Lehmann L, Buckin V
Department of Chemistry, University College Dublin, Belfield, Dublin 4.
J Dairy Sci. 2005 Sep;88(9):3121-9. doi: 10.3168/jds.S0022-0302(05)72994-5.
In the present work, we used high resolution ultrasonic spectroscopy for the analysis of heat coagulation process in milks. Two skim milks recombined from powder samples differing by their preheating treatments as well as dairy ingredients (phosphocasein, whey protein isolate, and a milk model) were monitored while submitted to a temperature of 120 degrees C. Three stages in the precoagulation and coagulation processes can be distinguished in the ultrasonic velocity and attenuation profiles. The first stage shows a very sharp decrease in ultrasonic velocity and increase in ultrasonic attenuation. This could be attributed to fast denaturation and aggregation of whey proteins and precipitation of calcium phosphate. In the second stage, small changes in ultrasonic velocity are observed, the rate of which depends on the nature and pH of the samples. In the third stage, a sharp decrease in ultrasonic velocity and attenuation is recorded for all samples, which corresponds to the coagulation and formation of the gel network. After coagulation, a very small change in ultrasonic velocity and attenuation was observed. From the ultrasonic measurements, we determined the heat stability vs. pH profiles of the different samples studied. The observed ultrasonic attenuation profiles were also interpreted in terms of changes in the size of casein micelles and coagulation processes.
在本研究中，我们使用高分辨率超声光谱法分析牛奶中的热凝固过程。对两种由预热处理不同的粉末样品重新组合而成的脱脂牛奶以及乳制品成分（磷酸酪蛋白、乳清蛋白分离物和一种牛奶模型）在120℃的温度下进行监测。在超声速度和衰减曲线中，可以区分出预凝固和凝固过程的三个阶段。第一阶段显示超声速度急剧下降，超声衰减增加。这可能归因于乳清蛋白的快速变性和聚集以及磷酸钙的沉淀。在第二阶段，观察到超声速度有微小变化，其变化速率取决于样品的性质和pH值。在第三阶段，所有样品的超声速度和衰减都急剧下降，这对应于凝胶网络的凝固和形成。凝固后，观察到超声速度和衰减有非常小的变化。通过超声测量，我们确定了所研究的不同样品的热稳定性与pH值的关系曲线。观察到的超声衰减曲线也根据酪蛋白胶粒大小的变化和凝固过程进行了解释。
