Answer:  

Omu Milk Products Co.,Ltd. kindly answered the above question.
 
 
Milk is usually pasteurized at 120°C for 15 seconds. As there is very little change in the milk proteins (as denaturation and precipitation barely happen) when milk is pasteurized at the above condition, no special treatment is required.
  
Milk protein consists of two major proteins: casein (~ 80%) and whey protein. Casein is very stable in the presence of heat, while whey protein is unstable in the presence of heat. Therefore, it is whey protein that is denatured when milk is heated during pasteurization. The whey protein is coagulated by heat and forms tiny particles. Since the amount of the tiny particles is very small, precipitation barely happens. 
 
The denaturation of whey protein affects the whiteness of milk. Milk becomes whiter after being pasteurized. The increased whiteness of milk is due to the change in its refractive index caused by the denaturation of whey protein.
 
Additional information regarding milk protein:
What is the white membrane which forms on hot milk?
Usually, when milk is heated at above 40°C, a white membrane forms on the top of the milk. This is called the Ramsden effect. When milk is heated, water evaporates at the surface of the milk causing irreversible protein precipitation. It is also caused by the milk fats that adhere to the protein precipitation. The membrane seems to consist of more than 70% fat and 20 – 25 % protein most of which is lactalbumin (whey protein). The membrane has a rich taste and is also high in nutrition because it contains fat and protein.
 
Why does milk seem to coagulate sometimes when it is added to soup?
Milk sometimes coagulates by heat or acid. In both cases, coagulation is caused by the denaturation of milk protein. When the milk is heated to very high temperature during cooking, the denaturation of whey protein may increase resulting in coagulation. In addition, when milk is headed at high temperature for a long period, lactose may break down, generating lactic acid. This change can increase the acidity and may change the color to brown because of separating milk fat. The milk fat will float on the top, and the milk will become unstable causing coagulation and/or precipitation.
  
For example, when we use milk for cooking food with low pH such as tomato soup, the milk protein (casein) is sometimes denatured by the acid in the food. Therefore, in rare cases milk coagulates when it is added to soup.
 
 
Acknowledgement
We would like to thank Omu Milk Products Co.,Ltd. for his kind answer. 
 
 

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