The juice of the grape, when subjected to chemi cal analysis, is found to consist of the following principal ingredients; namely, a considerable portion of water and sugar; extractive matter analogous to mucilage, some tannin, vegetable gelatin, accidula ted tartrate of potash, tartrate of lime, phosphate of magnesia, muriate of soda, sulphate of potash, and a particular liquid substance, generally re garded as an elementary constituent on which the fermentative process depends. These, however, are not always found to exist in the same vine, and they vary considerably in different vines. But some of them, such as tartarous acid, sugar, ex tractive matter, and the liquid substance already referred to, arc essentially necessary to fermenta tion: and it is by possessing these ingredients in proper proportions (for other fruits and vegetables possess them in some degree), that the grape is peculiarly fitted for making wine.
When the grapes are ripe, and the saccharine principle is developed, they are then pressed, and the juice thus obtained, or must, as it is called, is received into proper vessels, in which the fermen tative process is meant to take place. " When the must is exposed to the temperature of 65° of Fahrenheit's scale, it speedily begins to ferment: small bubbles first collect on the top, and may be seen gradually issuing from the central parts of the liquor, and bringing up the husks, stones, and other grosser matters which it contains. As the disen gagement of gas proceeds, a hissing noise is pro duced by the bursting of the bubbles; and a frothy crust or scum is formed by the viscid particles which they have carried to the surface. An in crease of the temperature and bulk of the ferment ing mass now takes place: the must loses its ori ginal consistency and its saccharine taste, acquiring a deeper colour and a vinous flavour, with an odour of spirit of wine, which becomes more perceptible as the process advances. At length these commo tions of the fluid abate spontaneously; and, after a Few hours rapid fermentation, the ebullition ceases altogether, the mass subsides to its former bulk, and the crust and solid particles which disturbed the transparency of the liquor, are precipitated to the bottom of the vessel." (Henderson's History of lineicnt and Modern Trines, In order to insure a regular and complete fer mentation, it is necessary that the grapes should be all equally trodden, and that the vat which is to con tain them, should be filled as speedily as possible; within twenty-four hours at farthest. Fermenta tion proceeds with incredible rapidity, insomuch that the juice, as it flows from the grape, will of ten work and ferment before it arrives at the vat, owing to the pressure and motion to which, when being carried from the vineyard, it has been sub jected. The most favourable temperature, as al ready stated, is 65° of Fahrenheit. Below that de
gree, fermentation is languid; above, it is violent; and when very high or very low, it ceases alto gether. When the process has commenced, how ever, the temperature quickly rises, even so high as 30° above the point in question. Another circum stance necessary to fermentation, is contact with the external air: without this, indeed, no fermenta tion can take place; but after the process has been established, such contact is not necessary. Finally, fermentation is brisk in proportion to the extent of the fermenting mass. In a cask it proceeds more slowly than in a vat: in short, the preference is to be given to large vessels. It may here be men tioned that when, from the deficiency or inactivity of the ingredients necessary to fermentation, that process does not go on successfully, means must be used to supply the defect. If, for example, the must is too thin and watery, and fermentation pro ceeds with difficulty, the objection may he removed by one of three ways: 1. By boiling the must and allowing the superabundant water to evaporate. 2. By merely throwing in a portion of must which has been thus thickened; or, 3. By adding a quan tity of sugar, as is the common practice in cold climates where the grapes seldom reach their full maturity. In some places baked gypsum is used, in order to absorb the excess of humidity: in others, the grapes are partially dried before they are trod den. The addition of tartar accelerates fermenta tion, particularly if the grapes abound in the sac charine principle. The stalks of the grapes, when added to the mash, act as a powerful leaven, and augment the strength of the wine; but to the weaker sort, they are apt to communicate a harsh and austere flavour, owing to the tannin and ex tractive matter which they contain. In the case of Port, they are always used; while in the manufac ture of the more delicate red wines of Bourdeaux, they are generally excluded. For the white wines of the same district, however, they are thought to he advantageous, rendering them less apt to spoil. Yet the wines of the Rhine, which are distinguished by their great durability, are not fermented with the stalks, and would probably not bear the ad dition. It is chiefly when the saccharine principle predominates that they contribute to the strength of the wine. The exact time required to complete fermentation varies, being dependant on the quality of the grapes and of the wine sought to be ob tained. Weak wines require but a short time; the lighter wines, in truth, will not bear the vat longer than from twenty to thirty hours. If a brisk wine is wished, the contents of the vat must be drawn out into casks before the fermentation subsides. In general, the process may be regarded as termi nated, when the saccharine flavour of the must has disappeared and the liquor has acquired a distinct vinous taste.