29. Carbohydrates. If we knead a little wheaten dough in a considerable quantity of water, the latter becomes milky from the presence of a pure white substance which washes out from the dough, while there is left behind a curious, elastic, pale-colored mass sometimes called "wheat gum."

If we allow the milky water to stand for some time, a large part of the white substance will settle, thus showing that it is a fine powder which was merely suspended in the water, and not really dissolved. This white material is starch, as may be proved by adding to some of it a little iodine solution; this will turn it a dark bluish color, and starch is the only substance known to be thus affected.

If starch be boiled with a dilute acid for a sufficient time it becomes mainly converted into a kind of sugar known as glucose, or grape-sugar, an important constituent of the commercial "glucose" of which large quantities are used in confectionery. Chemistry teaches us that this change is made possible by the fact that both starch and glucose consist of the same elements,-namely, carbon, hydrogen, and oxygen in nearly the same proportions,-the composition of starch being carbon, six parts; hydrogen, ten; and oxygen, five; as expressed by the formula C6H10O5; while for pure glucose the formula is C6H12O6. It will be noticed that in each there is twice as much hydrogen as oxygen; that is to say these elements are present in just the same,proportion as in water, which, as is well known, has the chemical formula H2O. A substance which is thus composed of carbon united with the elements of water is called a carbohydrate.1 Not only do starch and glucose come under this head, but also other kinds of sugar, various sorts of true gum (such for example as that on postage stamps), and the substance known as cellulose of which wood, cotton, and paper are mainly composed. Among the cereal grains, although sugar is sometimes present to a notable degree, as in "sweet corn," the amount of digestible carbohydrate as given in the tables may be understood as being almost entirely starch.

1 Car-bo-hy'drate - L. carbo, coal; Gr. hydor, water.

During the process of digestion in man and other animals starch is converted into sugar, and as such is absorbed into the blood and carried all over the system to serve either for making fat or for giving warmth and strength. Since only fluids can be absorbed, and since starch is composed of solid insoluble particles, the necessity of somehow converting the starch of our food into sugar, is obvious.

Similarly, when grains sprout, the starch in them undergoes a sort of digestion and becomes converted into sugar, largely maltose or "malt sugar" (formula C12H22O11). This being soluble in the sap of the young plant, may be carried to the regions of growth where food is needed. This change of the insoluble starch into the soluble sugar is accomplished through the action of a substance called diastase, one of a remarkable class of substances known as enzymes2 that have the power of bringing about such changes by their presence in comparatively minute amount. The process of malting consists in causing grain to sprout and allowing the conversion of starch to proceed until as much sugar as possible is produced. At this point the plantlets are killed by heat so that they will not use up any of the sugar they have made. The sweet substance is then dissolved out by soaking the malted grains in water. From the liquid so sweetened, lager beer and other malt liquors are made by subsequent fermentation with yeast. Diastase separated from malt may be used instead of an acid to convert starch into sugar.

2 En'zyme< Gr. en, in; zyme, leaven; so called because acting like the substance in leaven or yeast which produces similar changes.