In Open Vats, other conditions being equally favorable, fermentation commences more promptly and is sooner ended, owing to the free access of the air, a certain amount of oxygen, as already shown, being necessary to fermentation. Although fermentation will continue away from the air when once started, it will be slow. The objections to open vats are, that although there is a layer of carbonic acid resting above the must, yet it is liable to be disturbed and become mixed with the air, and if the fermentation is long continued, a portion of the wine may become sour. Those who employ open tanks should also avail themselves of those conditions under which the wine will complete its fermentation in a few days, and should draw off promptly.
Closed Vats.—By using closed vats fermentation will be longer in commencing, and will proceed more slowly, but as already intimated, the wine can with safety be left longer in them than in open tanks. When it is necessary to develop much color, it would be advisable to use covered tanks, for the longer the wine is left in contact with the skins, the darker it becomes. The covering should be close enough to prevent the immediate contact of the open air, and yet allow the escape of gas—of close boards, but not luted, unless provided with a safety valve.
Fig. 5.
The Best Practice, however, in all cases, whether the vats are closed or not, is to have a false head resting directly upon the pomace, and which will keep the latter submerged during the whole process of fermentation. In this way good color will be developed, and the marc will be kept from the air, and the danger of souring will be avoided. In [figure 5], A represents a fermenting vat with the front half removed, showing the false head in place.
This head is made of several pieces which can be laid one by one upon the pomace, and maybe perforated with auger holes as represented in C, or may be a wooden grating, D. These pieces or sections together constitute the head B, and are kept in place by two cross pieces, e e, which are held down by blocks bolted or pinned to the inside of the tank. G is a stave with a block, f, attached, and H the same, showing the cross piece, e, slipped under it. When the tank is filled to the required height, the false head is put in, resting on the pomace, the ends of the cross pieces are slipped under the blocks, and everything is ready. As soon as the fermentation becomes violent, the whole will be submerged in the bubbling wine.
Hermetically Sealed Tanks.—Closely covered tanks must be provided with a safety valve or pipe for the discharge of carbonic acid gas, leading and discharging into a vessel of water, which completely prevents contact with the air. Under pressure the fermentation is much slower, and is not so complete. Yet great advantages are claimed for this method by some writers who maintain that by keeping the cover cool with wet straw or cloth, or by using a safety tube in the form of a worm passing through a condenser on the top of the vat, the vapors are condensed and fall back into the liquid, preventing loss of alcohol, and increasing the aroma, and that the wine acquires a superior fineness and velvety smoothness under the pressure of the gas. Boireau says that this latter quality is caused by the complete dissolution of the mucilaginous matters; and Pasteur has shown that more glycerine is produced when the fermentation is slow, which may contribute to the mellowness and smoothness.
Practice in the Médoc.—Mr. Boireau says that the greater part of the grand red wines of the Médoc, the prime St. Emilion, and the prime Graves, are fermented in closed vats; though a certain number of the viniculturists still follow the old custom, and make their wine in open vats.