Origin of Ferments.—In order to produce the different kinds of fermentation, the necessary ferment must be added, unless it is already contained in the fermentable matter or in the air. In the manufacture of beer and bread, yeast must be used; the other kinds of fermentation, except alcoholic, can generally be produced by the ferments or their spores furnished by the atmosphere; but Pasteur, in the course of his investigations, never produced alcoholic fermentation from spores found in the air. But the germs of the Saccharomyces cerevisiæ and of Mycoderma vini seem to be found only on the surface of fruits, and their stems.[2]
These different germs, however, are all found in the must of grapes, and in wine, and are ready to develop whenever favorable conditions offer themselves, and produce diseases in the wine. It is found that these germs are killed by raising the temperature of the liquid to 140° F., and hence the process of heating wines to preserve them (which see).
Leaving the germ theory of fermentation, we will pass to what is of more practical importance.
ALCOHOLIC FERMENTATION
IN WINE MAKING.
Vinous or Alcoholic Fermentation transforms the juice of the grape into wine, and, as already shown, is caused by the yeast or ferment, which finds its way into the must; and by this fermentation the sugar of the grape is changed principally into alcohol, and carbon dioxide, or carbonic acid gas. And in order to show the relations between the sugar and the alcohol produced, it is necessary to say something about the chemical constituents of each.
Sugar.—In general terms, cane sugar may be expressed by the chemical formula, C₁₂H₂₂O₁₁, or, in other words, one molecule contains 12 atoms of carbon, 22 of hydrogen, and 11 of oxygen.
And the general term glucose, or grape sugar, may be expressed by the formula C₆H₁₂O₁₆, or one molecule contains 6 atoms of carbon, 12 of hydrogen, and 6 of oxygen.
If, instead of using the word atoms, we use the word pounds, the chemical formula may be made clear to the unscientific. Taking the formula for cane sugar, already given, it simply means that 342 pounds contain the following ingredients, in the following proportions:
| lbs. | lbs. | ||||
| 12 | parts | carbon, each | weighing | 12, | 144 |
| 22 | “ | hydrogen, | “ | 1, | 22 |
| 11 | “ | oxygen, | “ | 16, | 176 |
| 342 | |||||
And the formula for glucose means that 180 pounds contain: