Why Does Milk Form a Skin When it is Heated?

After you’ve heated a glass of milk or hot chocolate, sometimes the milk forms a skin on top of the liquid. The skin is comprised of solid proteins that combine with the milk’s fat molecules, which begin to evaporate as the milk is heated. These proteins, casein and beta, clump together when the liquid reaches a temperature of around 113 to 122 degrees Fahrenheit (45 to 50 Celsius). As the heating continues, the soft protein layer begins to dry out, which is why the milk forms a skin on the liquid’s surface. This layer of skin forms a hard barrier, causing steam to build up beneath it and increase the liquid’s temperature. When left alone, this often causes the milk to boil over.