Wine is a slow product to make. I found this out when I first took the brave step of risking a reasonable amount of home-grown fruit for a chance of making something equally tasty. Some suggested a fruit wine could be drunk as soon as one month after starting fermentation, but most worthwhile sources generally suggested far longer fermentation times. Since I was intending on making still wines and using bottles that were not built to withstand pressure, it was imperative I did not bottle before the wine fermentation was complete. So I decided I would wait as long as I needed to for the ferment to finish. I have done this many times since, and like it because it allows me to observe the process to completion. Patience also acts with the added benefit of extra age for your wine.
The first and most obvious thing you can do to tell if your wine fermentation is still in progress is to look at it. If it’s fermenting, you will see small bubbles rising from the bottom to the top, much like a carbonated drink in a clear glass. If it’s actively fermenting, you may even see small fragments of fruit or grape pulp being thrown about in the wine. Also look for bubbles on the top of the wine, particularly around the edges. If you are using an airlock, bubbles moving through it are a sure sign that the pressure inside your fermentation vessel is likely higher than the pressure outside it. While this isn’t always caused by excess CO2 (more about that later) if the bubbles occur at regular intervals, it’s a good sign that fermentation is still underway. The ‘bubbles through airlock’ method can be a little contentious, as many insist it cannot be relied upon as an indicator of fermentation. While it doesn’t always tell you when your fermentation is complete, it will give a fairly reliable indication that it is not complete, and in my view, it deserves mention for this reason alone. Cloudy wine is also a good indicator that fermentation is still occurring. And you may notice when your wine is still actively fermenting is that it’s never fully clear. While wines may even still be cloudy when fermentation is finished, but I’ve never observed the reverse phenomenon (when a wine that is still fermenting is fully clear). The yeast in suspension during an active fermentation always seem to add a degree of cloudiness to wine. And once the yeast has done it's job than it typically starts to fall out to the bottom of the fermentation vessel.
The surest way to establish whether or not a fermentation is complete is by measuring the specific gravity. You can do this using either a hydrometer or a refractometer. Rather than looking for a given value such as 1.000, you need to take successive readings at regular intervals and make sure all readings show the same value before stabilizing and bottling. Brewers are often advised to do this every day for three days. However, I would caution a time frame so short with wine making. Wine ferments so slowly that you may not notice a difference over just three days. There’s no rush to get a wine into the bottle, since you’re usually looking at an aging period of around 6 months. So why risk it? I would suggest waiting several days, or even a week between readings, especially after the initial bulk of wine fermentation has finished.
From what I’ve observed, amateur winemakers seem far less concerned about temperature control than their beer brewing counterparts. Wines are often started in summer, when fruit trees are at their most prolific, and continue fermenting well beyond the end of the season. Yeast also prefer warmer temperatures (but not too warm), so the rate of fermentation can slow, not only because the sugar content of the wine has reduced, but also because the ambient temperature has dropped. As such, all of the above tests can fail those living in colder climates if the wine is gets so cool that the yeast go dormant before they finish fermenting. The best thing you can do to avoid being caught out by a false measurement of completeness is to move your wine to somewhere warm for a week or so before starting to take readings. You might start seeing bubbles through the airlock shortly after this, but it’s probably just the air inside the carboy expanding as it warms. It should settle down within a few hours. If the bubbles continue for days, chances are you’ve woken the yeast up and they are happily eating sugars again. If you take successive readings days or weeks apart and they all show the same value, then your wine fermentation is finished. Whether or not you choose to bottle, or simply age in the carboy is up to you. I generally leave mine for a month or two in the carboy, or until I’m 100% certain it’s as clear as it will ever be. The more diverse may wish to try their hand at carbonating or back-sweetening their wines, but that’s another story for another day.
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