52 FOODS AND NUTRITION 



fundamental food units are spoken of as food stuffs, ali- 

 mentary principles, or nutrients. We shall adopt the last 

 term in speaking of them. 



Nutrients. — Nutrients are usually classified under the 

 following names: water, inorganic salts, proteids, carbo- 

 hydrates, fats, and albuminoids. Since the importance 

 of a food and its value as nourishment depends upon the 

 amounts and kinds of these nutrients present, it will be 

 well to consider some of their properties and the means 

 by which their presence may be detected. 



Proteids or nitrogenous compounds. — Different vari- 

 eties and forms of proteid are found in nearly all kinds of 

 animal and vegetable foods. The exact composition of 

 proteid is unknown, but the elements of which it is built 

 are nitrogen, carbon, sulphur, hydrogen, and oxygen. It 

 occurs in a great variety of forms which receive different 

 names; for example, myosin and syntonin in meats; egg 

 albumin; casein of milk and cheese; gluten of grain. 

 White of egg or egg albumin illustrates well the charac- 

 teristic properties of proteids. 



When egg albumin is heated or treated with acids (see 

 Ex. XIII.), it is changed from a soluble, sticky liquid to 

 a white solid, insoluble in water. This change to a solid 

 form, due to the heat or acids, is called coagulation, and 

 is a characteristic of many proteids. The solidifying of a 

 piece of meat under heat is due to the coagulation of the 

 proteid in it by the heat, while the curdling of milk when 

 sour is another case of coagulating proteid due to the 

 i;)resence of lactic acid in the milk. 



There are a number of ways in which the presence of 

 proteid in a food may be detected. The principal chem- 

 ical tests are as follows (see Ex. XIII, a-d.): 



