WINE. The chemistry of wine presents many points of interest, md may be treated of independently of its history, manufacture, com merce, and uses.
In the juice of the grape, fermentation is excited by the access of air, alcohol and carbonic acid being formed by the decomposition of the sugar contained in the fluid c„it„o„ = 4CO, Sngar. Alcohol. Carbonic acid.
fho process onto commenced, continues independently of any further influence of the air. In addition to the alcohol and carbonic acid formed by the fermentation of the . juice, there is also produced a yellow or gray insoluble substance, containing a large quantity of nitrogen. It is this body which possesses the power of inducing fermentation in a new solution of sugar, and which has in consequence received the name of ferment. [FERMENT.] The alcohol and carbonic acid are produced, as above indicated, from the elements of the sugar ; rut the feement is formed from those azotised constituents of the grape juice which have collectively been termed gluten. [GLUTEN.] Gluten liseolved in pure water undergoes a process of decomposition; but the lecomposition which it suffers in an isolated state, and that which it undergoes when dissolved iu a vegetable juice, belong to two different kinds of transformations. There is reason to believe that its change to the insoluble state depends ou an absorption of oxygen ; for its sepa ration in this state may be effected under certain conditions by free exposure to the air without the presence of fermenting sugar. It is known also that the juice of grapes, or vegetable juices, in general become turbid when in contact with air before fermentation com mences ; and this turbidity is owing to the formation of an insoluble precipitate of the same nature as ferment. The oxygen consumed in the fermentation of wine or beer is not taken from the atmosphere, theugh the access of this is necessary to excite it in the first instance.
Gluten seems to act towards sugar as diastase does towards starch, namely, imparts that impetus to it which enables it to alter its con dition. When both gluten and sugar are present in a liquid, fermen tation will go on till the decomposition of one or other be complete.
When the quantity of ferment is too small in proportion to that of the sugar, its fermentative putrefaction will be completed before the trans formation of all the sugar is effected. Some sugar here remains undo composed, as the cause of its transformation is absent, namely, contact with a body in a state of decomposition : this happens in the vies dc liqueurs, the fruity or sweet wines. But when the quantity of ferment predominates, a certain quantity of it remains after all the sugar has fermented, its decomposition proceeding very slowly on account of its insolubility in water. This residue is still able to induce fermentation when introduced into a fresh solution of sugar, and retains the same power until it has passed through all the stages of its own transforma tion. Hence a certain quantity of yeast is necessary in order to effect the transformation of a certain portion of sugar ; not because it acts by its quantity in increasing any affinity, but because its influence depends solely on its presence, and its presence is necessary until the last atom of sugar is decomposed.
Climate and soil greatly modify wine both iu kind and quality. Differences, however, in the vintage and manufacture often determine the character of wines made in the same district, and cause them to widely differ from each other in odour, flavour, and even colour. Such differences often occur spontaneously, as in the case of amontillado.
The quantity of azotised matter in the juice seems to be the same in whatever part the grapes may grow ; at least no difference has been observed in the amount of yeast formed during fermentation in the south of France and on the Rhine. The grapes grown in hot climates, as well as the boiled juice obtained &mil them, are proportionally rich in sugar. Hence, during the fermentation of the juice, the complete decomposition of its azotised matters, and their separation in the insoluble state, are effected before all the sugar has been converted into alcohol and carbonic acid. A certain quantity of the sugar conse quently remains mixed with the wine in an undecomposed state, the condition necessary for its further decomposition being absent. The azotiscd matters in the juice of grapes of the temperate zones, on the contrary, are not completely separated in the insoluble state when the entire transformation of the sugar is effected. The wiue of these grapes, therefore, does not contain sugar. but variable quantities of undecomposed gluten in solution. This gluten gives the wine the property of becoming spontaneously.converted into vinegar when the access of air is not prevented ; for it absorbs oxygen and becomes insoluble, and its oxidation is communicated to the alcohol, which is converted into acetic acid. By allowing the wine to remain at rest in casks with a very limited access of air. and at the lowest possible tem perature, the oxidation of this azotised matter is effected without the alcohol undergoing the same change, a higher temperature being neces sary to enable alcohol to combine with oxygen. As long as the wine in the stilling-casks deposits yeast, it can still be caused to ferment by the addition of sugar ; but old well-layed wine has lost this property, because the condition necessary for fermentation—namely, a substance in the act of decomposition or putrefaction—is no longer present in it. In hotels and other places, where wine is drawn gradually from a cask, and a proportional quantity of air necessarily introduced, its erema eausis—that is, its conversion into acetic acid—is prevented by the addition of a small quantity of sulphurous acid. This acid, by entering into combination with the oxygen of the air contained in the cask or dissolved in the wine, prevents the oxidation of the organic matter.