Milk is a suspension of tiny fat droplets in water. The droplets are coated with a thin layer of protein which helps it to remain suspended in the water. The water in milk also contains protein, sugar (lactose), mineral salts and water-soluble vitamins. The average size of a fat globules is about four micometers across.

The largest structure in the fluid portion of milk are casein protein micelles (several thousand protein molecules bonded together in a roughly spherical shape of about a tenth of a micrometer across). There are four different types of casein proteins in milk and they make up 80 percent of the protein in milk by weight.