A more scientific nomenclature has recently been proposed by Fischer, in which glucose is used as the equivalent of dextrose and fructose as the proper name for levulose. All sugars are further classified by Fischer into groups according to the number of carbon atoms found in the molecule. We have thus trioses, tetroses, pentoses, hexoses, etc. Such a sugar as sucrose is called hexobiose by reason of the fact that it appears to be formed of two molecules of hexose sugars. For a similar reason raffinose would belong to the hexotriose group.[24]
Again, the two great classes of sugars as determined by the structure of the molecule are termed aldoses and ketoses according to their relationship to the aldehyd or ketone bodies.
Since sugars may be optically twinned, that is composed of equal molecules of right and left-handed polarizing matter it may happen that apparently the same body may deflect the plane of polarization to the right, to the left, or show perfect neutrality.
Natural sugars, as a rule, are optically active, but synthetic sugars being optically twinned are apt to be neutral to polarized light.
To designate the original optical properties of the body therefore the symbols d, l, and i, meaning dextrogyratory, levogyratory, and inactive, respectively, are prefixed to the name. Thus we may have d, l, or i glucose, d, l, or i fructose, and so on.
The sugars that are of interest here belong altogether to the pentose and hexose groups; viz., C₅H₁₀O₅ and C₆H₁₂O₆, respectively. Of the hexobioses, sucrose, maltose, and lactose are the most important, and of the hexotrioses, raffinose. In this manual, unless otherwise stated, the term dextrose corresponds to d glucose, and levulose to d fructose. In this connection, however, it should be noted that the levulose of nature, or that which is formed by the hydrolysis of inulin or sucrose is not identical in its optical properties with the l fructose of Fischer.
46. Preparation of Pure Sugar.—In using the polariscope or in testing solutions for the chemical analysis of samples, the analyst will be required to keep always on hand some pure sugar. Several methods of preparing pure sugar have been proposed. The finest granulated sugar of commerce is almost pure. In securing samples for examination those should be selected which have had a minimum treatment with bluing in manufacture. The best quality of granulated sugar when pulverized, washed with ninety-five per cent and then with absolute alcohol and dried over sulfuric acid at a temperature not exceeding 50° will be found nearly pure. Such a sugar will, as a rule, not contain more than one-tenth per cent of impurities, and can be safely used for all analytical purposes. It is assumed in the above that the granulated sugar is made from sugar cane.
Granulated beet sugars may contain raffinose and so may show a polarization in excess of 100. This sugar may be purified by dissolving seventy parts by weight in thirty parts of water. The sugar is precipitated by adding slowly an equal volume of ninety-six per cent alcohol with constant stirring, the temperature of the mixture being kept at 60°. While still warm the supernatant liquor is decanted and the precipitated sugar washed by decantation several times with strong warm alcohol. The sugar, on a filter, is finally washed with absolute alcohol and dried in a thin layer over sulfuric acid at from 35° to 40°. By this process any raffinose which the sugar may have contained is completely removed by the warm alcohol. Since beet sugar is gradually coming into use in this country it is safer to follow the above method with all samples.[25] In former times it was customary to prepare pure sugar from the whitest crystals of rock candy. These crystals are powdered, dissolved in water, filtered, precipitated with alcohol, washed and dried in the manner described above.
47. Classification of Methods.—In the quantitive determination of pure sugar the various processes employed may all be grouped into three classes. In the first class are included all those which deduce the percentage of sugar present from the specific gravity of its aqueous solution. The accuracy of this process depends on the purity of the material, the proper control of the temperature, and the reliability of the instruments employed. The results are obtained either directly from the scale of the instruments employed or are calculated from the arbitrary or specific gravity numbers observed. It is evident that any impurity in the solution would serve to introduce an error of a magnitude depending on the percentage of impurity and the deviation of the density from that of sugar. The different classes of sugars, having different densities in solution, give also different readings on the instruments employed. It is evident, therefore, that a series of tables of percentages corresponding to the specific gravities of the solutions of different sugars would be necessary for exact work. Practically, however, the sugar which is most abundant, viz., sucrose, may be taken as a representative of the others and for rapid control work the densimetric method is highly useful.
In the second class of methods are grouped all those processes which depend upon the property of sugar solutions to rotate the plane of polarized light. Natural sugars all have this property and if their solutions be found neutral to polarized light it is because they contain sugars of opposite polarizing powers of equal intensity. Some sugars turn the polarized plane to the right and others to the left, and the degree of rotation in each case depends, at equal temperatures and densities of the solutions, on the percentages of sugars present. In order that the optical examination of a sugar may give correct results the solution must be of a known density and free of other bodies capable of affecting the plane of polarized light. In the following paragraphs an attempt will be made to give in sufficient detail the methods of practice of these different processes in so far as they are of interest to the agricultural analyst. The number of variations, however, in these processes is so great as to make the attempt to fully discuss them here impracticable. The searcher for additional details should consult the standard works on sugar analysis.[26]