Safe Pressure in Wine Bottles

 

updated 07/27/2024

Inexperienced winemakers (and sometimes experienced winemakers) may get a refermentation in the bottle, producing a light spritz or pushing out the cork. This post addresses what is safe and what is not safe in terms of bottles and pressure.

Fermentation in the Bottle?

Fermentation in the bottle is caused by the presence of a fermentable sugar and live yeast. The yeast does its thing (eating-n-excreting), producing a bit more alcohol and CO2. In a fermenter, the CO2 bubbles off. In any closed container, it remains dissolved in the wine.

The typical causes are:

Fermentation was ongoing. The wine was bottled too early as the fermentation was not complete, so it continued in the bottle.

Stuck fermentation restarted in the bottle. A ferment can get stuck (stopped) for numerous reasons. The winemaker thought the ferment was complete, but it wasn’t. The simple act of racking (or bottling) can cause the fermentation to restart in the bottle.

Wine not properly stabilized. It’s necessary to “stabilize” a wine when backsweetening. Home winemakers commonly use potassium sorbate and potassium metabisulfite (K-meta) to prevent a renewed fermentation, some folks pasteurize the wine, add a spirit to raise the ABV to a level the yeast cannot tolerate, or sterile filter the wine.

Depending on the amount of sugar in the bottle, it may produce a light spritz when the cork is pulled, it may push the cork out, or if the bottle is sealed in some fashion it may cause the bottle to explode.

Bottle Pressure Rating

Bottles are either unrated or rated for a specific pressure.

Typical wine bottles are unrated, so they are unsafe for any amount of pressure, although one source stated that typical corkable wine bottles are rated for 3 to 5 psi to allow for some dissolved CO2 in the wine. Please note that this rating is a LOT less than beer or champagne bottles (see below).

Most beer bottles are rated for 40 psi (2.7 atmospheres [Atm]), and some sources state that American beer is typically bottled at 35 psi (2.4 Atm). Some special beer bottles are rated up to 60 psi (4.1 Atm), but it’s safest to assume all common beer bottles are rated for 40 psi.

Champagne bottles are typically rated for 70 psi (4.8 Atm) to 90 psi (6.1 Atm), roughly double the rating of beer bottles.

Boom!

If a renewed fermentation happens in a corkable bottle, depending on the amount of pressure, the most likely result is the cork will be pushed out, resulting in a mess and a loss of wine.

However, if the cork is secured in some fashion, e.g., it’s wired down, the cork cannot give way. This is a dangerous situation, since if the pressure exceeds the strength of the weakest point in the bottle, the bottle will fail at that point. Bluntly speaking, it will shatter, and if the pressure is great enough it is an explosion.

A person I knew bottled in 1 gallon jugs, which are not strong glass. He brought a bottle up from his cellar one evening and placed it on the counter. The next morning he discovered the jug had exploded, leaving only the bottom on the counter. Jagged chunks of glass were embedded in the walls. He considered himself so fortunate that no one was present when it happened.

His cellar was much colder than  his kitchen, and when the wine warmed up over night, the wine and CO2 expanded, resulting in an explosion.

Beer bottles are far safer than a corkable wine bottle, but represent a greater danger: Beer bottles are crown capped, so if the pressure is too great, the glass will fail. There’s no cork to push out. The likelihood of a dangerous explosion is far greater.

Champagne bottles are the safest, since they can handle the most pressure, but at the same time they are the most dangerous if the pressure limit is exceeded.

Avoiding the Problem

The last section makes it sound like winemaking is insanely dangerous.

It’s not.

The purpose of the last section is to explain the possible danger and to make readers think before doing. I’ve been making wine since 1981 and beer since 1984, and during that time only one person I know had a dangerous situation. Also during that time I’ve had:

• 1 batch of wine that pushed corks
• 1 batch of wine with a very light spritz
• 1 batch of over-carbonated beer that each time a bottle was opened, the bottle overflowed no matter how cold we chilled it

It’s totally safe IF we pay attention to what we’re doing.

Making a still wine? Make sure the fermentation is complete and if backsweetening, that the wine is stabilized. This includes ensuring the sorbate is still good (see this post).

Making beer or sparkling wine? Use the correct container and use the correct amount of carbonating sugar for the type of product made. Research before you do.

Fixing the Problem

If you have a problem, how to fix it?

It depends on the situation. I tried an experiment in 2023 where I bottled a 13+ month old wine with no sorbate, as supposedly the yeast is dead after 9+ months. It didn’t work, so I have 4 bottles that have a light spritz. The corks are not being pushed out and the sparkling in the glass only last for about twenty seconds. I judge the situation as non-dangerous and am using those bottles first, chilling them before opening.

If the spritz was much greater or especially if the corks were being pushed out?

First, chill the wine (or beer) in a refrigerator or on ice. That will cause the wine and embedded CO2 to contract, reducing the likelihood of an adverse reaction.

Then unbottle into a primary fermenter and add K-meta to address any O2 exposure. Record the SG.

The safest method is to let the wine ferment to completion in a carboy — your hydrometer will help identify when the ferment is complete. If backsweetening, add sorbate + K-meta, and let the wine rest one to two weeks to check that the ferment doesn’t restart. If it doesn’t, rebottle. If it does, I’d bin the existing sorbate and buy fresh.

Alternately, add sorbate + K-meta while the wine is cold. Let it rest one to two weeks in a carboy to check that the ferment doesn’t restart. If it doesn’t, rebottle.

Yet another choice is to let the wine ferment dry, then bottle dry. Keep a bottle of sugar syrup or Agave nectar on hand, and sweeten by the glass when serving the wine.