This variety of sugar is chiefly obtained from the juice of the sugar-cane, but it is also abundantly present in the juices of certain species of maple and of beet-root, all of which yield this substance as a commercial product; it is also contained in sugar-grass (sorghum saccharatum), whose juice yields 13 per cent of sugar; in carrots and turnips, in the pumpkin, the chestnut, the young shoots of maize, in the flowering buds of the cocos palm, and in a large number of tropical fruits. Its use as an article of diet has been already mentioned underDIET. Several articles of food contain some form of sugar in considerable quantity. In peas, there are 2 per cent of sugar; in rye-meal and wheaten bread, about 3+ per cent; in cows' milk, 4 per cent; iu goats' milk and in barley-meal, 51 per cent; in human milk, in asses' milk, ripe gooseberries, and ripe pears, about 6 per cent; in oatmeal, about 8 per cent; in wheaten flour, from 4 to 8 per cent; in beet-root, from 5 to 10 per cent; in ripe peaches, 16i per cent; in ripe cherries, 18 per cent; and in dried figs, upwards of 60 per cent. Although sugar is commonly regarded as a luxury, it is in reality a very valuable article of food (as, indeed, might be inferred from its presence in milk, and in both the yelk and white of eggs), since it is very rapidly digested, and supplies heat-forming or respiratory food to the system. "When, however," says Dr. E. Smith, "it is compared with wheaten flour, it is a very dear food, since three or four times more carbon will be obtained for ld. in flour, besides nitrogen, none of which is found in sugar. It has also been proved by Messrs. Lawes and Gilbert that even its fattening properties—that is to say, its power to form fat in the system, when it is supplied in excess of the quantity which the daily wants of the body require to produce heat—are not greater than those of starch as found in the cheapest grain."—Practical Dietary, 1863, p. 63. In consequence of sugar being a fat-forming substance, it should be taken very sparingly in cases of excessive obesity. There are certain forms of dyspepsia in which sugar should be avoided, as exciting increased gastric uneasiness; and in diabetes, all articles of food containing or (like starch) yikld hag sugar, should be rigidly prohibited. Although prone to fermentation when in , dilute state, in its concentrated form sugar possesses great antiseptic power, and is extensively employed to preserve both vegetable and animal substances from decay. The sugar naturally existing in some fruits is often sufficient to insure their preservation in a dry state, while in other cases it is added, as in preserves and jellies. A mixture of salt and sugar applied to meat, fish, etc., preserves more of the natural flavor than mere salting does. Sugar converted into caramel is much used by cooks and confectioners as a coloring matter.
Closely allied in their chemical characters to sucrose are the following comparatively rare forms of sugar: (1) Trehalose so called from Trehala, or Turkish manna (the product of a coleopterous insect, Larinus nidifwans), from which this variety of sugar is extracted, differs from sucrose in the following points—it crystallizes in brilliant rectangular octahedra; contains water of crystallization; fuses at 212°, and loses its water of crystallization; is very soluble in hot alcohol; possesses about three times as great a rotatory power on polarized light; and when heated to 356° does not undergo further change. (2) 3[ycose, obtained from ergot of rye, possesses the same composition as trehalose, from which it mainly differs in crystallizing in rhombic prisms, and in exhibiting a somewhat weaker rotatory power. (3) Melezitose obtained from larch manna, differs from cane-sugar in its less sweet taste, and in exhibit. ing a less powerful rotatory action. (4) Melitose the chief ingredient in theAustralian manna yielded by the eucalyptus tree, crystallizes in acicular prisms, is feebly sweet, undergoes fermentation with yeast; but yields only half as much alcohol and carbonic acid as would be obtained from an equal weight of glucose, one half of this sugar being converted into an unfermentable syrupy body, known as eucalyn (C12H12012).
More important than any of the above varieties, and differing from cane-sugar in a distinctive physical property, is the substance formerly known as fruit sugar, but now often described as inverted cane-sugar. The objection to the former name is, that the sugar contained in many ripe acidulous fruits, and formerly regarded as a distinct variety, is merely a mixture of cane-sugar, with more or less of the inverted sugar which has already been noticed as resulting from the action of prolonged boiling, or of a little acid on cane-sugar. The same change occurs in many ripening fruits, in consequence of the presence of a peculiar albuminous ferment. Inverted sugar is not crystallizable, is soluble in dilute alcohol, and produces left handed rotation; hence its name. By chemical means, it is convertible into grape-sugar, a change which sometimes occurs spontaneously, as is seen in the gradual crystallization of the sugar in dried fruits.
constituting the hard granular sweet masses occurring in old dried fruits, such as raisins, figs, etc., has already been described in the article GLUCOSE, or GLYCOSE, under which names it is commonly known to chemists. It is also known as starch-sugar, because it is readily obtained by the action of a dilute acid on a hot solution of starch, and is identical with the sugar occurring in the urine in diabetes.
Milk -sugar, known also as lactine and lactose (C2DII,Oi. ±5Aq, or, according to some chemists, +Aq), is a purely animal product. It exists in considerable quan tity in the milk, especially of the herbivorous animals, and is one of the most important and essential ingredients in that secretion. It may be obtained on a large scale by separating the curd from the milk, and evaporating the whey till it is ready to crystal lize; when, on the introduction of small pieces of wood, the crystals of sugar are deposited on them. These crystals are four-sided prisms of a milk-white color, and so hard that they crunch between the teeth. This variety of sugar is only moderately sweet (vide supra), requires about six times its weight of cold water for its solution, but dissolves readily in boiling water, while it is insoluble in alcohol or ether. If it be gradually heated to 281°, two equivalents of water are expelled, whereas, if it be sud denly heated to about 400°, all five equivalents are given off. When pure, milk-sugar is insusceptible of fermentation; but when boiled with dilute acids, it is converted into a directly fermentable sugar, in many respects very similar to grape-sugar, and to which some chemists have given the name of lactose, a term commonly applied to milk-sugar itself. On treating a moderately diluted acid solution of milk-sugar with yeast, this variety is first formed, and then yields carbonic acid and alcohol; if, however, decom posing matters, as, for example, casein in the act of disintegration, are present, it under goes lactic and butyric fermentation; and hence we understand how milk after exposure for a time to the air becomes sour. The intoxicating character of the drink prepared by the Kalmucks and Tartars from sour mares' milk, is due to this indirected vinous fermentation of sugar of milk. Regarding the uses of this variety of sugar, it may be observed that it is probably the most important of the constituents of whey (which is milk deprived of the whole of its casein except a mere trace held in solution), and hence that it is the active ingredient in the whey-cure, which is so popular in Switzerland. (The whey in these cases is usually obtained from goats' milk.) It is also the chief constituent of the globules used in homeopathy.