Very sweet grapes yield sweet and full-bodied wine, because the ferment is not in sufficient quan tity to decompose all the sugar; less sweet grapes may require sugar to be added to the must to nourish tbe action of the ferment and to employ the whole of it in producing alcohol. Very thin must is as difficult te ferment as very thick must. The mean consistence should be 10°.5 to 11'5 B. (about P080 sp. gr.). When the must is very watery, the resulting wine is weak and very liable to change. Cold countries, wet lauds, and rainy seasons produce grapes containing more water and ferment than is necessary to decompose the sugar formed in the fruit, and the wine is liable to turn sour in consequence of the superabundance of ferment remaining after the spirituous fermentation. Mention has already been made of a means adopted for correcting this evil. In sugaring the must, it is necessary to remember that cane-sugar does not undergo alcoholic fermentation till it has arrived at a more hydrated condition than the sugars of fruit, or glucose. To avoid the delay thus occasioned in the fermentation, it is well to transform the crystallizable sugar before introducing it. Tartaric acid is preferably empleyed for this purpose, and the sugar solution should be boiled in the must for two hours in presence of 2 per cent. of this acid. If the grapes were insipid and flat, only a partial neutralization by the aid of chalk is needed, while with very acidulated musts the tartaric acid should be completely neutralized. By this plan is obviated the fault ascribed to the sweetening of some musts that it leaves a sickly taste in the mouth, due to the fact that the sugar used has not been entirely decomposed, because then all the added sugar is transformed into alcohol as rapidly as the grape sugar itself. Neither need there be any fear that the addition of the sugar will postpone the commencement of the fermentation ; therefore it may be added as early as desired, instead of waiting till towards the end of the operation. Obviously the sugar may also be converted by means of sulphuric acid or phosphoric acid on condition that the subsequent neutralization be complete ; but tartaric acid is specially nained, as it is normally found in the fruit of the vine, and a solution of it may be economically prepared by treating dry lees, which abound in vine countries.
The fermentation first manifests itself by little bubbles which appear on the surface of the must : little by little it raises the centre of the mass, agitates the whole, and produces more or less effer vescence, due to the liberation of carbonie acid. The suspended matters are distributed, raised, and precipitated until one portion settles on the bottom and another portion collects on the surface to form the "bead." The fermentation eommenees as soon as the vat is filled, and lasts ordinarily from twenty-four to thirty hours, with a temperature of 30° to 32° (86° to 89^ . 3 F.) in the centre. The volume of the mass increases considerably. The " head " raises itself, cracks open, and disengages abundance of froth ; heat develops in the liquor in proportion to the mass, and the colour is in tensified, Then the symptoms diminish, the head sinks, the liquor clarifies itself, and the work is ne,arly terminated. The heat, being greatest in the centre, must be equalized by pressing down and agitating the mass. The effervescence, agitation, and heat observable in the must are more intense when the skins, pippins, and stems are present. If the movement relaxes, the vat is mashed
and the head precipitated several times. This is done by means of a stirrer, but it must be dispensed with when the skins exhibit arty change. Acidity in the head is prevented by protesting it from the action of the air ; and by preeipitating the froth in the bath, the ferment it contains becomes mixed with the liquid and nourishes the fermentation. The moreeeompletely the grapes have been crushed and the more carefully the saccharine fluid has been extracted from the oells, the more regularly the alcoholic fermentation develops itself.
Sometimes instead of ncingling the skins with the must in the vat, a lid piereed with hales is fitted to it in such a way that the skins are thus kept immersed in the must. This arrangement has some advantages and 'some drawbacks. Clearly it avoids the necessity for agitating the mass, sinee the solid particles are plunged into the liquid and the wine reaches the surface. A second cover surmounts the vat, but if it does not close hermetically and the air obtains free access to the wine, there is danger of its causing acidity. The fermentation may be left to complete itself without any manipulation, provided the second cover be perfectly closed, preserving only an exit for the generated gas by means of a bunghole of suffieient size. This plan, however, is not in favour with the best authorities, who declare that considerable advantage is derived from mashing the mass while in the vat, since the wines gain in quality by the agitation.
The earbonie acid which disengages itself from the mass, and the deleterious effects of which are well known, displaces the atmospheric air resting on the vat, and then falls to the bottom of the room by reason of its density. Ventilation must he provided with the utmost care iu order to avoid the risk of suffocation to the workmen. Milk of lime and alkaline solutions absorb it.
The proportion of alcohol produced is in accordance with the sugar which the fruit contains, and is, in eonsequenee, very variable, since the musts may have any density from about 8° to 18° B. (about 1.060 to 1.143 sp. gr.). The proper moment for drawing off the wine and placing it in casks is when the fermentation has rendered the presence of the sugar insensible and replaced it by aleohol, though all the sugar has not absolutely disappeared, but sufficient still remains to excite the complementary fermentation in the casks. Concerning the colour of the wine, it may be remarked that it is more intense according as the fruit has heen more pressed and longer fermented, as the grapes are riper, and as the wine is more alcoholic. As a general principle, lively and prolonged fermentation is destructive of bouquet and aroma, which e,onstitute the merit of some wines; on the contrary, a very complete fermentation should be allowed to wines whose principal quality is alcoholie strength. Finally, energetic mashing, often repeated, prevents ebange in the " head," provided that the must has entered freely into fermentation under the infiuenee of a suffieient temperature and with a suitable proportion of sugar. The disengage ment of carbonic acid will be such 8.8 to obviate any reason to fear the access of atmospheric air, if the mass does not remain in the vat too long after the liberation of the gas is fioished.