76. Glycogen

This is the only uncombined carbohydrate found in the animal body in appreciable quantity outside the forms that are in the blood circulation. It is sometimes called animal starch. It is a white powder, soluble in water, and may be extracted in small amounts from the muscles and liver. (See p. 139.) It is formed out of the sugars that are taken into the circulation from the digestive tract, and, as we shall see, is a reserve store of fuel for the maintenance of muscular energy, and in this way it performs a very important office in nourishing the animal body. It was formerly believed that another carbohydrate exists in muscle called ino&ite, but it is now known that this substance belongs to a different class of compounds.

77. The Pentosans

These bodies are very widely distributed in nature, being found in the leaves, stem, roots, and seeds of a great variety of plants, in algae and in beets and turnips. Some pentosans are known as gums, such as gum arabic, gum tragacanth, and cherry gum. Pentosans, on hydrolysis, yield pentose sugars, among which are arabinose and zylose. These gum-like substances exist in such human foods as beets and turnips, spinach, cabbage, and other vegetables that serve more or less as human food.

78. Galactans, Mannans, Levulans, Dextrans

These are compounds of little importance in human nutrition that are more or less associated in the framework of a great variety of plants or parts of plants, including seeds, beets, and turnips, tubers and bulbs, algae, lichens, molds, and the wood and bark of many species of trees. On hydrolysis they yield galactose, mannose, levulose, and dextrose, respectively.

Together with the pentosans these compounds make up the least valuable part of certain vegetable foods.

79. The Pectin Bodies

Another class of compounds much like the gums, and perhaps related to them chemically, is the pectin bodies. Some of these substances are gelatinous in appearance. The jellying of fruits, such as apples and currants, is made possible by their presence. They exist in greater abundance in unripe fruit than in the ripe, consequently the former is selected for jelly-making. When such fruits are cooked, the pectin which they contain takes up water chemically and is transformed into a gelatinous substance, and the secret of jelly-making is in stopping the cooking process before the chemical transformations have passed beyond a certain point. Mucilages not greatly unlike the gums and pectins exist in certain seeds and roots, the most notable instance being flaxseed.

80. Dextrin, which is sometimes spoken of as a gum, is made by heating starch to about 200° C. It may also be produced by treating starch with a dilute acid. Dextrin i& undoubtedly formed on the outer part of the loaf when wheat bread is baked. It is soluble in water.

81. Cellulose

This is found in the tough or woody portion of plant tissue. In tables of food analyses we find the term crude fiber, which consists largely of cellulose, a familiar example of which in a nearly pure form is the cotton fiber used in making cloth. Crude fiber is separated from associated compounds by the successive treatment of vegetable substance with weak acids and alkalies, and as so determined is sometimes improperly taken to represent the amount of cellulose in a plant. While crude fiber is mainly cellulose, it contains a small proportion of other compounds, and, besides, more or less cellulose is dissolved by the acid and alkali treatment, so that the percentages of crude fiber given in food tables only approximately measure the cellulose present.

All plant tissue is made up of cells, the walls of which are chiefly or wholly cellulose. It is this substance out of which is built the framework of the plant, and which gives toughness and rigidity to certain of its parts. The more of this plant tissue contains, the more tenacious it is, other things being equal, and the more difficult of mastication.

The proportions of cellulose in the different parts of a plant are greatly unlike. It is usually most abundant in the stem, with less in the foliage and least in the fruit. With vegetables like potatoes and turnips, the leaves are much richer in fiber than the tubers or roots, which contain a comparatively small proportion. Of the grains or seeds considerable is present in the outer coatings, while but little is found in the interior. Considering human foods of plant origin, we find that vegetables such as celery, lettuce, beets, and turnips are relatively rich in crude fiber, while tubeFs, flours, and meals contain only small amounts. In certain by-products from the grains, like bran, which is made up mostly of the seed coatings, fiber is present in fairly large proportions, while in flour derived from the inner parts of the grain the percentage is almost negligible.

The stage of growth at which a plant is used for food purposes has a marked influence upon the proportion of crude fiber. In young, actively growing vegetable tissue, the cell-walls are thin, but, as the plant increases in age, these thicken chiefly through the deposition of cellulose. In general, the toughness and hardness of mature plants, as compared with young, is due to the increased proportion of woody fiber, although the decrease in the relative amount of water in the tissues and the deposition of other substances have more or less effect. The rapid toughening of young asparagus tips and the tenderness of young beets as compared with old ones are cases in point.