Why does a skin form on the surface of heated milk?

The formation of skin on heated milk is primarily due to the Maillard reaction, which is a chemical reaction between reducing sugars and amino acids. Here is a more detailed explanation of the process:

1. Protein Denaturation: When milk is heated, the proteins present in the milk (mainly casein) start to denature. This means their molecular structure changes, causing the proteins to unfold and break down into smaller molecules, including peptides and amino acids.

2. Lactose Browning: As milk continues to heat, the lactose (a natural milk sugar) undergoes caramelization, which is the browning reaction of sugars. Lactose breaks down into smaller sugars, such as glucose and galactose, which then undergo a series of chemical reactions to produce a variety of compounds with a brown color and distinctive flavors.

3. Maillard Reaction: The Maillard reaction occurs when the free amino acids produced by protein denaturation and the reducing sugars (such as glucose and galactose from lactose breakdown) interact. This reaction is catalyzed by heat and involves a complex series of chemical changes. It results in the formation of various compounds responsible for the characteristic color and flavor of heated milk.

The Maillard reaction products include:

- Browning pigments: These are brown-colored compounds responsible for the formation of the skin on the surface of heated milk.

- Flavor compounds: These compounds contribute to the toasty, nutty, and caramel-like flavors associated with heated milk.

4. Evaporation and Fat Accumulation: As the milk continues to heat, some of the water evaporates, concentrating the remaining milk solids. This leads to the accumulation of fat and protein particles at the surface, further contributing to the formation of the skin.

5. Cooling and Gel Formation: Upon cooling, the heated milk proteins, along with the fat particles and Maillard reaction products, form a network that traps air bubbles near the surface. This creates a gel-like layer, commonly referred to as the "skin" on heated milk.

The skin on heated milk can vary in thickness and texture depending on the heating temperature and duration. It is also influenced by the composition of the milk, particularly its protein and lactose content.