GALACTOSE-GRAPE OR STARCH SUGAR.

99

produces right-handed rotation of a ray of polarized light; the fresh solution has a higher rotatory power than it exhibits after standing for a few hours, or after heating it to 212°. If lactose be heated gradually to 284° (140° C.), I atom of water is expelled; but it is remarkable that if the temperature be suddenly raised to about 400°, the crystals melt, and then lose 24 H2O. When boiled with dilute acids it is very slowly converted into a very soluble, crystallizable and fermentable sugar, galactose, which exerts a righthanded rotatory action upon polarized light, and furnishes twice as much mucic acid when treated with nitric acid as an equal weight of lactose. When pure, milk-sugar is not susceptible of fermentation, but milk itself may be fermented. In this case, Berthelot was unable to find any evidence of the conversion of the lactose into glucose before it underwent fermentation. If the milk be allowed to ferment spontaneously in contact with chalk, a considerable quantity of calcic lactate is formed simultaneously with the alcohol. Lactose may be combined with oxide of lead, which it dissolves freely; it likewise forms with it a white insoluble compound, 4€12H24→12+5PbO=Є48H8Pb ̧→49+ 5H,. It also yields compounds with the alkalies and with the alkaline earths, and absorbs both ammoniacal and hydrochloric acid gases. An alkaline solution of lactose when boiled with the salts of copper reduces them, and precipitates the red oxide of the metal. It also reduces the salts of mercury and of silver when heated with their solutions. The reaction upon the salts of silver has been made the basis of a process for silvering glass (936). When lactose is oxidized by nitric acid it yields mucic acid, with small quantities of saccharic, tartaric, racemic, and oxalic acids.

86

(1084) 3. GRAPE OR STARCH SUGAR; Glucose,* Dextrose (H12H,O=180+18.—Preparation.-Starch sugar is prepared upon a considerable scale by allowing a mixture of starch, with a sufficient quantity of water to render it liquid, to flow gradually at a temperature of about 131° (55° C.), into a vat containing water acidulated with 1 per cent. of sulphuric acid; this liquor is kept constantly at the boiling point, by which means

Under the name of glucose (from yλukus, sweet) several distinct modifications of sugar have been comprised, such as linen sugar, uncrystallizable diabetic sugar, and honey sugar, as well as sugar from malt, modified cane sugar, and the modification of milk sugar produced by acids; but these bodies, though closely resembling each other, present differences in their molecular structure, as may be proved by the differences in the effects produced by them on polarized light. In order to avoid this ambiguity it will be convenient to designate starch sugar by the term dextrose, proposed by Kekulé, whilst the term glucose may still be retained as generic for the group.

100

GLUCOSE, GRAPE OR STARCH SUGAR.

the starch is at once altered so as not to produce a mucilage. The liquid, after all the starch has been added, is maintained in a state of ebullition for about half an hour, at the end of which time the starch is usually completely converted into sugar. The portion of starch used amounts to one half of the weight of the water employed. The liquid is drawn off, and the acid is neutralized, by adding chalk in small quantities at a time, until it ceases to occasion any effervescence; the precipitate is allowed to subside, and the clear solution is concentrated by evaporation until it acquires a density of 128. It is drawn off from the deposited calcic sulphate, after which it is set aside for some days to crystallize. The molasses is allowed to drain off, and the sugar is dried at a gentle heat in a current of air.

109

6 12 6 2

The formula assigned to starch is €12H200, and since that of starch sugar may be represented as (EH12,H,O), the conversion of starch into sugar may be regarded in the light of a combination of starch with the elements of water, I atom of starch and 4 atoms of water yielding 2 atoms of dextrose.

By the action of diastase (1096) starch is also converted partially into a fermentable sugar, which is one of the varieties of glucose.

A similar conversion of ligneous fibre into this form of sugar may be effected under the influence of acids, but in this case the acid is employed in a more concentrated form; 2 parts of clean linen or calico cut into shreds, are added gradually to 3 parts of oil of vitriol, and allowed to stand for 24 hours. The mixture is afterwards diluted largely with water, and boiled for a few hours. The acid may then be neutralized by the addition of chalk, or what is better, of barytic carbonate, and the sugar can afterwards be separated from the insoluble calcic or barytic sulphate.

True dextrose is also produced from glycogen (1095) by prolonged boiling with diluted acids, as well as from trehalose and melezitose when similarly treated. One of the forms of glucose is also furnished by amygdalin, salicin, phloridzin, and other glucosides (1492) when decomposed by acids. Tunicin (1106), or the horny covering of beetles and other insects, also yields it when treated with dilute acids. Cane sugar furnishes a mixture of dextrose with lævulose in equal proportions.

Glucose, usually in the form of dextrose, is the variety of sugar met with as a morbid constituent of the urine in cases of diabetes. It has been shown by Bernard to be rapidly produced from one of the normal constituents of the liver (glycogen), and the same physiologist has remarked, that by irritating with a needle the

GRAPE SUGAR-DERIVED PRODUCTS.

101

fourth ventricle of the brain in a dog or a rabbit, sugar is developed in the blood and in the urine, after a few minutes.

Properties.-Grape sugar is distinguished from cane sugar by several characters; it crystallizes with difficulty in warty concretions, composed of hard transparent cubes. It requires about its own weight of water for solution, so that it is less soluble in water than cane sugar, though it is more readily taken up by alcohol; 100 parts of alcohol of 83 per cent. dissolve 2 of dextrose at 68° (20° C.), and about 21 parts at the boiling point. It crystallizes from a hot solution in alcohol containing not more than 5 per cent. of water, in anhydrous acicular prisms (EH) fusible at 295°. It requires nearly 24 parts of glucose to produce the same sweetening effect as is produced by I part of cane sugar. The action of sulphuric acid upon grape sugar is quite different from its effect upon cane sugar, since, instead of decomposing it, as occurs with sucrose,' it forms with glucose a definite compound acid, which has been termed sulphosaccharic acid, and which, according to Péligot, yields a soluble salt with calcium and with barium.

12

6 12

Dextrose forms with common salt a compound that crystallizes with facility (NaCl,H,O,2,H12), thus furnishing another distinctive character of this variety of saccharine matter. This compound when heated to 212° gives off H2O, and if the temperature of the residue be raised to 320° (160° C.) it loses an additional atom of water. When grape sugar is heated, it begins to soften at about 140° (60° C.) and at 212° it melts and loses H.; at about 340° it loses another atom of water and is converted into glucosane* H100s, and at a still higher temperature an additional atom of water is expelled, and the residue consists of caramel, which, when heated still further, is decomposed into gascous products.

6

Grape sugar forms definite but unstable combinations with the alkaline bases. These compounds, even at ordinary temperatures, gradually undergo change; their solution, which is at first strongly alkaline, becomes neutral, owing to the formation of a powerful, colourless, but uncrystallizable acid, glucic acid (H,Є12H15, Péligot), which remains in combination with the

3

base.

Glucic acid, when boiled with water, absorbs oxygen and becomes brown; a new acid is formed (H,,H,;), which Mulder terms apoglucic acid, (from ano, at a distance from, and glucose):

2

9

* This is a colourless uncrystalline, unfermentable substance, not sweet, but reconvertible into dextrose by boiling with dilute acids.

102

QUANTITATIVE ESTIMATION OF SUGAR.

it is irot crystallizable. If an alkaline solution of grape sugar be heated, the sugar is rapidly destroyed, and a different body of dark brown colour, also possessed of acid characters, is produced; this substance is not crystallizable; it has been termed melassic acid. This body has a powerful tendency to combine with oxygen, and when boiled with an alkaline solution, to which a cupric salt has been added, speedily reduces the cupric to a cupreous salt, whilst the red oxide is precipitated. The compounds obtained by the combination of dextrose with the acids and the alcohols, will be described hereafter (1246, 1247).

4

(1085) Estimation of Sugar.-Upon the foregoing facts Barreswil has founded a simple method of estimating the amount of sugar which any mixture may contain. For this purpose he prepares an alkaline solution of potassio-cupric tartrate (K,Єu 2€,H,0), of known strength. A given volume of this solution is placed in a porcelain capsule, and heated to near its boiling point; the solution of sugar to be tested is then added by degrees from a burette, until the addition of the syrup ceases to produce any further precipitate. The proportion of sugar which is present in the liquid will be inversely as the volume of the saccharine solution consumed. Cane sugar, however, does not reduce the alkaline solution of cupric tartrate; consequently no indication of the presence of cane sugar is afforded by this reagent. Nevertheless, a quantitative determination even of this variety of sugar may be effected by means of this method, provided that a given volume of the saccharine liquid, acidulated slightly with sulphuric acid, be diluted with water, and boiled for a short time; the cane sugar is thus converted into inverted sugar (1086), and on diluting this liquid with water till it occupies a definite volume, the proportion of altered sugar which it contains may be ascertained. Supposing both kinds of sugar to be present, a preliminary experiment is made before boiling with acid, in order to estimate the amount of inverted or of grape sugar, and by deducting this from the quantity found after acidulating the liquid and boiling, the proportion of cane sugar is ascertained. Fehling recommends for the preparation of the standard copper solution, the following proportions, as reduced to English weights-1 ounce of crystallized cupric sulphate, 3 ounces of cream of tartar, 1 ounce of pure potassic carbonate, 14 or 16 ounces of a solution of caustic soda (sp. gr. 1'12), and water, which is added until the solution measures 12630 water grains. Two hundred measured grains of this solution contain a quantity of copper, which would be reduced by 1 grain of sugar (H1);

12

FRUIT SUGAR-INVERTED SUGAR.

103

each atom of sugar reducing 5 atoms of cupric oxide to the state of cupreous oxide. The test is often conveniently resorted to as a preliminary trial for sugar in urine: little or no action occurs when a solution of grape or diabetic sugar is mixed with an alkaline solution of copper in the cold; but on the application of a gentle heat, the yellow hydrated cupreous oxide is thrown down, and on raising the temperature to the boiling point the anhydrous red oxide is deposited.

When a solution of any of the varieties of glucose is mixed with an ammoniacal solution of acetate of lead, a precipitate is formed, containing (2 ЄH12,3Pb). Peroxide of lead converts glucose into a mixture of formiate and carbonate of lead, whilst water is eliminated.

6 12

(1086) 4. FRUIT SUGAR-LEVULOSE (H12O6).-The researches of Buignet (Ann. de Chimie, III. lxi. 268) have shown that many ripe acidulous fruits, such as apricots, peaches, pine-apples, citrons, plums, and strawberries, contain cane sugar mixed with a variable proportion of inverted sugar; other fruits, among which are grapes, cherries, gooseberries, and figs, contain inverted sugar only.

Inverted sugar has received this appellation from its inverted action upon polarized light, which distinguishes it from sucrose; it appears to be procurable only from sucrose, by the action either of acids or of a peculiar albuminous ferment, present in the juice of many ripening fruits. Honey and manna contain inverted

sugar.

Inverted sugar is soluble in alcohol of 85 per cent. It is not crystallizable, nevertheless it gradually separates into crystallized starch sugar, and an uncrystallized syrup. It is, in fact, a mixture of two different kinds of sugar; and if a solution of 5 parts of this sugar in 50 of water be mixed intimately with 3 parts of slaked lime, the solution, which is at first very liquid, becomes pasty: on straining it through linen, and expressing the fluid portion, the sugar is separated into two equal portions of different properties. One of these varieties, ordinary starch sugar, remains in solution, and exerts a right-handed rotation upon a beam of polarized light, as may be ascertained by removing the lime by means of oxalic acid; whilst the other, which remains combined with the lime, exerts a left-handed rotation, and may be in like manner separated from the lime by suspending the compound in water and decomposing it by means of oxalic acid (Dubrunfaut).