This group includes a large number of carbohydrates that may be considered as complexes of the simple sugars already described. Indeed, they make up the principal bulk of the carbohydrate content of the raw materials from which human food is prepared. The poly-saccharides may be divided into three sub-groups, the starch group, the gum and vegetable mucilage group, and the cellulose group, the first being the one of greatest importance in human nutrition.

75. The Starches

Starch is a widely distributed and abundant constituent of vegetable tissue. Food plants, especially those most used by the human family, contain it in generous proportions, in some seeds as much as 60 or 70 per cent being present. Probably only water and cellulose are more abundant in the vegetable world.

Starch does not exist in solution in the sap, but is found in the interior of plant cells in the form of minute grains which have a shape, size, and structure characteristic of the seed in which they are found. Potato starch grains are large, about 1/300 of an inch in diameter, and are kidney-shaped, while those of the wheat are smaller, about 1/1000 of an inch in diameter, and resemble in outline a thick burning-glass. Corn starch grains are angular, being somewhat six-sided, and those of other seeds show marked and specific characteristics. These differences in size and shape furnish the most important means of detecting adulterations of one ground grain with another, as, for instance, when corn flour is mixed with wheat flour.

Unless modified by some chemical change, starch is not dissolved by water. The starch grains are not affected at all by cold water, and, in hot water, at first only swell and burst. Prolonged treatment with hot water causes chemical changes to more soluble substances. For this reason the simple leaching of a food material removes no starch by solution. At the same time the cooking of a ground grain so breaks up and liberates the starch grains that they are probably acted upon more promptly by ferments in the digestive fluids.

The proportion of starch in plant substances used for human food varies greatly. The dry matter of many seeds, such as rice and the cereal grains, wheat, maize, barley, or oats, is largely made up of starch. The same is true of potatoes and other tubers. Johnson quotes the following figures from Dragendorff:l

Table XVII. Amount Of Starch In Dry Matter

1"How Crops Grow " p. 52.

It appears that in grain plants starch forms most abundantly during the later development of the seed. At the Maine Station none could be found in very immature field corn cut August 15, while on September 21 the dry matter of the whole plant on which the kernels had matured to the hardening stage contained 15.4 per cent. In general, the stem and leaves of forage plants are poor in starch.

The distribution of starch in seeds is worthy of note. The grain of wheat has been carefully studied in this particular, and it is found that this body does not normally exist in the seed coatings, this tissue consisting largely of mineral matters, proteins, cellulose, and gums. On the contrary, the germ and the interior material deposited around it are rich in starch. To be sure, wheat bran, which is now very largely the outer seed coats of the grain, has more or less, but this is due to imperfect milling.

Starch is an important commercial article, and for this purpose is mainly obtained from corn and potatoes. Special forms of starch used in cookery are sago, tapioca, and arrowroot. It is used as human food, as a source of dextrin and in other ways. By treatment with an acid, corn starch is converted into the glucose of our markets, dextrin and maltose being intermediate products.