15 



Action of acids on calcium casein. — Wlien an acid is added to milk, 

 its first action is to unite with certain salts of the milk and then with 

 the calcium of calcium casein, forming; a precipitate of free casein. 

 On further addition the acid unites with the casein, formino; a casein 

 salt of the acid used, as in the case of sour milk, when casein lactate is 

 formed. If a large amount of acid is added to milk, the casein is first 

 coagulated and then dissolved, as in the case of the Babcock test. 



Action of alkalis on casein compounds. — Alkalis, like ammonia, caus- 

 tic soda, carbonate of soda, etc., unite with casein compounds, forming 

 compounds soluble in water. They also dissolve casein coagulated 

 by acids. 



Action of heat on casein compounds. — Heat alone under ordinary 

 conditions, even at the boiling point, coagulates calcium casein little, 

 if any. The skim formed on the surface of milk heated above 140° F. 

 contains practically all the constituents of milk and is primarily due 

 to the presence of calcium casein and not to albumin, as formerly 

 supposed. 



Action of rennet on calcium casein. — Coagulation by rennet in the 

 presence of soluble lime salts is one of the most characteristic prop- 

 erties of calcium casein. The manufacture of cheese from milk 

 depends upon this property. The compound or curd formed by treat- 

 ing milk with rennet is called calcium paracasein, but does not differ 

 much from calcium casein (reference No. 1, p. 30). The coagulation 

 of calcium casein by rennet is entirely different from that produced 

 by acids (reference No. 1, pp. 22 and 30). The properties of rennet 

 and the conditions that affect its power to coagulate calcium casein 

 will be considered in Lecture 6. 



3. Milk fat or butter fat. — Milk fat is a mixture of several different 

 compounds, chief of which are palmitin, olein, myristin, and butyrin. 

 Each of these compounds contains glycerin united with an acid (ref- 

 erence No. 3, p. 13), and each varies in its properties from the others 

 (reference No. 3, p. 14). Milk fat in pure, fresh condition is a soft 

 whitish to yellowish mass, of mild taste and only slight odor. It melts 

 easily and is lighter than water. Milk fat is present in milk in the 

 form of minute, transparent globules, averaging about one ten-thou- 

 sandth of an inch in diameter (reference No. 3, p. 14). The fat 

 globules are not surrounded by any kind of special covering (reference 

 No. 3, p. 14), but are simply minute particles of fat floating free in. 

 the form of an emulsion. 



The amount of fat in milk is affected by many conditions, among 

 which are individuality, breed, age, stage of lactation, time of milk- 

 ing, and so forth (reference No. 3, pp. 15-17). The direct relation of 

 milk fat to cheese yield will be considered later. 



4. Milk sugar. — Milk sugar is present in cow's milk in solution. Its 

 importance in relation to cheese making is due to the readiness with 



