Heating Milk

When milk is heated to near the boiling point, the whey proteins lactalbumin and lactoglobulin become insoluble, mesh with the milk’s calcium phosphate, and precipitate, forming a film on the bottom and sides of the pan. This film can scorch easily. Scorching can be prevented by constant stirring, slow temperature increases, or use of a double boiler.

The skin that forms on the surface of the milk during heating is caused by the evaporation of water, which is accompanied by an increased concentration of casein, fat, and mineral salts. This thin skin also scorches easily; in addition, it can trap steam that is trying to escape and cause the milk to boil over.

Several steps can be taken to avoid this problem, including using a lid, continual stirring during heating, floating a small pat of butter on top of the milk, or, in the case of hot chocolate, adding whipped cream or marshmallows. Casein will not coagulate with heat unless it is boiled for long periods of time. Canned evaporated milk, however, which contains higher concentrations of casein, may coagulate during the high heats of sterilization. This is prevented by warming the milk prior to sterilization.