GENERAL CHEMISTRY OF MILK 83 



Casein is precipitated by dilute acids. When small quanti- 

 ties of acid are used the precipitate is fine and flaky, while 

 sufficiently large amounts of acid produce a more or less compact 

 curd. Usually this phenomenon is explained by the combination 

 of acid with the calcium which takes place, forming salts, thus 

 rendering the casein insoluble. The process, however, is probably 

 not so simple. 



In the ordinary souring of milk the acid is chiefly lactic acid 

 and is produced by bacterial action. This acid causes precipita- 

 tion of the casein; a coagulum or curd is formed. The process 

 takes place in two phases : the first phase is the Uberation of casein 

 after the calcium has combined with the acid; the second phase 

 consists in actual combination of the acid with the casein. Thus, 

 finally, the curd is formed. When milk is heated less acid is re- 

 quired for its precipitation than at low temperature. 



Casein is prepared on a large scale commercially for various 

 purposes. The commercial uses of casein are given by Allen as 

 follows: 



"1. Patent foods, such as Sanatogen. 



"2. For administering medicinal agents, salicylates, alkaloids, 

 lithium, mercury, silver, iron, bismuth, and others. 



"3. Cheese. 



"In the arts casein is used — 



"1. In painting materials. 



"2. Adhesives. 



"3. Plaster materials, as substitutes for horn, bone, and in mak- 

 ing rods, picture frames, etc. 



"4. As a medium for mixing colors in textiles. 



"5. For waterproofing colored papers, art papers, drawing 

 papers, cartridge cases, cardboard, etc. 



"6. Mixed with asbestos paper to form waterproof and fire- 

 proof materials. 



"7. Miscellaneous uses, such as paint removers, shoe polishes, 

 photographic plates, roofing pulp, glazing for inside of casks, 

 preparation of artists' canvasses, solidifying mineral oils, billiard 

 balls, etc." 



The preparation of casein requires considerable care in order 

 to precipitate all the casein and not lose part of it by an insufficient 

 amount or excess of acid. It is advantageous to make a trial 

 experiment before a large amount of milk is coagulated. A pre- 

 liminary experiment is made as follows: Ten flasks are filled with 

 20 c.c. of skimmed milk each, and then 0.01 N. acetic acid is added 

 in rising quantity to each flask. The first flask may receive 

 100 c.c. of the acid; the second flask, 120 c.c, etc. The precipi- 

 tate is filtered off, and the flask which leaves a water-clear filtrate 



