An airlock is a small device, that when filled partially with water, acts as a water trap. It attaches to the top of a fermentation vessel and allows the gases caused by a fermentation to percolate through it and out of the vessel without allowing contaminants to get to the must.
The water that is put in the airlock creates an effective environmental barrier. It keeps unwanted, air-born matter and bugs, regardless of how small, from getting through the airlock and to your precious wine.
How to Use an Airlock
Regardless of which airlock you choose, both are used in the same manner. They are attached to a hole or opening of a fermentation vessel with the aid of a Rubber Stopper.
The size of rubber stopper you will need is determined by the size of opening to which the airlock is being fitted. Once you have the correct size rubber stopper, you can attach the airlock by placing its stem into the hole of the rubber stopper and then putting the rubber stopper into the hole or opening of the container.
After you have the airlock in place, fill it approximately half full with water. Be sure to put the dust cap back on the airlock. These caps have either ridges or pin-holes that still allows the gasses to make their way out.
When to Use an Airlock
One of the major misconceptions surrounding airlocks is that they should be used at the very beginning of a fermentation. But, nothing could be further from the truth.
For the first few days of fermentation, also known as the "primary fermentation", your juice should be exposed to air, not isolated from it. Just cover your fermenter with a secured lent-free towel or something similar during this time.
There are two major reasons why an airlock should not be used during the primary fermentation:
The first reason is that there is so much gas being produced during these first few days that an airlock could not keep up anyway. The massive amounts of gas would simply blow the water out of the airlock.
In fact, the current of gas is so high during the primary fermentation that small air-born particles and other contaminants can not even get to the juice. Bugs will not crawl into this type of environment either as this amount of gas is simply not hospitable to their survival.
The second and more important reason for not using an airlock during the primary stage of fermentation is because air is vital to the start-up of a healthy, vigorous fermentation. Yeast needs air to multiply in to great enough numbers to ferment properly.
Wine yeast has been programmed to multiply during the primary fermentation. Later on in the fermentation they will multiply some, but nothing to the extent they do during the primary fermentation. Typically during the first 3 to 5 days of fermentation, yeast will multiply to around 100 to 150 times the amount you originally added.
If an airlock is used during this growth stage, the yeast becomes stymied by the lack of oxygen and unable to multiply to sufficient numbers. The result is a long drawn-out, sluggish fermentation that can produce some slight off-flavors in your wine. All symptoms of over-worked yeast. In other words, too few yeast cells trying to do too much work.
For these reasons we recommend using an airlock only after the first racking of the juice. This is usually around the 5th to 7th day of fermentation. Most recipes will have a recommended number of days that will specifically apply to the wine you are making.
Or, you can use a hydrometer to judge when the first racking should be performed. Usually the optimum time to rack your wine is when it reaches a Specific Gravity reading of 1.035 to 1.025 on your hydrometer.
Anytime after this first racking, an airlock should be used. It plays an extremely vital role in protecting your must as the fermentation starts to slow down in its later stages. It is during this 3 to 5 weeks of slower "secondary fermentation" that your juice becomes most susceptible to contamination from outside sources.