Food substances fall into five distinct groups according to their chemical composition:

1. Those valuable for their mineral salts.

2. Those containing considerable protein: milk, eggs, meat, etc.

3. Those characterized by their starch content: flours, cereals, breadstuffs, etc.

4. Those with a preponderance of sugar.

5. Those largely fat.

The plan is to study the foods of each group separately, beginning with the protein group because of its importance in the diet and the simplicity which governs its cooking processes.

Of the protein group milk and eggs are by far the most important members. It is only necessary to think of the purpose for which nature intended them, to realize how great their value is. In both cases they provide the entire food of the young animal until such time as that animal is able to find food for itself; that is, the milk and the egg not only are sufficient to keep the young animal alive and warm, but are also able to provide the material for building up the body. In the case of the egg, the chicken is fully developed inside the shell and gets no further sustenance than what was originally there until it breaks out of its shell.