The wines formed from other firths, as currants, goose berries, apples, pears, &c. are greatly inferior to those ob tained from the grape. The juices of these fruits abound too much with acid, and too little with saccharine matter, to afford of themselves even tolerable wines ; and they must, therefore, be improved by the addition of sugar, be fore fermentation. The wines from the juices of the apple and pear, denominated cyder and perry, contain a large portion of the acids of these fruits, and a considerable quantity of carbonic acid : to the presence of the latter is owing their sharpness and sparkling property. Cherries furnish a very pleasant wine ; apricots, peaches, and prunes, afford wines of an indifferent quality.
The nutritive grains, and particularly barley, furnish a fermented liquor of a vinous nature, called ale or beer. The grain, after being converted into malt, is first reduced to a coarse powder in a mill, or bruised between rollers. It is then infused in hot water, at the temperature of about 160° or 170°, and allowed to macerate for a few hours ; after which the liquor is drawn off, and a fresh quantity of water is added. The infusion thus obtained. is denotninat edwgrt. Before being allowed to ferment, the wort is boiled with some bitter vegetable substance, commonly hops ; partly with a view of correcting any ascescent ten dency, and partly of improving the flavour of the liquor. To promote fermentation, a quantity of yeast is added to the infusion, after it has been allowed to cool ; but the process is usually checked before it has been completely finished, and the liquor is then drawn off. \V hen the ob ject of the fermentation is to obtain a wort for distillation, part of the grain, as we formerly observed, is used in a raw or unmalted state. The process, in that case, is con ducted more rapidly, and allowed to proceed to its utmost extent. See BREWING.
With respect to the theory of the chemical changes upon which the vinous fermentation depends, we have ;till no very precise knowledge. The subject was investigated by Lavoisicr ; but though he was the first who gave any thing like an accurate account of the process, the conclu sions which he deduced from his experiments, are by no means free from objections. He dissolved a quantity of pure sugar in water, and caused the solution to ferment by the addition of yeast. The products were the carbonic acid gas disengaged during the process, and the alcohol remaining after its termination ; and having previously de termined sugar to be a compound of oxygen, hydrogen, and carbon, he inferred, that as no portion of the water of the fermenting liquor suffered decomposition, part of the oxygen and carbon of the sugar must have united to form carbonic acid, while the remaining part of these elements combined with the hydrogen, and produced alcohol. In
this explanation, the operation of the yeast is entirely dis regarded ; and it seems to be taken for granted, that dur ing the fermentation that substance suffers no change whatever, a circumstance which is by no means probable.
According to Thenard, the yeast excites fermentation by its carbon abstracting from the saccharine matter part of its oxygen, and thus forming at least a portion of the car bonic acid disengaged during the process; while the re maining elements of the sugar enter into combination with the hydrogen and nitrogen of the ferment, and produce the vinous liquor. From some experiments which he in stituted with the view of supporting this opinion, he found that the nitrogen of the yeast disappeared during the fer mentation. Ile ascertained that it did not make its escape along with the carbonic acid, and he therefore inferred, that it must have entered into composition with the vinous pro duct, though he did not succeed in detecting it by any mode of analysis.
Seguin has proposed a theory of fermentation which dif fers considerably from that of Thenard. Ile is of opinion, that during the process water suffers decomposition ; and that its oxygen combines with the carbonaceous part of the yeast to form carbonic acid, while its hydrogen unites with the saccharine matter, and produces the fermented liquor. To this theory it may be objected, that, besides being liable to the difficulty of accounting for the great quantity of car bonic acid extricated by referring it entirely to the carbon of the ferment, a greater weight of alcohol ought to be procured by fermentation than that of the sugar subjected to the process ; which is contrary to experience, as little more than half the quantity is obtained. To this it may be ;Added, that alcohol contains less oxygen than sugar. Upon the whole, the hypothesis of Seguin is perhaps less probable than that of Thenard; though in the present state of our knowledge on this subject, it would be easy to sug gest various other theories of equal plausibility.