268 CHURNING 



Permanency of Fat-in-Skimmilk Emulsion. This emul- 

 sion, established by the secretion of the milk, is fairly per- 

 manent. The question here consistently arises: What causes 

 these fat globules to remain divided, what hinders them from 

 running together like oil? 



Earlier investigators claimed that each fat globule was 

 surrounded by a definite membrane. This was a mere assump- 

 tion which proved erroneous. The presence of such a membrane 

 has never been satisfactorily established. Later study failed to 

 demonstrate .its existence and yielded substantial evidence that 

 such a membrane does not exist. The only envelope that sur- 

 rounds the fat globules in milk is the skim milk in which they 

 are suspended. 



The forces that make it possible for the minute fat globules 

 to retain their identity as single units and that prevent them from 

 running together, are the difference in the surface tension be- 

 tween the fat globules and the skim milk, adsorption and the 

 viscosity of the milk. 



Surface Tension. The fat globules stay apart, they retain 

 'their individuality, they do not run together, primarily because 

 of the law of surface tension. By surface tension is under- 

 stood the attraction which the molecules of one and the same 

 substance have for each other. The molecules which are located 

 in the surface of a liquid, are attracted toward the interior of 

 the liquid by the molecules situated there, but there is no 

 similar attraction towards the exterior, because the molecules in 

 the interior of a liquid* are subject to .attraction from all sides. 

 The tension thus produced on the surface by this molecular at- 

 traction towards the interior is called the surface tension. It 

 obviously conveys to the surface the tendency to become re- 

 duced to the smallest possible dimensions. Hence, if a liquid 

 is placed in a position, where it is not affected by gravity, the 

 form of the liquid changes until it assumes the smallest possible 

 surface for a given volume. And this is the sphere. 



In the case of milk, in the secretion of which nature has 

 placed the fat in finely divided particles, the fat possesses a greater 

 surface tension than the surrounding skim milk. The minute, 



