A fermentation lock is an inexpensive but essential item for anyone making wine at home.
Fermentation is the process by which yeast breaks down sugar into alcohol and carbon dioxide. It begins at the primary fermentation stage, when grapes or other fruits are mixed with sugar and yeast in a large container. After a week or so the fruit pulp is removed, and the young wine is siphoned into carboys, large glass or plastic containers, which act as secondary fermentation chambers.
The carboy needs to be closed to prevent dust particles, bacteria, wild yeast, or insects such as fruit flies from contaminating the wine. If this happens the flavor of the wine may be spoiled as a result of excess oxidation, or the whole batch of young wine could turn to vinegar.
At the same time, since the wine is still fermenting, it is still producing carbon dioxide, which, if not allowed to escape, will cause a pressure build-up inside the carboy and create a potential explosion hazard.
Therefore, a method is needed to allow the carbon dioxide to escape, while at the same time protecting the wine from contamination.
The simplest solution is to cover the top of the carboy with an uninflated balloon by fastening the neck of the balloon firmly around the neck of the container. As carbon dioxide rises from the fermenting liquid, it will inflate the balloon, and the gas can be released by simply pulling open the neck of the balloon for a few seconds. However, this is not the best solution because the balloon may impart a rubbery flavor to the wine.
The preferred method is to use a fermentation lock, a bent glass tube, designed to hold liquid, which will form an airlock to protect the fermenting wine while allowing carbon dioxide to bubble out.
A fermentation lock can be purchased from a beer- and wine-making supplier and is quite simple to use. It comes with a plug that fits into the neck of the carboy, and which has a hole in the center. The fermentation lock is inserted into the hole in the plug, and then half filled with liquid.
A wine-maker who does not have access to a wine-making supplier can make a home made fermentation lock by running a length of flexible plastic tubing from the carboy into a liquid-filled glass jar.
For wine-making, the recommended liquid for the fermentation lock is the same sanitizing mixture, such as sulphite solution, which is used to sterilize wine-making equipment. Another option is glycerine, which evaporates more slowly than sanitizing solution. However, wine-makers who are concerned that the liquid in the fermentation lock might be sucked back into the wine may prefer to use vodka, a sanitary alternative which will not adversely affect the flavor of the wine.
Once the fermentation lock is in place, it needs to be kept half full of liquid, so it should be checked regularly and topped up as necessary to compensate for evaporation.
When the carbon dioxide stops bubbling out, fermentation is complete, and the wine is ready to bottle.