The nature and action of yeast have been the subject of chemical investigation. Lavoisier ascertained, that, be sides the other elements which are usually found in vege tables, it contained a quantity of nitrogen in its composition, and so far evinced a connection with animalized matter. The researches of Fabroni, Thenard, and Seguin have de monstrated that the fermenting property of yeast is owing to the presence of a substance resembling gluten or albu men, which is derived from certain vegetable infusions, ca pable of spontaneous fermentation.
When circumstances are sufficiently favourable for the vinous fermentation, the liquor, in passing into it, becomes somewhat turbid, and manifests a kind of commotion throughout its whole mass. Air bubbles begin to be sepa rated, and being entangled by the flocculent part of the li quid, occasion a frothy appearance on the surface. In the meantime, the temperature gradually increases, and some times requires to be checked. The gas which is disen gaged is found to consist chiefly of carbonic acid, mixed occasionally with a portion of hydrogen. At length, the extrication of air, and the intestine commotion with which it is accompanied, gradually diminish, and the process ter minates by the liquor recovering its transparency. If the fermented substance be now examined, it is found to have exchanged its sweet taste for one of considerable pungency, and to have acquired the property of acting as a powerful stimulant on the animal system.
The vinous fermentation depending in a great measure upon the separation of carbon, the process may be conduc ted without the aid of atmospheric air ; and, indeed, it has been found by experiment, that, if the air be excludedi while the gas disengaged by the fermentation is permitted to make itAkescape, the vinous product is stronger than when the process is carried on in open vessels. In that case, however, as Chaptal remarks, the fermentation ad vances more slowly.
The products to which the vinous fermentation gives birth, resemble one another by possessing an intoxicating quality, and yielding, by distillation, a portion of alcohol ; but they differ considerably in their strength, odour and flavour. Their differences, in these respects, are owing, partly to the essential oils, and other proximate principles which they derive from the substances submitted to the vi nous process, and partly to the manner of conducting the fermentation. They may be considered as of two general descriptions : Wines, properly so called, and the various kinds of ale or beer.
Wines are obtained by subjecting to fermentation the sweet juices of fruits, particularly that of the grape. The products are extremely diversified, and vary in flavour and appearance with the climate, soil, and the nature and cul ture of the vine, as well as with the manner of carrying on the fermentative process. Hence wines are sweetish, and weaker or stronger, according as the saccharine matter of the grape is more or less abundant, and the fermentation more or less complete ; and they are sharp and sparkling, when part of the carbonic acid, which is generated during the process, is retained. Astringent wines derive their pe culiar flavour from the astringent principle contained in the grape from which they arc Formed. The colour is com municated by the external pellicle of the fruit, and might easily be prevented, if necessary, by removing the husks, before fermentation. Wines of every description contain a certain portion of supertartratc of potash, which gradually separates from them, when they are left undisturbed ; and to this is owing, in a great degree, the improvement of wines by age.
France produces a great variety of excellent wines. For daily use, Fourcroy prefers those of Burgundy, because all their principles are duly combined, and none of them pre dominates, so as to communicate a peculiarity of flavour. The wines of Orleans, after being matured by age, resem ble those of Burgundy. The red wines of Champagne are highly prized for their excellence and delicacy ; though, in sonic cases, they possess a pungent and sourish taste, from being bottled before the carbonic acid is sufficiently disengaged by fermentation. The wines of Languedoc and Guienne are greatly esteemed, on account of their tonic qualities, particularly when they are mellowed by age. Those of Anjou are strong, spirituous, and intoxi cating.
The Rhenish and Moselle wines have a cool, sharp taste, and readily intoxicate. Some Italian wines, such as those of Orviette, Vicenza, and Lacryma Christi, arc well fer mented, and resemble the French wines. Those of Spain are in general boiled, sweet, and being but partially fer mented, cannot be reckoned wholesome. The wines of Rota and Alicant must, however, be excepted, and are justly considered as useful stomachics. The wines of Por tugal have been long in great demand in this country ; and, indeed, no wine can be accounted superior to good old port.