460 APPLIED BIOLOGY 



FOODS 



375. What are Foods? A convenient definition for our 

 present purposes is that foods are solid or liquid substances 

 which when taken into the alimentary canal are useful in the 

 life-processes of our bodies. In most cases substances able 

 to serve as our foods must be capable of being digested and 

 absorbed as materials for energy, repair, and growth ; but 

 a certain amount of plant cell-walls is useful, although not 

 digestible, in the human alimentary canal. 



Sources and Kinds of Human Foods. (L) Write in your note- 

 book a list of some common foods, arranging in three columns those 

 of animal, plant, and mineral origin. 



376. Nutrients. We might consult cook-books and make 

 a very long list of the names of prepared foods which are 

 served on our tables ; but these are made by combinations of 

 such common ingredients as meats, vegetables, milk, butter, 

 lard, sugar, flour, starch, chocolate, salt, etc. Chemists 

 have shown that these common things which are used in 

 every kitchen in combining our foods are composed of cer- 

 tain chemical compounds known as sugar, starch, fat (oil), 

 proteins, and minerals. These compounds, from which all 

 the combinations of human foods are made, are known in 

 physiology as nutrients. 



In order to prove that various foods are made up of these 

 few nutrients, we need to know some method of identifying 

 each. Fortunately chemists have discovered some simple 

 tests which are easily applied, as directed below. 



377. Chemical Tests for Nutrients. Certain chemicals produce 

 characteristic reactions on nutrients and hence may be used to detect 

 their presence in mixtures of foods. 



Starch Test. (D or L) Repeat the iodine test with diluted 

 starch paste ( 100). 



Sugar Test. (D or L) Boil a few grapes, raisins, or prunes in 

 water in a test-tube for two minutes, and this will extract some sugar. 



