Learn the simple reason why a skin forms on top of milk when it's left in an open mug in the fridge. Find out how to easily prevent it.
1 Answer
Milk develops skin when stored in open mugs due to the evaporation of water and the denaturation of proteins at the surface, which then interact to form a thin film.
Detailed Explanation:
The 'skin' that forms on milk left in an open mug is a complex phenomenon involving several factors. Firstly, water evaporates from the surface of the milk, concentrating the proteins and fats present. Secondly, proteins, particularly casein and whey proteins, denature at the air-liquid interface. Denaturation means the proteins unfold and change their structure. This unfolding allows them to interact with each other and with other components of the milk, such as calcium and fats. These interactions create a network or film on the surface. The higher the fat content of the milk, the thicker the skin tends to be. The process is accelerated by warmer temperatures, which increase both evaporation and protein denaturation. Leaving milk uncovered exposes it to air, facilitating evaporation and allowing the protein interactions to occur freely, leading to the formation of the noticeable skin.
Pro Tip:
To prevent milk skin from forming, cover the mug tightly with plastic wrap or a lid. This minimizes evaporation and reduces the surface exposure to air, slowing down the protein denaturation process.