Structure Characteristics Of Milk Proteins Complexes With Altered Casein To Whey Protein Ratios

The structure change on milk proteins at different temperature and different pH had already been studied now, but with regard to these conditions, milk protein structure changed by casein to whey protein ratios were seldom investigated. However these changed of milk protein ratios had great impact on its macrostructure physical characteristics, according to our prophase research achievements had known that casein to whey ratio was effect on quality of yoghurt gel, the hardness, viscosity and water-retaining capability and microstructure of yoghurt gel would increase with the decreasing of the ratios of casein to whey. Therefore, the project was amd to reach the changes of surface hydrophobic, free sulfydryl, particles size, microstructure etal with casein to whey protein ratios at different temperature and pH.According to our prophase research achievements, protein content was fixed 4%, casein to whey protein ratios were 4.35:1,3:1,2:1, 1:1 respectively, studied the structure change of milk protein with the ratios of casein to whey protein at different heating temperature, time and pH.The amd of analysisd the change of surface hydrophobic, free sulfydryl, particles size, zeta-potential, free milk protein, microstructure ect. Reached to change of milk protein structure with the ratio of casein to whey protein at the course of fermentation, the change of curd time and the extent of pH decreased.Surface hydrophobic Surface hydrophobic and the ratio of casein to whey protein had a significant correlation at different temperature and different pH. Surface hydrophobic changed with the ratio of casein to whey protein had on significant impact at different pH and at low temperature(≤50℃); Surface hydrophobic increased with the reduceing of casein to whey protein ratio at 68℃, the change of pH had on significant impact. Surface hydrophobic significant increased, the extent of surface hydrophobic increased with the reduceing of casein to whey protein ratio at high temperature (90℃and 100℃). At the same time there was the great effect of pH on surface hydrophobic, the extent of surface hydrophobic increased greatest at acid condition (pH6.2), and the order as follows: pH6.2>pH6.5>pH6.7>pH6.9>pH7.1.Free sulfydryl Free sulfydryl and the ratio of casein to whey protein had a significant correlation at different temperature and different pH. Free sulfydryl changed with the ratio of casein to whey protein had on significant impact at different pH and at low temperature (≤50℃); Free sulfydryl decreased with the reduceing of casein to whey protein ratio at 68℃, the change of pH had on significant impact. Free sulfydryl significant decreased, the extent of free sulfydryl decreased with the reduceing of casein to whey protein ratio at high temperature(90℃and 100℃). At the same time there was the great effect of pH on free sulfydryl, the extent of free sulfydryl decreased greatest on acid condition (pH6.2), and the order as follows: pH6.2>pH6.5>pH6.7>pH6.9>pH7.1.Particles size Particles size and the ratio of casein to whey protein had a significant correlation at different temperature and different pH. Particles size increased with the improving temperature at normal pH(pH6.7), the order as follows: 90℃>68℃>room temperature; Particles size increased with the improving pH at 90℃, the order as follows: pH6.2>pH6.7>pH7.1; At the same time, the particles size significant increased on acid condition (pH6.2), the particles size less increased at pH6.7, the ratio of casein to whey protein had on significant impact on particles at alkalinity condition (pH7.1).Free milk protein contents The free milk protein contents and the ratio of casein to whey protein had a significant correlation at different pH and 90℃. Free milk protein contents reduced with pH reducing, the volume fraction of particles size of free milk proein under 80nm decreased, the order as follows: pH6.2>pH6.7>pH7.1; The free milk protein contents significant decreased on acid condition (pH6.2), the free milk protein contents less decreased at pH6.7, the ratio of casein to whey protein had on significant impact on free milk protein contents at alkalinity condition(pH7.1)Microstructure texture Microstructure texture and the ratio of casein to whey protein had a significant correlation at different temperature and different pH. Polymerization capacity increased with the improving temperature at normal pH(pH6.7), the order as follows: 90℃>68℃>room temperature; Polymerization capacity increased with the improving pH at 90℃, the order as follows: pH6.2>pH6.7>pH7.1; More whey protein and casein micelle polymerization, whey protein formed network structure with the ratio of casein to whey protein decreasing at pH6.2; Whey protein polymerization capacity enhancement with the ratio of casein to whey protein decreasing at pH6.7; The ratio of casein to whey protein had on significant impact on milk protein polymerization capacity at alkalinity condition(pH7.1).Structure changes of milk protein at the process of fermentation The time of curd reduced with the ratio of casein to whey protein decreasing, the extent of pH reduced at the same fermentation time, the extent of surface hydrophobic increased improving ,the extent of free sulfydryl reduced improving. milk protein polymerization capacity improving at same pH.