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 action of enzymes, which follows similar lines with the - and -glucosides, i.e. the compounds formed by the interaction of glucose with substances generally containing hydroxyl groups (see ).

Fermentation of Glucose.—Glucose is readily fermentable. Of the greatest importance is the alcoholic fermentation brought about by yeast cells (Saccharomyces cerevisiae seu vini); this follows the equation C6H12O6 = 2C2H6O + 2CO2, Pasteur considering 94 to 95% of the sugar to be so changed. This character is the base of the plan of adding glucose to wine and beer wort before fermenting, the alcohol content of the liquid after fermentation being increased. Some fusel oil, glycerin and succinic acid appear to be formed simultaneously, but in small amount. Glucose also undergoes fermentation into (q.v.) in the presence of the lactic acid bacillus, and into butyric acid if the action of the preceding ferment be continued, or by other bacilli. It also yields, by the so-called mucous fermentation, a mucous, gummy mass, mixed with mannitol and lactic acid.

We may here notice the frequent production of glucose by the action of enzymes upon other carbohydrates. Of especial note is the transformation of maltose by maltase into glucose, and of cane sugar by invertase into a mixture of glucose and fructose (invert sugar); other instances are: lactose by lactase into galactose and glucose; trehalose by trehalase into glucose; melibiose by melibiase into galactose and glucose; and of melizitose by melizitase into touranose and glucose, touranose yielding glucose also when acted upon by the enzyme touranase.

Commercial Glucose.—The glucose of commerce, which may be regarded as a mixture of grape sugar, maltose and dextrins, is prepared by hydrolysing starch by boiling with a dilute mineral acid. In Europe, potato starch is generally employed; in America, corn starch. The acid employed may be hydrochloric, which gives the best results, or sulphuric, which is used in Germany; sulphuric acid is more readily separated from the product than hydrochloric, since the addition of powdered chalk precipitates it as calcium sulphate, which may be removed by a filter press. The processes of manufacture have much in common, although varying in detail. The following is an outline of the process when hydrochloric acid is used: Starch (“green” starch in America) is made into a “milk” with water, and the milk pumped into boiling dilute acid contained in a closed “converter,” generally made of copper or cast iron; steam is led in at the same time, and the pressure is kept up to about 25 &#8468; to the sq. in. When the converter is full the pressure is raised somewhat, and the heating continued until the conversion is complete. The liquid is now run into neutralizing tanks containing sodium carbonate, and, after settling, the supernatant liquid, termed “light liquor,” is run through bag filters and then on to bone-char filters, which have been previously used for the “heavy liquor.” The colourless or amber-coloured filtrate is concentrated to 27° to 28° B., when it forms the “heavy liquor,” just mentioned. This is filtered through fresh bone-char filters, from which it is discharged as a practically colourless liquid. This liquid is concentrated in vacuum pans to a specific gravity of 40° to 44° B., a small quantity of sodium bisulphite solution being added to bleach it, to prevent fermentation, and to inhibit browning. “Syrup glucose” is the commercial name of the product; by continuing the concentration further solid glucose or grape sugar is obtained.

Several brands are recognized: “Mixing glucose” is used by syrup and molasses manufacturers, “jelly glucose” by makers of jellies, “confectioners’ glucose” in confectionery, “brewers’ glucose” in brewing, &c.

GLUCOSIDE, in chemistry, the generic name of an extensive group of substances characterized by the property of yielding a sugar, more commonly glucose, when hydrolysed by purely chemical means, or decomposed by a ferment or enzyme. The name was originally given to vegetable products of this nature, in which the other part of the molecule was, in the greater number of cases, an aromatic aldehydic or phenolic compound (exceptions are sinigrin and jalapin or scammonin). It has now been extended to include synthetic ethers, such as those obtained by acting on alcoholic glucose solutions with hydrochloric acid, and also the polysaccharoses, e.g. cane sugar, which appear to be ethers also. Although glucose is the commonest sugar present in glucosides, many are known which yield rhamnose or iso-dulcite; these may be termed pentosides. Much attention has been given to the non-sugar parts of the molecules; the constitutions of many have been determined, and the compounds synthesized; and in some cases the preparation of the synthetic glucoside effected.

The simplest glucosides are the alkyl esters which E. Fischer (Ber., 28, pp. 1151, 3081) obtained by acting with hydrochloric acid on alcoholic glucose solutions. A better method of preparation is due to E. F. Armstrong and S. L. Courtauld (Proc. Phys. Soc., 1905, July 1), who dissolve solid anhydrous glucose in methyl alcohol containing hydrochloric acid. A mixture of - and -glucose result, which are then etherified, and if the solution be neutralized before the -form isomerizes and the solvent removed, a mixture of the - and -methyl ethers is obtained. These may be separated by the action of suitable ferments. Fischer found that these ethers did not reduce Fehling’s solution, neither did they combine with phenyl hydrazine at 100°; they appear to be stereo-isomeric -oxidic compounds of the formulae I., II.: The difference between the - and -forms is best shown by the selective action of enzymes. Fischer found that maltase, an enzyme occurring in yeast cells, hydrolysed -glucosides but not the ; while emulsin, an enzyme occurring in bitter almonds, hydrolyses the but not the. The ethers of non-fermentable sugars are themselves non-fermentable. By acting with these enzymes on the natural glucosides, it is found that the majority are of the -form; e.g. emulsin hydrolyses salicin, helicin, aesculin, coniferin, syringin, &c.

Classification of the glucosides is a matter of some difficulty. One based on the chemical constitution of the non-glucose part of the molecules has been proposed by Umney, who framed four groups: (1) ethylene derivatives, (2) benzene derivatives, (3) styrolene derivatives, (4) anthracene derivatives. A group may also be made to include the cyanogenetic glucosides, i.e. those containing prussic acid. J. J. L. van Rijn (Die Glykoside, 1900) follows a botanical classification, which has several advantages; in particular, plants of allied genera contain similar compounds. In this article the chemical classification will be followed. Only the more important compounds will be noticed, the reader being referred to van Rijn (loc. cit.) and to Beilstein’s Handbuch der organischen Chemie for further details.

1. Ethylene Derivatives.—These are generally mustard oils, and are characterized by a burning taste; their principal occurrence is in mustard and Tropaeolum seeds. Sinigrin or the potassium salt of myronic acid, C10H16NS2KO9·H2O, occurs in black pepper and in horse-radish root. Hydrolysis with baryta, or decomposition by the ferment myrosin, gives glucose, allyl mustard oil and potassium bisulphate. Sinalbin, C30H42N2S2O15, occurs in white pepper; it decomposes to the mustard oil HO·C6H4·CH2·NCS, glucose and sinapin, a compound of choline and sinapinic acid. Jalapin or scammonin, C34H56O16, occurs in scammony; it hydrolyses to glucose and jalapinolic acid. The formulae of sinigrin, sinalbin, sinapin and jalapinolic acid are:—



2. Benzene Derivatives.—These are generally oxy and oxyaldehydic compounds. Arbutin, C12H16O7, which occurs in bearberry along with methyl arbutin, hydrolyses to hydroquinone and glucose. Pharmacologically it acts as a urinary antiseptic and diuretic; the benzoyl derivative, cellotropin, has been used for tuberculosis. Salicin, also termed “saligenin” and “glucose,” C13H18O7, occurs in the willow. The enzymes ptyalin and emulsin convert it into glucose and saligenin, ortho-oxybenzylalcohol, HO·C6H4·CH2OH. Oxidation gives the aldehyde helicin. Populin, C20H22O8, which occurs in the leaves and bark of Populus tremula, is benzoyl salicin.

3. Styrolene Derivatives.—This group contains a benzene and also an ethylene group, being derived from styrolene C6H5·CH:CH2. Coniferin, C16H22O8, occurs in the cambium of coniferous woods. Emulsin converts it into glucose and coniferyl alcohol, while oxidation gives glycovanillin, which yields with emulsin glucose and vanillin (see and ). Syringin, which occurs in the bark of Syringa vulgaris, is methoxyconiferin. Phloridzin, C21H24O10, occurs in the root-bark of various fruit trees; it hydrolyses to glucose and phloretin, which is the phloroglucin ester of para-oxyhydratropic acid. It is related to the pentosides naringin, C21H26O11, which hydrolyses to rhamnose and naringenin, the phloroglucin ester of para-oxycinnamic acid, and hesperidin, 