Casein occurs in fresh milk in the form of a:

Casein is a type of protein found in milk that plays a crucial role in its structure and nutritional value. In fresh milk, casein exists as a colloidal solution. This means it is dispersed in liquid but not dissolved in the way that sugar or salt would be. The colloidal nature of casein allows for tiny particles to remain suspended within the liquid, giving milk its characteristic white color and creamy texture.

When milk is homogenized, the protein molecules become finely dispersed throughout the liquid, creating a stable colloid. This is distinct from a solid solution, where one substance is completely dissolved in another, or a suspension, where larger particles are mixed in but can eventually settle out, and an aqueous solution, which involves solutes being fully dissolved in water. The stability and properties of casein in the colloidal form are essential for the digestion and absorption of milk proteins in the human body.

Understanding this characteristic of casein is important in fields such as food science and public health, where the nutritional aspects of milk and dairy products are often evaluated.