Foods may be classified in various ways, according to:

1. Their physical properties.

2. Their source.

3. Their composition.

4. The role which they perform in the animal body.

Foods are classed in accordance with their general physical properties: First, into solid, semisolid, and liquid foods; secondly, into fibrous, gelatinous, starchy, oleaginous, crystalline, and albuminous foods.

A subdivision sometimes used is that of the "complete" foods, such as eggs or milk, which in a single article comprise all the necessary ingredients and elements to support life, and "incomplete" foods, which are capable of maintaining life but a compara-tively short time.

Foods may be classed as to their source primarily into animal and vegetable foods.

Animal foods consist of meats, fish, shellfish, and crustaceans, eggs, milk and its products, animal fats, gelatin. The vegetable foods are subdivided into cereals, vegetables proper, fruits, sugars, vegetable oils.

The simplest chemical classification possible is that advocated by Baron von Liebig, who was the first to suggest a really scientific division of foods. He grouped all foods into two classes a. Nitrogenous, b. Non-nitrogenous.

Each of these classes contains food materials derived from both the animal and vegetable kingdoms, although the majority of the animal substances belong to the nitrogenous, and the majority of vegetable substances to the non-nitrogenous group.

A. The Nitrogenous Group

The Nitrogenous Group von Liebig regarded as containing "plastic" elements - i. e., they are essentially "tissue builders" or "flesh formers".

Nitrogenous foods are sometimes called "azotised foods " or "albuminoids" - that is, substances resembling albumin. They consist chiefly of the four elements carbon, oxygen, hydrogen, and nitrogen, to which a small proportion of sulphur and phosphorus are usually joined. These elements for the most part are combined as some form of albumin.

Nitrogenous or proteid foods are non-crystallisable, but coagula-ble, principally fluid or semisolid substances. They are fermentable, and under some conditions will putrefy.

The nitrogenous group comprises all forms of animal food, excepting fats, glycogen, and such substances as milk - sugar and honey. It includes, therefore, albumins and gelatins. Its chief representatives are milk, eggs, crustaceans, fish, shellfish, flesh, and fowl. It also contains such nitrogenous substances as exist in the vegetable kingdom or "vegetable albuminoids".

B. The Second Or Non-Nitrogenous Group

The Second Or Non-Nitrogenous Group von Liebig called "respiratory or calorifacient foods," because their function in the body is to furnish fuel or maintain animal heat. Since this original classification was suggested it has been established that the non-nitrogenous aliments supply energy as force, manifested through muscular action, hence they are also called "force producers," in distinction from the nitrogenous or proteid "tissue builders".

This is a convenient distinction to adopt, but it must not be held too absolutely, for in emergencies the tissue builders are used as force producers and heat producers as well.

The non-nitrogenous group contains strictly only the three elements carbon, hydrogen, and oxygen, although various salts are mixed with both vegetable and animal foods. It includes vegetables, fruits, cereals, starches, sugars, gums, fats and oils (which latter are both animal and vegetable), and organic acids. Many vegetables, besides some fruits, contain considerable nitrogen, but the "carbohydrates" - i. e., starches and sugars - constitute their main bulk.