Curious about the skin that forms on top of milk when you heat it? Learn the simple science behind what's happening to the proteins and fats.
1 Answer
Milk skin forms when milk is heated because the proteins in the milk denature and coagulate, combining with calcium and fat to create a film on the surface. This is exacerbated by evaporation at the surface.
Detailed Explanation:
When milk is heated, several things happen that contribute to skin formation. First, the proteins in milk, primarily casein and whey proteins, begin to denature. Denaturing means that the proteins unfold from their natural three-dimensional structure. This unfolding exposes hydrophobic (water-repelling) regions of the protein molecules.
These denatured proteins then aggregate or clump together. They also interact with other components of the milk, such as calcium and fat. The fat molecules, being lighter, tend to rise to the surface. The denatured proteins, calcium, and fat combine to form a film or skin on the surface of the milk.
Evaporation also plays a role. As the milk heats, water evaporates from the surface, concentrating the proteins and other solids. This increased concentration further promotes the formation of the skin. The skin acts as a barrier, slowing down further evaporation, but also becoming thicker and more noticeable over time.
Pro Tip:
To prevent milk skin from forming, try stirring the milk frequently while heating, or cover the milk with a lid to reduce evaporation. Alternatively, heat the milk gently over low heat, as higher temperatures accelerate protein denaturation.