Cane sugar, when pure, consists of minute transparent crystals. It is 16⁄10 heavier than water, and is soluble in one-third of its weight of that fluid. By long-continued boiling in water it is changed into uncrystallizable sugar, or treacle, by which its flavor is altered, but its sweetening power increased.
Grape sugar crystallizes in very small cubes, of inferior color as compared with cane sugar crystals. It dissolves in its own weight of water, being three times less soluble than sucrose. In sweetening power one part of cane sugar is equal to 2½ parts of grape sugar; but there is probably little if any difference, between the nutritive power of the two substances.
Inulin is a substance somewhat resembling starch. It does not occur in large quantities. It is met with in the roots of the dandelion, chicory, and many other plants.
Gum is an abundant constituent of plants. The kind termed gum arabic, so largely employed in the arts, is a very pure variety of this substance. Common gums are said to be essentially composed of a very weak acid—gummic, or arabic acid—united with lime and potash. The solution of gum is very slightly acid, and has a mucilaginous, ropy consistence: it is almost tasteless. Mucilage, or bassorin, is simply a modified form of gum, which, though insoluble in water, forms a gelatinous mixture with that fluid. It exudes from certain trees—the cherry for example—and exists largely in linseed and other seeds. Gums are nutritious foods, but it is probable that they are not equal in alimental power to equal weights of starch or sugar.
Vegetable jelly, or pectin, is almost universally diffused throughout the vegetable kingdom. It is owing to its presence that the juices of many fruits and roots possess the property of gelatinizing. It is soluble in water, but prolonged boiling destroys its viscous property. Pectose is a modification of pectin; it is insoluble in water. According to Fremy, the hardness of green fruits is due to the presence of pectose; which is also found in the cellular tissue of turnips, carrots, and various other roots.
Cellulose is a fibrous or cellular tissue, allied in composition to starch. It is the most abundant constituent of plants, and forms the very ground-work of the vegetable mechanism. Linen, cotton, and the pith of the elder and other trees are nearly pure forms of cellulose. Ligneous, or woody tissue (lignin) is indurated cellulose, hardened by age. It is almost identical in composition with cellulose. Pure cellulose is white, colorless, tasteless, insoluble in water, oil, alcohol, or ether. It is heavier than water. Sulphuric acid is capable of converting it into grape, or starch sugar. In its fresh and succulent state cellulose is digestible and nutritious; but in the form of ligneous tissue it opposes a very great resistance to the action of the digestive fluids. Digestible cellulose is probably equal in nutritive power to starch.
Oils and fats occur abundantly in vegetables, more particularly in their seeds. In the seeds of many cruciferous plants the proportion of fat and oil exceeds 35 per cent. The oils and fats termed fixed are those which possess the greatest interest to agriculturists; the volatile oils being those which confer on certain plants their fragrant odour. There are a great variety of vegetable oils, but the proximate constituents of most of them are chiefly stearin, margarin, olein, and palmitin.
Stearin is a white crystalline substance, sparingly soluble in alcohol and ether, but insoluble in water. There are two or three modifications of this substance, but they do not essentially differ from each other. The melting point varies from 130° to 160° Fahr. Stearin is the most abundant of the fats.
Margarin presents the appearance of pearly scales. It is the solid fat present in olive oil, and it is also met with in a great variety of fats and oils. It melts at 116° Fahr.
Olein is the fluid constituent of oils and fatty substances. It resists an extreme degree of cold, without solidifying. There are several modifications of this body—the olein of olive oil being somewhat different from that of castor oil; the olein of linseed is sometimes termed linolien.