Casein is the largest protein in cow’s milk by quantity, making up about four fifths of the total milk protein, while the rest is whey. When milk is acidified or treated with rennet, casein precipitates and forms the solid basis for quark and many types of cheese.
In practice, casein is often sold as a protein powder in its own right, because it forms a gel-like structure in the stomach and is therefore digested much more slowly than whey. For this reason, casein is occasionally mentioned in sports science for taking before longer gaps between meals, such as before sleep, although no blanket recommendation should be derived from this.
The entry Whey protein explains more about its counterpart whey, the Casein guide offers a detailed comparison, and the guide Protein timing covers the timing of protein intake in general.
