It’s a fairly common mistake to use airlocks only during fermentation and switch to a solid plug or bung for bulk aging in a carboy. Why wouldn’t you? After all fermentation is over so there really shouldn’t be anything going on in there.

The truth is that there are other forces at work that can cause problems when using a solid plug on a carboy.

The Trouble With Solid Plugs on Carboys

Typically we like to leave a bit of headspace in our carboys when we rack our wines. While it’s best to minimize this headspace we often do still have some space (i.e. trapped air) in there.

As you can imagine a glass or plastic carboy does not “breath” and it’s certainly not flexible. Solid plugs or bungs also do not breath, nor can they accommodate any changes in pressure within that headspace.

Airlocks are the best protection for your wine in the carboy.You put the two together and there’s no room for pressure changes or for suspended carbon dioxide to come out of suspension. So what happens when a carboy warms up a few degrees? Or when a bit of carbon dioxide finds its way out of suspension?

The plug pops out leaving your wine unprotected.

Let’s say the opposite happens and the temperature drops. This creates a vacuum in the headspace causing the plug to get sucked down tightly into the neck of the carboy and can be quite difficult to remove.

This happened to me once while making a one gallon batch of mead. The plug was so far down into the neck of the carboy that there was nothing left to grab hold of to pull it out. I decided to go after it with a screw driver and try to pry it out. This resulted in a lot of chipped glass and a decent size hole in my hand.

I also tried drilling a screw into the plug and pulling on the screw. This valiant effort merely put a hole in my plug. It didn’t move at all.

After wrestling it for an hour, and eventually got it out. At that point I vowed never to use a solid plug again.

It’s actually a pretty common occurrence for winemakers to have trouble with solid plugs in carboys. On average I get two to three emails per month from winemakers who are trying to figure out if their wine has gone bad or not because they came home to find that the plug has popped out of the carboy left their wine exposed for three days or more. It happens all the time.

Always Use An Airlock With Carboys

It doesn’t matter how long fermentation has been over, always use an airlock. Yes, you do have to check the water level from time to time as evaporation, however, this is a small price to pay. The alternative is coming home from a week long vacation to find that your plug is on the floor and your wine has been sitting there for who knows how many days open and unprotected.

Airlocks provide a flexible barrier that can give with pressure changes. They can also tip you off if spoilage micro-organisms have taken hold as the airlock will start to bubble again after being in active.

That being said airlocks certainly aren’t perfect. They do need some maintenance and your attention from time to time. Evaporation is your biggest concern. If the water level gets too low the airlock will no longer protect your wine.

It’s also possible that the water could be come contaminated and cause problems if it comes into contact with your wine. During temperature drops it’s even possible for the water to get sucked down into your wine. These are rare occurrences though.

In my opinion keeping tabs on an airlock is a small price to pay for the protection they offer. Solid plugs just aren’t reliable enough when used on glass or plastic carboys.

3 Ways to Fool Proof Your Airlock

Here are some ways you can make your airlock safer for your wine and more resistant to the issues mentioned above.

  • Add a splash of vodka to the water. This will protect the water against spoilage micro-organisms and even if it does get into your wine it won’t be noticeable. Keep an eye on it though as alcohol evaporates fairly quickly.
  • Fill your airlock with mineral oil instead of water. Mineral oil is food grade, doesn’t evaporate, doesn’t spoil, and if it falls into the carboy it floats and can be separated through racking.
  • Fill your airlock most of the way with water and top it off with mineral oil. This reduces the amount of oil you need and prevents the water from evaporating or harbouring spoilage micro-organisms.

So skip the solid plugs and bungs if you age your wine in carboys. Always use an airlock.

Photograph by: Marco Assini (license)

  • Rancho Fuego

    Greetings! First batch of wine here. Can or should you use One Step as the solution in your airlock? Some say water, a little vodka; others recommend sanitizer. I’m just a week into the second fermentation.

    • Hello Rancho Fuego! I recommend only using liquids that won’t harm your wine if they end up in the carboy. It can happen during temperature drops or when removing the airlock.

      Plain water is what I usually use but adding vodka can be done to help keep spoilage micro-organisms at bay. Another option is to add water and then just enough mineral oil to cover the water. Mineral oil is food grade, doesn’t evaporate, and doesn’t spoil. Because it is oil, if it does end up in your wine it will float on top and is easily racked off of.

      Most sanitizers aren’t something you want in your wine thus I recommend avoiding those.

      • Jerry B

        I’ve used potassium metabisulfite at sanitizing strength in my air locks, not sure why a few drops into the carboy is an issue, as it’s added in the winemaking process as a stabilizer/preservative?

        • You’re quite right Jerry! Potassium metabisulfite would be fine to use in the airlock. The sulfites do tend to bind and atomize in a fairly short time period so it’s not a great long term, bulk aging option but when you’re in the throws of fermentation I’d think it’s just fine.

          -Matt