SUMMARY 351 



three of which are soluble in water and volatile in steam. 

 It is liquid at the animal temperature, but solid at ordinary 

 temperature. The minute fat globules occur in milk in the 

 form of a true emulsion. Milk protein consists principally 

 of caseinogen, which contains sulphur and phosphorus in 

 addition to the usual protein elements. Caseinogen exists 

 in milk combined with a calcium compound, probably calcium 

 phosphate. It is precipitated by rennin, an enzyme of the 

 gastric juice, by being split into two different soluble pro- 

 teins, one of which unites with calcium phosphate to form 

 the insoluble curd. Acids coagulate casein by removing the 

 calcium compound and precipitating the protein. The other 

 protein is lactalbumin which contains sulphur but no phos- 

 phorus. It is coagulated by heat. Inorganic salts are 

 present in milk, principally chlorides and phosphates, with 

 some citrates. Milk is secreted in certain glands of the 

 animal especially adapted for the purpose, and some of its 

 constituents such as lactose and caseinogen are not found 

 elsewhere in the animal body. 



Milk is subject to some adulteration, principally with 

 water, but since this reduces the percentage of fat in milk 

 the consumer is protected by law, which in most States 

 specifies the minimum amount of fat allowable. Since milk 

 is an excellent nutrient for bacteria, great care must be 

 taken in keeping and handling the product. Immediate 

 cooling and keeping cold serves to a considerable extent 

 to prevent the action of bacteria, but still further protection 

 is assured by pasteurization which consists in heating the 

 milk to 60° or 80° C, for twenty minutes and cooling. This 

 destroys most of the bacteria and does not interfere with 

 the taste and flavor of milk. Sterilization consists in heating 

 the milk to 115° C, a process which kills all bacteria, but 

 which interferes with the taste and flavor of the milk. Various 

 chemicals, such as formaldehyde and boric acid, which kill 

 bacteria, are unfit for use because of their harmful effect 

 on man. 



Of the various milk products, cream is an important one, 

 consisting principally of fat which has been allowed to rise 

 and is skimmed off, or separated by centrifugal force. It is 



