Reducing Headspace in Secondary Containers

updated 08/19/2023

Yet another common question in winemaking is “how do I reduce the headspace in my wine?”

For those new to winemaking, “headspace” is the air between the wine and the stopper/bung in secondary storage, e.g., carboys, barrels, jugs, and tanks. This must be a relatively small space, else the O2 in the air will oxidize the wine and ruin it.

This post explains my ideas regarding the management of headspace.

Why is Headspace Bad?

Oxygen is wine’s enemy — it oxidizes wine, which has an effect analogous to rust (iron + O2). During fermentation O2 is not a problem — yeast needs O2 for reproduction. But after fermentation, O2 is wine’s main enemy. A large headspace means there’s more air, and more O2. Reducing the space as much as feasible reduces the problem.

Potassium metabisulfite (K-meta) is also used, as it “scavenges” O2, e.g., binding to it and rendering it harmless. This is among the reasons I add K-meta at all rackings, and at bottling time.

The smaller the headspace, the smaller the amount of O2, which means the K-meta has less to do.

How Much Headspace?

There is disagreement among winemakers about how much headspace is allowable. Ask a question of 10 winemakers, you’ll get at least 11 answers …

Allowable headspace depends on container shape. Carboys and most jugs become very narrow at the top, presenting an opening that’s 3/4″ to 1.5″ across (1.9 to 3.8 cm). This is a desirable shape for winemaking, as the headspace can be made very small.

Contrary to this is the typical primary fermenter for home winemakers, a food grade bucket that is a foot across (mine are 11.5″ to 13.5″ [29 to 35 cm]), has a wide cross-section. Because of the wide area, the headspace is very large in relation to the volume of wine, even when the fermenter is filled to the brim. This is among the reasons why primary fermenters should not be used for secondary (post-fermentation) storage.

Maximum Headspace

My choice is to fill the secondary container to within about 2″ (5 cm) of the bung. Depending on container shape, I may use a space as much as 3″ to 4″ (7.5 to 10 cm), although I prefer less.

Minimum Headspace

So why not fill the container to the bung? Two reasons — temperature and air pressure.

As temperature varies, so does the volume of the wine. Especially with larger containers, when the ambient temperature rises a few degrees, the wine expands and can overflow the container.

When the air pressure drops, it pulls on the air inside the carboy through an airlock, and can pull wine into the airlock if the containers is too full.

For these reasons I never fill a container to within 1″ (2.5 cm) of the airlock.

Caveat to Headspace Size

The one caveat to the above is if the wine is still fermenting OR if it’s degassing. In both situations, emitted or trapped CO2 can cause the wine to overflow the container. I may have as much as 8″ headspace in a 19 to 23 liter carboy in this situation. This typically lasts 1 to 3 weeks, and at the next racking I reduce headspace to 2″.

Note: I degas all wines by 1 minute of stirring — NOT whipping — post fermentation. This releases a lot of the trapped CO2 and while it doesn’t fully degas the wine, it shortens the degassing period. Which also facilitates clearing, as suspended CO2 will hold solids in suspension.

Reducing Headspace

There are numerous ways to reduce headspace to protect the wine. This is a key point — you are protecting your investment by reducing headspace. The investment is not just money — it includes time, effort, and pride.

I don’t use all these methods, but am including them for completeness.

Marbles

Folks will drop sanitized marbles into their wine to reduce headspace. This certainly works, but it’s critical to ensure the marbles are lead-free glass. My understanding is that lead can be leached from leaded glass, and that’s a very bad thing.

Additionally, folks will use other objects to fill the space. Practically speaking, any object can be used if it will not interact with with wine and can be sanitized. Please note that you need to get objects into and out of the carboy (or other container), and the precludes a lot of objects.

Vacuum

Those who own vacuum pumps may produce a vacuum inside a container to eliminate headspace. The space is still there, but there’s nothing in it.

This requires having a vacuum pump and the equipment to establish and maintain a vacuum.

It also limits the containers that may be used. Standard 12, 19, and 23 liter carboys are of sufficient construction to safely accept a vacuum. Demijohns from 4 liters and up (I have 25 and 54 liter demijohns) are NOT safe for vacuum and may implode.

I have an additional concern that a vacuum looks like air, so without a pressure gauge, there’s no way to tell if the wine is safe or not.

Inert Gas

The headspace may be flushed with an inert gas, typically argon or CO2. This also requires special equipment. For CO2, this is common among home beer makers.

My concern with this method is similar to that for a vacuum — air, argon, and CO2 are colorless, so it’s impossible to determine what’s in the headspace.

Downsize Containers

If the secondary container is not full, downsizing to a smaller container may solve the problem.

It’s common among wine kit makers to downsize. Most kits reconstitute to 23 liters, but after the first racking the volume is typically 1 to 2 bottles less than that. Folks will downsize to a 19 liter carboy, with the excess in smaller bottles.

This requires having a collection of smaller bottles, plus drilled stoppers to fit them and a number of airlocks.

I have bottles from 125 ml and up, along with #2 and #3 drilled stopped and a drawer full of airlocks, so I’m prepared to for this situation.

Topup with Compatible Wine

My preferred solution is to topup with a compatible wine. Especially when making 23 liter kits, I will topup the 23 liter carboy with 1 to 2 bottles of a compatible wine, so I get the full 23 liters of volume. This has the advantage that for that wine, I have only one container to manage.

What is a compatible wine? That all depends on what the batch is. For red Vinifera grapes I’ll use the same varietal, if I can find it. If I don’t have the same varietal, I’ll often use a Merlot. Any wine that tastes good will work.

For whites I’ll use the same varietal, or one as neutral in taste as I can find.

Anything is better than nothing.

Water

Some sources recommend topping with water. I do not. This is an absolutely last resort. Adding water dilutes the wine, and while it protects it, it reduces quality.

Summary

My preferred solutions are to downsize containers and/or topup with compatible wine.

Why? Neither requires any special equipment, and I’m 100% certain of what’s in the container.

Regardless of what method is chosen, choose one. Protect your investment.