living in a liquid medium in order to secure sufficient oxygen procured it from the sugar, thus, as we have said, producing from the latter and alcohol. The production of alcohol hence resulted as a product of metabolism in the body of a living organism.
It has been more recently shown, however, that the active cause of fermentation is to be found not in the yeast itself, but in a ferment (or enzyme) produced by the yeast cell. This ferment Buchner has succeeded in freeing from the cell, so that it is now possible to produce alcoholic fermentation without the presence of the living yeast.
But this discovery does not detract from the work of Pasteur, to whom is due the great credit of definitely showing the importance of living organisms, the yeasts, in the production of alcohol, since without the yeast cell the ferment or enzyme would not be produced.
The nature of the experiments by which Pasteur demonstrated the importance of the yeast is of interest. In the first place he showed that grape juice filtered and kept from contact with the air is not subject to alcoholic fermentation. In the second case he demonstrated that grape juice sterilized by heat is, if similarly protected, unfermentable. In the third case he showed that if the yeasts caught on the filter used in the first series of experiments be added to the sterile juice of the second series, fermentation ensued.
Pasteur was asked the origin of the yeasts which make the alcohol in wine. The question was answered by an experiment. Taking the grapes and completely removing from them the fuzz or "bloom," he extracted the juice free from contact with the air. No fermentation followed, consequently no alcohol resulted. From this it was learned that the yeasts necessary for the production of the alcohol of wine live in nature in the air and are found in abundance on the outside of the grape. If the grapes be crushed the sweet juices serve as food for the yeast plants. These when well fed grow rapidly and, by a simple process of budding, produce myriads of yeast plants. These, like their parents, give rise to ferments which break down the sugar into and alcohol.
It was later found that although these yeasts may increase greatly in numbers, a strong percentage of alcohol is impossible in nature. This is due to the singular fact that when the strength of alcohol increases perceptibly the organisms forming it are unable to thrive in their own product. Hence they increase more slowly. When a strength of 12 per cent, of alcohol is reached reproduction is manifestly checked, and at 14 per cent, all cell activity ceases.
To increase the strength and purity of the alcohol thus formed in nature, man, as we have seen, has resorted to the processes of distillation and rectification by which alcohols practically free from impurities may be obtained in concentration.
