Inulin replaces starch as the reserve food carbohydrate in a considerable number of natural orders of plants, particularly in the Compositae. It is the carbohydrate of the tubers of the dahlia and artichoke and of the fleshy roots of chicory. It is often found associated with starch in monocotyledonous plants, such as many species of Iris, Hyacinthus, and Muscari. Among the monocotyledons, starch seems to be the characteristic carbohydrate reserve of aquatic, or moisture-loving, species, while inulin is more common among those which prefer dry situations.
Inulin may be prepared from the tubers of dahlias or artichokes, by boiling the crushed tubers with water containing a little chalk (to precipitate mineral salts, albumins, etc.) filtering and cooling the filtrate practically to the freezing point, which precipitates the inulin.
Inulin is a white, tasteless, semi-crystalline powder, which is soluble in hot water, from which it may be precipitated by alcohol or by freezing. It forms no paste like that of starch or dextrin, and gives no color with iodine. It is levorotatory, and when hydrolyzed by acids or by the enzyme inulinase yields fructose; in fact, inulin bears the same relation to fructose that starch does to glucose.
Graminin, irisin, phlein, sinistrin, and triticin are all inulin-like polysaccharides, which have been found in the plants after which they are named. Their solutions are, as a rule, sticky or gummy in consistency, which suggests that these compounds bear the same relation to inulin that dextrins do to starch.
(C) Mannosans, or Mannans
Mannan bears the same relation to mannose that starch does to glucose and inulin to fructose. It occurs as a reserve food substance in many plants. It has been reported as present in moulds, and in ergot; in the roots of asparagus, chicory, etc.; in the leaves and wood of many trees, such as the chestnut, apple, mulberry, and many conifers; also as a part of the so-called "hemi-celluloses" which are present in the seeds of many plants, notably the palms, the elders, cedar, larch, etc.
It is a white, amorphous powder, which is difficultly soluble in water, is strongly dextrorotatory (specific rotatory power +285°), and when hydrolyzed yields mannose.
Secalin (or carubin) is a substance which is found in the seeds of barley, rye, etc., which is similar to mannan, but is optically inactive.
(D) Galactans
These bear the same relation to galactose that the preceding dextrosans do to their constituent hexoses. Four different galactans have been isolated from plant tissues; they are all white, amorphous solids which dissolve with difficulty in water, forming gummy solutions.