Casein is the main protein found in milk. When milk curdles or separates into thick and thin parts, the thick white chunks are mostly casein. This protein helps give milk its creamy color and helps your body build strong muscles and bones.

Casein has remarkable properties that make it useful far beyond nutrition. When treated with chemicals, it becomes incredibly strong and can be molded into hard objects. Before plastics were invented, manufacturers used casein to make buttons, knitting needles, fountain pens, and even jewelry. Some violin makers still use casein glue because it creates such a strong bond. The protein also works as a powerful adhesive in wood glue and appears in many paints because it helps colors stick to surfaces.

In your own body, casein digests slowly, which means it releases nutrients gradually over several hours. Athletes sometimes drink milk before bed because the casein keeps feeding their muscles while they sleep. Cheese is packed with casein, which explains why it melts and stretches the way it does when heated.

People with milk allergies often react specifically to casein, which is why they must avoid not just milk but also cheese, yogurt, and many processed foods that contain milk proteins.