124 PHYSIOLOGICAL CHEMISTRY. 



albumen, the caseine remaining liquid. The addition of any acid, how- 

 ever, such as acetic or tartaric acid, will precipitate the caseine and 

 curdle the milk. If milk be allowed to remain exposed to the air at a 

 moderately warm temperature, it curdles spontaneously, owing to the 

 development of lactic acid, from transformation of its sugar ; and the 

 same change will occur instantaneously from electric disturbance, during 

 a thunder-storm. 



The caseine of milk, artificially coagulated by the action of rennet, 

 constitutes cheese. Rennet is the dried contents and mucous membrane 

 of the stomach of the calf, the animal being killed and the stomach 

 taken out while digestion is in full activity and the gastric fluids abun- 

 dantly secreted. An infusion of this substance even in small quantity, 

 added to fresh milk at the temperature of 30 C. produces coagulation 

 in fifteen or twenty minutes. The coagulum is drained from the watery 

 serum or " whey," and afterward pressed into the form of cheese. The 

 variety in consistency and flavor of different cheeses depends mainly 

 on the proportion of fatty matter retained in the coagulum, and on 

 certain slow changes, in the nature of fermentations, which go on in it 

 subsequently. 



The fatty matter of milk is suspended in its serous portion under the 

 form of minute spheroidal masses. These masses or " milk-globules " 

 are not quite fluid at ordinary temperatures, but have a semi-solid con- 

 sistency owing to their containing a considerable proportion of palmitine. 

 The fat globules, separated by churning from the other ingredients of 

 the milk, and united into a coherent mass, constitute butter. This sub- 

 stance, accordingly, represents the oleaginous ingredients of the milk ; 

 and when purified from the watery portions entangled with it, consists 

 mainly of palmitine and oleine, with certain flavoring ingredients, the 

 principal of which has received the name of "butyrine." These sub- 

 stances are usually mingled in the following proportions : 



Palmitine 68 parts. 



Oleine 30 " 



Butyrine and other flavoring matters ... 2 " 



loo 



When well prepared and in good condition, butter constitutes one of 

 the most valuable and easily assimilated forms of oleaginous food. If 

 contaminated with the nitrogenous matter of the milk, its fatty ingre- 

 dients after a time become decomposed with the development of volatile 

 fatty acids; in which condition it is said to be "rancid," and is no 

 longer fit for food. 



Bread. The cereal grains resemble each other more or less in their 

 constitution, all of them containing starch, nitrogenous matter, dextrine 

 or sugar, fat, and mineral salts in various proportions. Wheat is dis- 

 tinguished by containing a larger quantity of nitrogenous matter as 

 compared with the other ingredients, and by the peculiarly adhesive 

 quality of this substance, which has received accordingly the name of 



