Study On Thermal Stability Of Goat Milk,Structure And Functional Properties Of Protein Induced By PH And Heat

In this paper,Laoshan dairy goat milk was used as experimental material.The thermal solidification time of goat milk and homogeneous goat milk under different pH conditions was analyzed to evaluate its thermal stability.Compared with milk and homogenized milk,the difference of thermal stability of milk from different milk sources were discussed.The heat-induced acidification of goat milk during heating was determined.The effects of different pH-heat induction conditions on the structure and functional properties of goat milk protein were systematically studied.The purpose of this study is to provide theoretical basis for optimizing the processing conditions of goat milk and the application of goat milk protein.The results of the thermal stability of milk showed that the goat milk of Laoshan milk goat belonged to B type milk.The heat stability and pH value of the goat milk homogenized milk were positively correlated,and the heat stability of the homogeneous goat milk was higher than that of the fresh goat milk.Milk was of A type milk,the most stable pH of fresh milk was 6.9,while the most stable pH of homogenized milk was 6.7.During the 140 ℃ heat treatment,the pH of goat milk gradually decreased with the extension of heat treatment time.The titratable acidity of goat milk increased with the prolongation of heat treatment time.DSC results showed that the thermal denaturation temperature of milk protein decreased significantly with the increase of pH(P < 0.05),and the thermal denaturation temperature of the fresh milk protein was 124℃,and the thermal denaturation temperature of the milk protein at pH7.7 was the lowest at 115℃.It was found that the hydrophobicity of protein from goat milk heated by 85℃ was of the highest under the condition of weak acid(pH6.5)or weak alkaline(pH7.3 and 7.7).The average particle size of protein from goat milk of different pH decreased significantly at the temperature of 85 ℃(P<0.05),as the minmum value.The zeta potential of the goat milk protein solution after adjusting the pH of milk was the lowest at 85°C.These results indicate that the pH change of goat milk was more unstable due to the temperature change,and 85°C was the key temperature for milk protein after pH of milk changes.When the pH of the milk was alkaline(pH 7.3 and 7.7),the turbidity of the goat milk protein at each temperature was lower than that of the milk pH(pH 6.5 and 6.9).The secondary structure of goat milk protein is mainly composed of β-sheets.After the goat milk was induced by different pH-heat conditions,the regular structure in the secondary structure of the goat milk decreased to varying degrees.When the raw milk’s heat treatment temperature reaches 85°C,the regular structure content is significantly reduced.The SEM results showed that changes in the pH and temperature of the goat milk could directly lead to a corresponding change in the microstructure of the milk protein.If the temperature or pH was too high,the milk protein showed an increasingly unstable trend.The milk particles of the goat’s milk were partially agglomerated together,with uneven size and irregular shape.Through the study of the functional properties of goat milk proteins by pH and temperature,it was found that milk pH 6.5 was the optimal pH for the emulsification activity of goat milk proteins,and the alkaline pH was the inferior pH.However,the emulsion stability and emulsifying activity of the goat milk protein were just inversely related.When the pH of the goat milk changed,the foaming ability of the milk protein was reduced to different degrees when the heat treatment was performed at different temperatures.The pH value of the unmodified pH of fresh milk protein was inversely proportional to the foam stability,and the change in the pH of the goat milk improves the foaming properties of the milk protein at different temperatures.When the pH of the milk was acidic(pH6.5 and 6.9),goat milk protein not only had good foaming ability but also had good foam stability when goat milk treated at different temperatures.The milk had lower water holding capacity under weak alkaline condition,while higher under water holding capacity acidic conditions.The oil holding capacity of goat milk protein under the milk pH of neutral was better than that of other pH conditions.