50 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



by heat. The precipitate produced by saturation with salt floats to the 

 surface with the entangled fat, and the clear salted whey is seen below after 

 an hour or two. 



B. Flour. Mix some wheat flour with a little water into a stiff dough. 

 Wrap this up in a piece of muslin and knead it under a tap or in a capsule of 

 water. The starch grains come through the holes in the muslin (identify by 

 iodine test), and an elastic sticky mass remains behind. This is a protein 

 called gluten. Suspend a fragment of gluten in water; add nitric acid and 

 boil ; it turns yellow ; cool and add ammonia ; it turns orange (xanthoproteic 

 reaction). Boil another fragment with Millon's reagent ; it turns a brick-red 

 colour. 



C. Bread contains the same constituents as flour, except that some of the 

 starch has been converted into dextrin and dextrose during baking (most 

 flours, however, contain a small quantity of sugar). Extract breadcrust 

 with cold water, and test the extract for dextrin (iodine test) and for 

 dextrose (Trommer's test). If hot water is used, starch also passes into 

 solution. 



D. Meat. This is our main source of protein food. Cut up some lean 

 meat into fine shreds and grind these up with salt solution. Filter and test 

 for proteins. 



THE PRINCIPAL FOOD-STUFFS 



We can now proceed to apply the knowledge we have obtained of 

 the proteins, carbohydrates, and fats to the investigation of some 

 important foods. The chief proximate principles in food are : 



1. Proteins \ 



2. Carbohydrates ^organic. 



3. Fats J 



4. Water 



5. Salts I inorganic. 



In milk and in eggs, which form the exclusive foods of young 

 animals, all varieties of these proximate principles are present in 

 suitable proportions. Hence they are spoken of as perfect foods. 

 Eggs, though a perfect food for the developing bird, contain too 

 little carbohydrate for a mammal. In most vegetable foods carbo- 

 hydrates are in excess, while in animal food, like meat, the proteins. 

 Are predominant. In a suitable diet these should be mixed in 

 proper proportions, which must vary for herbivorous and carnivorous 

 animals. We must, however, limit ourselves to the omnivorous 

 animal, man. 



A healthy and suitable diet must possess the following cha- 

 racters : 



1. It must contain the proper amount and proportion of the 

 various proximate principles. 



2. It must be adapted to the climate, to the age of the individual, 

 and to the amount of work done by him. 



