The Effects Of Different Heat Treatments On The Physical And Chemical Properties Of Milk Protein

Heat treatment is a necessary process for dairy products,which has a great influence on the physical and chemical properties of milk.In this study,glycosylation and degree of hydrolysis on protein in milk powder was analysed,and effects of different heat treatments on physical and chemical properties of fresh milk protein were explored.The main exprement results are presented in the follows:(1)This study investigated glycosylated casein and whey protein in milk powder and explored its enzymatic property.The casein and whey protein in milk powder were extracted by different methods.The compositions of casein and whey proteins were analyzed by SDS-PAGE electrophoresis method,and glycosylated casein and whey proteins were identified by Periodate-schiff alkali dyeing and Morse test.Amino acid content was determined after papain hydrolysis on casein and whey proteins in milk powder and fresh milk.The most efficient method to extract casein and whey proteins from milk powder are acetic acid precipitation(pH 4.6)and lactic acid fermentation respectively.The results of SDS-PAGE showed that the molecular weights of the casein and whey proteins were 24,34,60 KDa,and 14,18 KDa,respectively.The casein of 24 and 34 KDa,and the whey protein of 14 ku were both glycoproteins by Periodate-schiff alkali dyeing.The results of Morse text of casein and whey proteins showed that milk powder exist glycoprotein.The results of milk protein hydrolysis by papain showed that the content of free amino acid in casein dehydrated from milk powder was significantly lower than that from fresh milk(P<0.05),and the same results to that of whey proteins.Milk powder contained glycosylated casein,both 24 and 34 KDa casein and 14 KDa whey protein were glycoprotein.The degree of milk protein hydrolysis from milk powder is lower than that from fresh milk.(2)The objective of this research was to explore the effects of different heat treatments on the surface characteristics and physical and chemical properties of milk protein.The milk was treated with different heat treatments(pasteurized at 85 ? for 30 min,high temperature sterilized at 121 ? for 15 min,concentrated at 85 ? for 6 h,and ultra-high temperature instantaneous sterilizated at 131-134 ? for 4-6 s,respectively).The changes of color value and micromorphology of milk particles were observed by Colour difference meter and Scanning electron microscopy.The indexes of functional groups,secondary structure,and molecular weight,were determined by Fourier transform infrared spectrometer,Circular dichroism spectroscopy,Matrix assisted laser desorption ionization time of flight mass spectrometry,respectively.Glycosylated casein and whey protein were identified by Periodate-schiff alkali dyeing and Morse test.Determination of free-amino acid casein and whey protein by Nihydrin colorimetry.The effects of different heat treatments on the microscopic characteristics(color value and surface micromorphology)of milk particles were analyzed.The results showed that the red values(a*)and yellow values(b*)were significantly increased after every heat treatment(P<0.05),while the brightness values(L*)decreased significantly(P<0.05).High temperature sterilization devastated the surface net structure worse than any others.The effects of different heat treatments on physicochemical properties(functional groups,secondary structure,molecular weight,glycosylation and hydrolysis degree)of casein were analyzed.The results showed that 1 and 3 new characteristic peaks were found in the infrared spectra of casein from high temperature sterilization and ultra-high temperature instantaneous sterilization,respectively.The content of a-helix structure of casein decreased via heat treatments,contrary to that of ?-antiparallel,?-parallel,?-turn and the random coil.The degree of hydrolysis of casein by high temperature sterilization and ultra-high temperature instantaneous sterilization was significantly lower than that of other treatments(P<0.05).The molecular weight of 36?30?18?14?10 KDa were glycosylated casein by Periodate-schiff alkali dyeing and Morse test.The heat treatment temperature is the key factor for functional groups of casein.Both of temperature and time are the critical factors for the color values and micromorphology of milk particles,the secondary structure,and hydrolysis degree of casein.There are glycosylated casein in milk,which molecular weight are 36?30?18?14?10 KDa,respectively.The effects of different heat treatments on physicochemical properties(functional groups,secondary structure,molecular weight,glycosylation and hydrolysis degree)of casein were analyzed.The results showed that there was no obvious change in the whey protein functional groups after heat treatments.The content of a-helix structure of casein increased via heat treatments,contrary to that of ?-antiparallel,?-parallel,?-turn and the random coil.The results showed that the content of free amino acids decreased significantly after heat treatments(P<0.05).The molecular weight of 18 KDa was a glycosylated whey protein by Periodate-schiff alkali dyeing and Morse experiment.Heat treatments have no effect on whey protein functional groups,but will change the secondary structure and reduce the degree of hydrolysis.There is glycosylated whey protein in milk,which molecular weight is 18 KDa.