This is a substance in milk which has a high value as a protein. The main protein in milk is caseinogen, which is turned
to casein by rennin, a substance found in rennet, and by acids. The casein is found in clots produced by the action of the rennin.
In most methods of cheese-making the milk is clotted with rennet and the casein formed in the clots traps the milk fats.
The cheese, therefore, contains the protein and the fat of the milk.
Casein produced by milk in the stomach reduces the acidity of the stomach, and it is probably this that makes milk such a valuable food to people with gastric ulcers.