Studies on Rennet and Rennin

Carl Anthon Ernstrom, University of Wisconsin


Rennet is a liquid, paste, or powder preparation containing the enzyme, rennin. It is usually prepared by extracting the fourth stomach (abomasum) of milk-fed calves with a sodium chloride solution, and is used in the manufacture of cheese, rennet casein, and various specialty products such as junket or rennet custard. Rennet is probably one of our oldest commercially-used enzyme preparations. Several hundred-thousand gallons of extract are produced each year in addition to large amounts of paste and powder. In spite of its long and extensive use, many properties of rennet, as well as rennin itself, are not fully understood. The milk-clotting action of rennet has been known for centuries, but the nature of this action has never been satisfactorily explained. Many theories of rennin action have been advanced, but none have been soundly established by experimental evidence. Much of the material on rennin in the literature is contradictory.

Because of the complexity of milk and the complexity of the clotting reaction, no completely satisfactory method has yet been devised for measuring rennin activity and studying the influence of pH, temperature, and possible activators and inhibitors on the activity of the enzyme itself. While it is known that calcium ions and other di- or trivalent cations are essential for the clotting of milk in the presence of rennin, the function of the ions in clot formation has never been demonstrated. Rennin has some proteolytic activity, but its importance in the proteolytic breakdown of cheese during ripening has been questioned from time to time. These are but a few of the unsolved problems which stand in our way of a better understanding of the nature of rennin and the role it plays in the cheese-making operation.