How does a mixture thicken when milk is added?

Milk contains proteins called caseins, which are responsible for thickening the mixture when milk is added. Caseins exist in the form of tiny suspended particles dispersed uniformly in the liquid, contributing to the even consistency and texture of milk.

When milk is added to a mixture, particularly sauces or soups, the heat causes the casein proteins to undergo several changes:

1. Denaturation: The heat denatures the casein proteins, causing them to lose their solubility in water. As the milk reaches a certain temperature, the casein molecules change their structure, exposing more reactive sites on the surface. This structural change is what allows them to interact and bind with other components in the mixture.

2. Aggregation: After denaturation, the exposed reactive sites on the casein molecules allow them to aggregate or clump together, forming larger and more complex protein structures. These aggregates create a network or mesh-like structure within the mixture.

3. Network Formation: The casein aggregates further interact and form a continuous network or matrix throughout the mixture. This network traps water molecules and other components within its structure, leading to the thickening and increased viscosity of the mixture.

In essence, the denaturation, aggregation, and network formation of casein proteins in response to heat is responsible for the thickening effect observed when milk is added to a mixture.