This section is from the book "Experimental Cookery From The Chemical And Physical Standpoint", by Belle Lowe. Also available from Amazon: Experimental cookery.
The physical and chemical properties of the constituents of milk account for the behavior of milk during its use in food preparation. Thus substances that lower surface tension become concentrated in the liquid/air interface. Proteins lower the surface tension of aqueous sols, hence accumulate in the surface. When milk is heated in an open pan, a scum or skin forms over the surface of the milk. At first this skin is rather thin and mobile but is gradually altered so that it becomes tenacious and tough enough to be removed with a stirring rod or spoon. This scum has been said to contain coagulated albumin and globulin. Tinkler and Masters state that if the scum is removed as formed, the total amount of protein that can be removed exceeds the total amount of albumin and globulin in the milk.
When foods are cooked in milk the milk not only foams readily but the scum tends to hold the steam formed in heating the milk; it is because of this that the milk "boils over" so readily.