Galactose occurs in the animal kingdom as one of the constituents of lactose, or milk-sugar. It is also one of the constituents of raffinose, a trisaccharide sugar found in plants, and occurs as "galactans" in many gums and sea-weeds. The d-galactose, obtained by the hydrolysis of any of these compounds, is a faintly sweet substance which resembles glucose in many of its properties; having one characteristic difference, however, in that it forms mucic acid instead of saccharic acid when oxidized by concentrated nitric acid. These oxidation products are very different in their physical properties and this difference serves to distinguish between the two sugars from which they are derived.
Fructose (levulose, honey sugar, or "diabetic" sugar) occurs along with glucose in the juices of many fruits, etc. It is a constituent of sucrose, of raffinose, and of the polysaccharide inulin, from which it may be obtained by hydrolysis. It is a ketose sugar, reduces Fehling's solution, forms the same osazone as glucose, and is easily fermentable by yeast. Its sweetness is slightly greater than that of ordinary cane sugar. d-fructose (the ordinary form) is easily soluble in water, and is strongly levorotatory, its specific rotatory power at 20° C. being -92.5°; it is unique in the very large effect which is produced in its rotatory power by increasing the temperature of the solution; at 87° its specific rotatory power is reduced to -52.7°, exactly equal to but in the opposite direction of the effect of glucose; hence, invert sugar, which is a mixture of an equal number of molecules of glucose and fructose, and which has a specific rotatory power of -19.4° at 20° C., becomes optically inactive at 82° C.
Sorbose is the only other ketohexose which has any importance in plant chemistry. It does not occur free in plants, but is the first oxidation product from the hexatomic alcohol, sorbitol, which is present in the juice of the berries of the mountain-ash. Sorbose is a crystalline solid, which is not fermentable by yeast, but which otherwise closely resembles fructose.
DISACCHARIDES
The disaccharides, having the formula C12H22O11, may be regarded as derived from the monosaccharides by the linking together of two hexose groups with the dropping out of a molecule of water, in the same way that many other organic compounds form such linkages. That this is a perfectly correct conception, is shown by the fact that, when hydrolyzed, the disaccharides break down into two hexose sugars, thus
C12H22O11 + H2O = C6H12O6 + C6H12O6.
With all known disaccharides, at least one of the hexoses obtained by hydrolysis is glucose; hence all disaccharides may be regarded as glucosides (C6H12O5·R) in which the R is another hexose group.
Since hexoses have both alcoholic and aldehyde groups, and since either of these types of groups may function in the linkage of the two hexoses to form a disaccharide, it is possible for two hexoses, both of which are reducing sugars to be linked together in three different ways: (1) through an alcoholic group of each hexose, (2) through an alcoholic group of one and the aldehyde group of the other, and (3) through the aldehyde group of each hexose. Disaccharides linked in either of the first two ways will be reducing sugars, since they still contain a potentially active aldehyde group; but those of the third type will not be reducing sugars, since the linkage through the aldehyde groups destroys their power of acting as reducing agents. Examples of each of these three types of linkage are found among the common disaccharides, as will be pointed out below.
The following table shows the general characteristics of the common disaccharides.
| Type 1.—Aldehyde group potentially active, reducing sugars: | |
| Sugar | Components |
|---|---|
| Maltose | Glucose and glucose |
| Gentiobiose | Glucose and glucose |
| Lactose | Glucose and galactose |
| Melibiose | Glucose and galactose |
| Turanose | Glucose and fructose |
| Type 2.—Non-reducing sugars: | |
| Sucrose | Glucose and fructose |
| Trehalose | Glucose and glucose |