90 DAIRY CHEMISTRY 



the casein and fat, and whey which contains the albu- 

 min and milk sugar. If the clear whey is heated, 

 a yellowish white, flakelike substance is formed. 

 This is coagulated albumin. The albumin in milk 

 is coagulated at temperatures ranging from 157° to 

 161° F., but in the ordinary processes of cheese 

 making the temperatures reached are not suffi- 

 ciently high for the coagulation of the albumin, 

 and as a result it remains in the whey. If higher 

 temperatures were attempted, excessive losses of 

 fats would occur, the casein would be unfavor- 

 ably affected, and a poor quality of cheese would 

 be produced. The chief distinctions between ca- 

 sein and albumin are that albumin is coagulated 

 by heat, while casein is not, and casein is coagu- 

 lated by dilute acids, but the albumin is not pre- 

 cipitated. 



85. Rennet. — In order to control the process of 

 fermentation and the coagulation of the milk, ren- 

 net is used. Rennet is an extract prepared from 

 the mucous membrane or lining of the fourth stomach 

 of the calf. It contains various ferment bodies, as 

 lactic and peptic ferments, which are the chief agents 

 in bringing about the changes necessary for the di- 

 gestion of milk. When rennet is added to ripened 

 milk, it produces the same effect as the addition of 

 dilute acid. A temperature of from 86° to 90° is 

 favorable for the action of the rennet ferments. 

 Prior to the addition of the rennet, the milk is 

 ripened with a starter, as in the ripening of cream. 



