"Calcium does not cause milk to clot under normal circumstances.
However, during digestion of milk in the stomach of infants, calcium is required for the precipitation of milk proteins, causing milk to curdle so it stays in the infants stomach long enough for digestion. The amount of calcium in milk is far in excess for this precipitation to take place.
This process is mediated by the enzyme Rennin (or chymosin), secreted in the stomach of infants. Most milk proteins are caesins, which have four major forms: alpha-s1, alpha-s2, beta and kappa caesins. Alpha and beta caesins, when left on their own would precipitate in the presence of calcium. However, when associated with unprecipitable kappa caesins, also present in milk, they form micelles which remain disolved.
Rennin digests kappa caesin, preventing it from aggregating the alpha and beta caesins into soluble micelles, effectively causing them to precipitate out naturally. And this precipitation requires calcium ions.
This same theory can be applied to the making of dairy products such as cheese, which also use enzymes similar to rennin to curdle milk."
