Casein

Casein is a family of phosphoproteins that are the major proteins in milk. In cow milk, caseins account for roughly 80% of the total protein and include several types, mainly αs1-casein, αs2-casein, β-casein, and κ-casein. In milk, caseins exist as large colloidal aggregates called casein micelles, which are stabilized on the surface by κ-casein and internally linked by colloidal calcium phosphate.

Biological role and properties: Casein micelles serve as a reservoir of calcium and phosphate and supply amino acids to offspring. The micelles are relatively stable at milk pH and ambient conditions but can be made to coagulate by acidification or by the enzyme rennet (rennin), which cleaves κ-casein and disrupts micelle stability. This coagulation forms curds, a key step in cheese production, while whey proteins remain soluble.

Nutrition and health: Caseins are slow-digesting proteins, providing a gradual release of amino acids. They are a good source of essential amino acids, though digestibility can vary with processing and individual digestion. Some people have dairy allergies to casein or experience intolerance, separate from lactose intolerance.

Industrial and culinary uses: In food processing, casein and its salts (caseinates, such as sodium caseinate and calcium caseinate) are used as emulsifiers, stabilizers, and binding agents in a wide range of products. Dried casein powder can be reconstituted for use in foods. Outside of food, casein has historically been used as a binder in paints and adhesives and has been explored as a vintage plastic material.