Casein contains a high number of proline residues that do not interact. There are also no disulfide bridges. As a result, casein has little tertiary structure. It is relatively hydrophobic, making it poorly soluble in water. Casein is found in milk as a suspension of particles, called casein micelles, which show only limited resemblance with surfactant-type micelles in a sense that the hydrophilic parts reside at the surface and they are spherical. However, in sharp contrast to surfactant micelles, a casein micelle interior is highly hydrated. The caseins in the micelles are held together by calcium ions and hydrophobic interactions. All molecular models consider micelles as colloidal particles formed by casein aggregates wrapped in soluble κ-casein molecules.

Casein is used in the manufacture of paint, glue, cheese, plastics, fiber, and protein supplements, as well as tooth remineralization products in dentistry.