Carboys come in many sizes, but all of them have generally the same shape and features, including a small neck. The small neck of the carboy may make them difficult to clean but it has one huge benefit. Small surface area. Six gallons of wine in a more traditional vessel, like a bucket will have a surface area of over 100 square inches, which is exposed any gas that happens to be in your carboy. Another six gallons of wine topped up to the neck of the carboy will have a surface area of less than 1.5 inches. This small surface area dramatically slows the rate of oxygen absorption into the wine.
But my carboy is full of CO2... what's the big deal if I leave some space?
A lot of home winemakers go by this train of thought but a lot of home winemakers occasionally make wine bordering on sherry or vinegar. While this is true for a while, don't bank on a pure carbon dioxide blanket for any extended period of aging. Wine produces enough inert carbon dioxide gas to protect it during the active fermentation, and it is highly saturated with CO2 for a short while after. During the first month or so of a wine's life, the yeast activity creates enough of a reductive environment and CO2 that oxidation is not much of a concern. The wine is essentially saturated with CO2 and contains a good bit of oxygen reactive elements like tannin and sulfides. Racking will degas much of this CO2, and by about the third month of aging, don't expect to have much of any CO2 in saturation. Once the wine is mostly degassed, it is extremely important to limit the exposure to oxygen to a minimal and topping up is one of the best strategies to do so.
You would think that by not ever opening a 3/4 full carboy, the heavy CO2 gas would protect the wine and never escape. This is true to some extent but don't bet on it for more than a month or two. Both the wine and the gas above the wine will expand and contract with small changes in temperature. This allows the airlock to push out CO2 and pull in air which becomes a problem over time. Though CO2 is heavier than air, it is also soluble with air. Any air pulled into the carboy will mix with the CO2, creating a blend which now includes a small amount of oxygen. Over time, this will cause negative oxidation effects in the wine. By topping up to the neck, you limit the total amount of air that can ever enter the carboy and also limit the surface area, making it much slower to dissolve into the wine. A topped up wine in bulk can last a very long without much concern of oxidation.
What is the best way to top up to the neck of the carboy?
There are some options. If you are close to the neck, you can use a similar wine from a previous vintage or a store bought wine to top up (just make sure that it is a healthy wine!). Another option is to add marbles to the carboy until the wine rises to the neck, but that is a bit of a clumsy solution. On some wines that often need watered back (Like Concord), you can usually get away with topping up with water or acidulated water. If your wine has more than a half gallon or so of airspace, usually the best bet is to use a combination of different sized containers rather than one un-topped carboy. I like to use various sized carboys (3gal, 5gal, 6gal, 6.5gal), growlers (1/2gal), 1 gallon jugs, and even wine bottles on occasion. For instance, if you have 5.75 gallons of wine, you can use a 5 gallon carboy, a growler, and a wine bottle with topper. This gives you some smaller batches to use for future top-ups which can be very handy.
Words of Caution...
Do not top up an actively fermenting wine unless you are looking to create a volcano. Wait to top up until after things have settled down. A little oxygen during fermentation is actually a good thing and will help to keep the fermentation healthy.
When topping up, be sure to leave 1 to 1.5 inches of space between the bottom of the bung and the wine. This allows ensures that any expansion does not push wine up into the airlock.
Be wary of natural cork and silicone bungs. Natural cork can be very permeable to air, depending on the grade. Silicone is one of the most air permeable elastomers, which is especially concerning when the silicone bung has a thin one way breather on top. Solid bungs are really the best, but just be careful that they don't blow off under pressure. I personally prefer standard rubber bungs with airlocks and just stay topped up. When I do open the carboy to check on things, I give a little spritz of my homemade sulfite and acid sanitizing solution to scavenge any air that I may have introduced.
Feel free to post any questions or comments about this post in the comments section below and make sure to click subscribe in the header for more wine tips to come.
Read Next: Homemade Sanitizer for Winemaking
For more information about home winemaking, subscribe to my youtube channel.
The Home Winemaking Channel