118 FOOD. 



Composition of Different Articles of Food. 



In the most valuable and nutritious kinds of food, which have been 

 adopted by the universal and instinctive choice of man, the first three 

 classes of proximate principles are all more or less abundantly repre- 

 sented. In all there exists naturally a certain proportion of saline 

 matter; and water and sodium chloride are generally taken in addition. 



Milk. In milk, the first food supplied to the infant, and largely 

 employed in various culinary preparations, all the important groups of 

 nutritive substances are present. It is a white, opaque fluid, consisting, 

 1st, of a serous portion, which contains albuminous matters, sugar, and 

 mineral salts in solution, and, 2d, of fatty globules suspended in the 

 watery liquid. It is this mixture of oleaginous particles with a serous 

 fluid which gives to the milk its opacity and its white color. Its rich- 

 ness in fatty matter may therefore be estimated from these physical 

 qualities. The ingredients in cow's milk are present in the following 

 proportions, according to Payen : 



COMPOSITION OF Cow's MILK IN 1000 PARTS. 



Water 864 



Nitrogenous matter (caseine and albumen) .... 43 



Sugar of milk . 52 



Fat 37 



Mineral salts 4 



1000 



Cow's milk resembles human milk in its general characters, but con- 

 tains a larger proportion of solid ingredients, especially of the nitro- 

 genous and saccharine matters, fat being present in nearly the same 

 amount in each. Sheep and goat's milk is richer in both nitrogenous 

 and fatty matters ; while the milk of the ass and the mare contains a 

 greater abundance of sugar, but is comparatively poor in nitrogenous 

 matter and fat. The nitrogenous matter of milk consists almost entirely 

 of caseine, associated with a very small proportion of albumen. Owing 

 to the relative quantity of these two substances, milk does not solidify 

 on boiling, but merely covers itself with a thin pellicle of coagulated 

 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, due to a transformation of its sugar ; and 

 the same change will sometimes 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. A faintly acidulated infusion of this substance even in 

 small quantity, added to fresh milk at the temperature of 30 (86 F.) 



