The carbohydrates are so called because they are composed of the elements carbon, hydrogen and oxygen, the last two in the proportion in which they are found in water. This last statement, although it is generally made in defining carbohydrates, is not strictly true, since a few of the less common members of the class are found to vary somewhat from this proportion.

The principal carbohydrates may be classed in three groups. The following table shows the chief members of these different groups, so far as our food is concerned.

Grains of Potato Starch.

Classification Of Carbohydrates

That the second and third groups bear a definite chemical relation to the first may be seen by a comparison of their formulae.

Dextrin

Dextrin is a substance having the same general composition as starch, but unlike it in some of its properties. It is chiefly important to us in that it is an intermediate product of the change of starch into sugar.

Glycogen is the form in which carbohydrate is stored in the body until it is needed for use. It is found chiefly in the liver and is sometimes called animal starch.

Diagram Representing the Supposed Structure of a Sphero-Crystal of Starch, Showing Radial and Concentric Arrangement.

From A. Meyer.

Cellulose

Cellulose is so slightly digested that we do not put it in the list of human foods, yet it is important from two standpoints. First, it gives the necessary bulk to food; and second, it so encloses the nutrients in vegetables and fruits that it must be definitely considered in cookery.

Composition

Allied to the gums are the pectose and pectin that are concerned in the making of jelly from fruit juice. The gelatinous substance obtained from Irish moss also belongs in this class. The sugars will be discussed under the special foods.