WHAT FOOD IS 51 



convenience. It is found less confusing to restrict the use 

 of the word food to such substances as can serve as building 

 material for protoplasm or as sources of energy through 

 oxidation. 



76. Food organic. Using the word food in this sense, then, 

 we must notice first of all that food materials are found in 

 nature only in the bodies of living things ; that is to say, they 

 are organic, to use the older term. From a chemical point of 

 view we may divide the foods into two main classes : those 

 that contain nitrogen and those that do not. The foods of 

 the first class are called proteins and are represented in our 

 familiar supplies by such substances as albumen, or white of 

 ^gg ; the curd or casein that is formed when milk sours ; and 

 the glnten, or pasty substance, in wheat flour or bread. Similar 

 nitrogen-containing substances are found in the cells of mus- 

 cles and are called myosin ; others, found in the seeds of the 

 plants belonging to the bean family, the Leguminosae, are 

 called legnmin. 



Of the non-nitrogenous foods there are two main divisions, 

 the fats and the carbohydrates. The fats are familiar to us in 

 such substances as butter, suet, lard, tallow, olive oil, cotton- 

 seed oil, peanut oil, and others. The carbohydrates comprise 

 all the sugars and all the starches. 



77. Food functions. In dividing the foods into the two 

 classes, nitrogenous and non-nitrogenous, we have at the same 

 time separated them in accordance with their true relations to 

 protoplasm. For the proteins are the foods that are necessary 

 for the building of protoplasm ; the protoplasm may be said to 

 consist fundamentally of protein. The fats and carbohydrates 

 are important in living cells as fuel, or oxidizable material. 



\\q find accordingly that all seeds contain protein, some in larger 

 proportions (beans, peas, lentils, for example) and some in smaller 

 proportions. In addition to this, all seeds contain either fat (as the 

 castor bean, peanut, cotton seed, flax seed) or some carbohydrate (as 

 the bean, cereals, date). 



