146 SCIENCE AND PRACTICE OF CHEESE-MAKING 



Some of these salts formed by casein with aJ.kaiis 

 are found in commerce in the form of dietetic and 

 medicinal preparations. 



Action of salts on milk-casein. — Milk-casein may 

 be precipitated, apparently unchanged chemically, 

 by saturating milk with common salt, magnesium 

 sulphate, ammonium sulphate, etc., at ordinary tem- 

 peratures. Milk-casein is also precipitated by small 

 amounts of solution of alum, zinc sulphate ai/d 

 many other metallic salts. Calcium chlorid and 

 some other salts coagulate casein in milk heated to 

 95°-ii3° F. 



Action of heat on milk-casein. — Heat alone under 

 ordinary conditions, even at the boiling point of 

 water, does not coagulate the casein in milk. How- 

 ever, heated under pressure to 26^°-2S^° F., casein 

 salts are changed in their properties and casein itself 

 is coagulated. The browning of milk heated under 

 pressure is more or less due to changes in casein. 

 The formation of a peculiar skin (haptogen mem- 

 brane) on milk heated above 140° F. is largely due 

 to the calcium casein of the milk and not, as was 

 formerly supposed, to albumin. The skin itself 

 contains practically all of the constitutents of the 

 milk and may be regarded as a kind of evaporated 

 milk. On removing the membrane, a new layer is 

 formed and, by removing these one after another, 

 practically all of the milk can be transformed into 

 the membrane condition. It appears to be due to 

 surface evaporation. 



Action of rennet on milk-casein. — One of the 

 most characteristic properties of milk-casein is its 

 coagulation by the enzym or chemical ferment con- 



