On the subject of the fermentation of the grape-juice we shall only offer a few remarks. It has been already stated that the saccharine contents 'of grape-juice range from 13 to 30 per cent. If we regard all this sugar as grape-sugar, 0,2E12012, with an equiva lent of 180, then each atom may be resolved into 2 atoms of alcohol, C4H602, with an equivalent of 46, and 4 of carbonic acid gas, CO,, with an equivalent of 22, according to the equation Alcohol Carbonic Acid.
C111112012 = 4(CO2), provided that there is no loss; or under the most favorable conditions of fermentation, 180 parts (by weight) of anhydrous grape-sugar, or 198 of the hydrated sugar (with the formula, 0,2E14014), may yield 92 parts of alcohol; or, roughly speaking, 2 parts of sugar yield 1 of alcohol. "According to this," says Mulder, the juice of French and German grapes gives, when analyzed, as a maximum, from 7 to 15 per cent of alcohol by weight. But sonic of the sugar remains undissolved, and. during fermentation, more alcohol is evaporated than water; therefore, for such grape-juice, or rather for the wine to be produced from it, the alcoholic contents must be under 15 per cent as a maximum, and 7 per cent as minimum."—Op. cit., pp, 49, 50. According to Mulder, sugar is found In all wine,* and its quantity depends to a considerable extent upon the treatment to which the grapes are subjected before pressure. Tokay wine, for example, is prepared frord de have been allowed not only to get over-ripe, but partly to dry on the vines; IL de prude is obtained from grapes dried on straw exposed to the sun ; and in both these cases, water is evaporated, and the concentrated juice yields a wine of extra strength. The strong heavy wines used by the ancients were thus prepared. When the grapes are dried on the vine, the wine is called yin sec; and when the juice has been evaporated by the aid of heat, the wine is called vin eotti.
In consequence of the close connection which exists between the amount of sugar in the grape-juice and the excellence of the wine which it yields, attempts are often made, especially in bad soasous (want of heat and light, and excess of rain), to introduce extraneous sugar into the juice; or, as it is technically called to doctor it. For this pur pose, a cheap fermentable sugar is added to the sour juice, an adulteration which cannot subsequently be detected by chemistry, although it may be suspected, from,the absence of the proper aroma from the wine. Similarly, sugar is often added to good grape-juice, in order to obtain a stronger wine than the natural product. Many imitations of port wine are thus manufactured. The character of the wine is much influenced by the extent to which the process of fermentation is allowed to proceed. If it goes on till all the sugar is converted into alcohol, a dry wine is produced; when it is checked before the change is completed, a rich fruity wine is produced; and when the wine is bottled •while the fermentation is still in progress, effervescent wine is formed.
Shortly after the must has passed from the wine-press symptoms of fermentation appear; the juice becomes more turbid, bubbles rise to the surface, and a froth soon settles there. This process in a moderate climate usually reaches its highest point in three or four days; and before it is quite finished the whole liquid mass is stirred up so as to re excite the process. For this purpose, in many districts a naked man used (we do not know if the custom generally still exists) to go into the wine-tub, who both accomplished the necessary stirring and promoted fermentation by his animal heat. Several persons have been killed in this way by suffocation from the atmosphere of carbonic acid gas.
In two or three weeks the fluid becomes comparatively clear, and a precipitate forms at the bottom. The wine is now removed from the sediment into another vessel, and a slow form of fermentation—after-fermentation., as it is termed—goes on for several months, sugar being constantly converted into alcohol and carbonic acid, and a fresh precipitate forming at the bottom. Several similar changes into other vessels are made, to get rid of the sediment, till it is fit for transferring into casks. That the process of fermenta tion may go on satisfactorily, not only must water, sugar, and a nitrogenous matter in a state of actual change be present, but there must be a certain temperature and a certain amount of atmospheric air present. " Although," says Mulder, " there is a wide inter val between the extremes of temperature at which fermentation is possible, the bound ary is very narrow which limits good and active fermentation in every kind of wine. The grapes of each country ripened under different degrees of summer warmth and very unequally rich in constituents, require very different temperatures during fermentation; and different temperatures are also required for grapes which are the product of a warmer or a colder summer. But on these points we have little accurate knowledge. All we know is that a high temperature during autumn promotes fermentation, and a low one is detrimental to it; and that inequality of temperature during fermentation is extremely injurious, and not unfrequently spoils the wine altogether."—Op. cit. p. 61. To what extent it is expedient to admit atmospheric air to the must, so that the fermen tation may go on most favorably, is a point regarding which there has been much dis cussion, and which is not definitely settled. While some have asserted that no air is necessary to the development of fermentation, others have maintained that the wine Is improved by the free admission of air during fermentation. Gay-Lussac proved experi mentally that air is essential to initiate fermentation, which would then be con tinued without any fresh supply; and for many years wine was made in France with an almost total exclusion of air from the fluid by an arrangement intended to prevent the escape of alcohol by evaporation; hut when the same chemist proved that by the use of open vats scarcely part of the alcohol was lost, this arrangement fell into disuse. Judging from the method of preparing Bavarian beer, in which air is allowed to enter freely, Liebig recommended the same in the ease of wine, and suggested that a large opening should be made in the casks in which fermentation takes place. This method has been tried on a large scale by Von Babo, Crasso, and others, with red wine, which was found to be of a better quality than that which underwent the same process in a cask which was closed, and only provided with a glass tube for the escape of the car bonic acid. But in other experiments made with white wine, the wine in open casks appeared to lose in aroma; and hence the solution of this question apparently depends on the kind of wine. Liebig's opinion has been very fiercely, and, as Mulder thinks, unfairly attacked; the probability is that wines containing much sugar may be allowed with advantage to ferment in closed vessels, while those less rich in that substance may be left in open casks, provided the temperature be low and equable. the main object is to increase the quantity of alcohol, the admission of much i air s injurious, since it promotes the formation of acetic acid, and causes a corresponding loss of lcohol.