Rennin is not a digestant, but is the milk-coagulating ferment of the gastric juice. It is obtained from the mucous membrane of the fourth stomach of the calf. Under its influence the case-inogen of milk changes to paracasein, and the latter takes calcium and forms an insoluble curd. The calcium is usually furnished by the calcium phosphate of the milk, but occasionally must be supplied by the addition of a small amount of calcium chloride or lime-water. The ordinary rennin curd contains 13 per cent. more calcium than the curd of hydrochloric acid (Harris), and is tougher and more cohesive, though less dense and more readily acted upon by pepsin. If the stomach contents are highly acid or more than very slightly alkaline, the rennet will not act. Hence if sodium bicarbonate or more than a very little lime-water is added to milk, no coagulation takes place at all; and in marked cases of hyperacidity the curd formed is the dense hydrochloric acid curd and not that of rennet. Its action is retarded by agitation unless in the presence of hydrochloric acid (Bernegau). It has been found to coagulate from 5000 to 166,000 times its weight of milk.

The use of rennet in medical practice is to prepare junket and whey. Junket is the whole coagulated milk, and is a valuable food for invalids. It is prepared by adding the commercial liquid rennet, or essence of pepsin, or junket tablets dissolved in water, to barely warm milk, and setting aside till the clotting takes place. The process is retarded if the milk is hot. The junket may be eaten plain or with cream and sugar; it may be flavored with sherry, nutmeg, etc.

Whey is the liquid portion of the milk after the rennet curd is removed. It is obtained by breaking up the junket and straining through cheese-cloth or linen. It contains some of the rennin ferment, a small amount of soluble protein (lactalbumin), a slight amount of fat, about 4 per cent. of milk-sugar, and the salts of the milk with the exception of the calcium phosphate. It is used as a nearly protein-free diluent of milk in infant feeding. Before it is added to milk it should be brought to the boiling-point to destroy the rennin; otherwise it will coagulate the new milk.

Rennet is used very extensively in cheese-making and in the preparation of junket for the table.