Tannic Acid is a very important ingredient in wine, and is frequently mentioned in this work. (See [Fining], [Tannin].)
Carbonic Acid.—It has been shown in the chapter on Fermentation that carbon dioxide is the gas produced by fermentation. This gas, CO₂, was known to the old chemists as carbonic acid, or carbonic acid gas, and the latter terms are frequently used in this work in the sense of carbon dioxide, in accordance with common usage. But modern chemistry teaches us that carbon dioxide, CO₂, is not an acid at all, but in connection with water it takes up a molecule of the latter, and becomes H₂CO₃, carbonic acid proper. The gas, however, as well as the acid, exists in all wines, and to the former, sparkling wines owe their effervescence. Its presence is important, exercising a preservative effect by preventing their oxidation, and also by keeping in dissolution substances which would otherwise cloud the wine. When the wine is first fermented it is saturated with carbon dioxide, and while it remains so, oxygen will not be absorbed, and hence its preservative effect. Mr. Maumené even recommends resort to artificial means to restore it, or to re-saturate the wine in case of its loss. If, however, the precautions heretofore indicated for keeping table wines are observed, the wine will be well preserved.
Acetic Acid is the result of oxidation, or acetic fermentation (which see), and lactic acid is derived from lactic fermentation, but is regarded as accidental in wine, probably not existing in the must, though it is found in some wines made from grapes which have been bruised and broken a long time before using.
Butyric Acid is the product of butyric fermentation.
Valeric Acid is supposed to exist in wine from the distinctive odor which is smelt in it under certain conditions.
Succinic Acid has been referred to as one of the products of alcoholic fermentation.
The Total Acids in wine vary a good deal, but four or five per mille is a fair average.
Space will not permit of more details on the various substances contained in wine, but those who desire further information are referred to the work of Maumené in French, and that of Thudichum and Dupré in English.
The Bouquet proper of wines is a perfume containing different odors, like that of a bouquet of flowers. It is very complex also in its origin, and the matters contained in the wine which give rise to it are but little known. It is variable, being different in different wines, and all the odorous matters doubtless contribute to its existence, such as œnanthic, and other ethers, the different alcohols, aldehydes, and perhaps even certain essential oils. The varieties of grapes, the season, and the soil, also have their effect, as well as the method of vinification. It is believed, however, that the bouquet is principally due to the ethers.
Artificial Bouquet.—In this connection, the experiments of Mr. Maumené will be found interesting. He prepared one litre of a liquid similar to wine, but without a sensible odor, by adding distilled water to the distillate from a red wine of Bordeaux.