the FC value was the highest at 85?/15 s.The effect on the WHCwas significant(P < 0.05).After treatment at 65?for 30 minutes,the WHC was the highest.The effect on the OHC was significant(P < 0.05).After 85?/15 s treatment,its OHC was the highest.Using SDS-PAGE to determine the whey protein at different lactation period,the results show that,the effect on the content was significant(P< 0.05).The denaturation temperature was greatly affected by lactation period,and the denaturation temperature increased with the extension of lactation period.The heat denaturation temperature of whey protein decreased first and then increased.There were significant differences in fluorescence intensity between different lactation periods.The effect on the turbiditywas significant(P < 0.05).The turbidity decreased gradually with the prolongation of lactation period.The effects on EAI,ES,FC,WHC and OHC were significant(P < 0.05).With the prolongation of lactation period,EAI decreased,foaming ability decreased,WHC of 1st day's whey protein was significantly lower than others.OHC showed a gradual downward trend.