To determine when alcoholic fermentation has finished, a simple measuring device called a hydrometer is used. It measures the density of a wine or must, giving an estimate of the remaining sugar content. Once the sugar content (or density) remains constant for two or more days, alcoholic fermentation is considered complete, and one of the following steps follows:
- Post-fermentation maceration
- Draining
Post-Fermentation Maceration
Once a red wine has finished fermenting, the winemaker may choose to leave the wine in contact with the pomace for anywhere from a couple of days to more than a month in order to extract more tannins from the grapes. This practice is known as post-fermentation maceration, and the decision whether to carry it out depends on the activity of acetic acid bacteria in the new wine, which is determined by chemical analysis. If the analyses show high bacterial activity, the post-fermentation maceration is shortened or skipped.
Draining
The term draining (descube) is usually used only for red wines, since it involves separating the pomace, which does not exist in white wine. Draining consists of separating the new wine (or free-run wine) from its pomace, either with pumps or simply by gravity. It begins by running the wine out through the bottom valve of the fermentation tank until the solids are reached. Once the pomace starts to come out, everything left in the tank is sent to the press to finish draining the free-run wine and begin pressing the pomace, producing what is commonly called press wine.
Because this wine is extracted under pressure from the inside and surface of the pomace, it usually has a very high concentration of poor-quality tannins and a large amount of suspended solids. Press wine is therefore not of very high quality, and it is advisable to keep it separate from the free-run wine obtained by gravity. There are, however, situations in which the quality of the press wine is good enough to blend a small percentage of it into the free-run wine to improve the latter’s structure and tannin content. This technique should only be used when the structure and tannin content of the free-run wine fall short of expectations and the tannins in the press wine are good enough, from a sensory point of view, for the addition to improve the blend as a whole.
For a long time, the pomace was removed from fermentation tanks by hand with shovels (and in some wineries it still is), which requires the worker to climb inside the tank. This procedure is extremely dangerous for the worker, however, since the oxygen-poor conditions inside, caused by the high CO2 content and the large amount of evaporated ethanol, can cause asphyxiation, fainting and even death. For this reason, large vessels must be fitted with a ventilation system so that the worker’s health is not put at risk. Today there are automated alternatives for removing the pomace without anyone entering the tank, but they tend to be quite expensive, especially considering how little they are used over the course of production.