Carbon dioxide is a friend when starting to make wine and unless making a sparkling wine like an elderflower or strawberry champagne an enemy when closer to bottling. As yeast ferments it creates carbon dioxide as well as ethanol as part of the process. These are made when the sugar molecule is broken to extract a little energy for the yeast cell and the new molecules are left

One molecule of fruit sugar becomes two molecules of alcohol and two of carbon dioxide.

We want to keep the ethanol to get us drunk and for while the carbon dioxide is allowed to bubble away out an air lock. Some carbon dioxide will dissolve into the young wine only to escape gently over time. It is said that wine takes at least nine months to fully degas but I imagine this is in ideal conditions and based on grapes. As an amateur it is difficult to tell when a wine will be fully degassed but any one from an amateur to professional drinker can taste carbon dioxide if it is still dissolved. It will leave a tingling sensation on the tongue or even a slight sparkling sensation if there in huge amounts. While some wines may benefit from this “petillant” character they will be rare, the only one I have seen so far was a Seville Orange wine I bottled too early. Generally we think of carbon dioxide as relatively inert and simply bubbling away during fermentation but carbon dioxide does however dissolve in water and thus the water in wine to create carbonic acid.

Carbon dioxide and a water molecule turn to a loose hydrogen molecule and a precursor to carbonic acid… I think…

This is not a huge issue and it is non-destructive and over time it will dissipate out through the airlock. If still present in a wine it will give an artificially acidic taste and dull the over all flavour profile reducing the “fruitiness” of the wine. It is often a reason wines are left to breathe a little before they are drunk… or left an age to breath if they were bottled too early. Its obviously best to de-gas fully before bottling to remove as much carbon dioxide as possible and thus any chance of carbonic acid on the tongue.

Carbon dioxide is a natural part of the wine making process so it has to managed. After secondary fermentation once fine lees starts to really build up a wine will be racked to clear it. At this point there is the highest concentration of carbon dioxide freshly dissolved from all the fermentation. As the wine is racked it is agitated and this allows dissolved carbon dioxide to escape. Splash back is a process that can add extra agitation as you rack by holding the end of the siphon tube at the mouth of the demijohn which allows the wine to fall into the demijohn from a height creating more splashes and bubbles.

There a many varied views on splash back and it may depend on the fruit, intended style and the current status of your wine to determine if you need to do it. There are no right and wrong answers in my view but some answers may be more right, more of the time in regards to this process. Generally it is though that white wines that naturally have a higher acidity (that’s oddly a lower pH) will potentially react with the oxygen that dissolves as you splash back so a white wine is better to roll down the side of the demijohn than splash in directly. The acidity of a white wine is believed to be more likely to oxidise and give the vinegary taste of ‘volatile acidity’

Again generally; it is thought that with the slightly lower acidity (that’s a high pH) of a red wine it reduces the chance of oxygen reacting with the acids present and that the oxygen is needed in a small quantities as it reacts with tannins allowing them to bind and mellow the wines taste and build mouth feel.

If you rack with or without a splash back the carbon dioxide liberated will collect in the demijohn so over oxidation does not occur as the wine continues to age. Carbon dioxide is heavier than oxygen so in the few minutes after racking and the air lock in place it will form a protective blanket to keep oxygen out. The small amount of oxygen that may have dissolved can bind with sulphur to create sulphur dioxide and allow this also to dissipate out as the wine ages.

Some even say that the extra little bit of oxygen actually allows carbon dioxide to be created and aids de-gassing – the jury is out on that… no doubt I will get conflicting emails threatening to punch me in the face. If any one does have some info by all means educate me… gently.

The longer a wine matures the less carbon dioxide will be present as it naturally escapes the wine so there is probably less need to splash back in later rackings. This does not mean that a splash back is or is not needed though. Sulphur dioxide may need to be liberated and some wines may want a slight oxidation to aid their character – I have seen a Pinot Noir left to slightly oxidise as it sits.

The easiest way to allow a wine to fully de-gas is time. The longer the bulk ageing in an air locked demijohn the more chance it has to de-gas. As the wine is together in large volume it allows more flavours to develop so it is a win win in my view.

Many people choose to manually de-gas a wine either to bottle early or as a safety to ensure there is absolutely no fizz left. A wine should only be de-gassed “artificially” just before bottling and any earlier is just creating space for new carbon dioxide to dissolve into it or exposing it to oxygen or microbes that could contaminate it.

To aid or replace natural de-gassing there are several methods you can use to speed the process or ensure it has occurred. A method I have not used is mechanical de-gassing with a wine whip. This kinky method uses a large spatula on an electric drill to stir the wine. The agitation allows gas to escape but to allow this you are exposing the wine to oxygen and a great big dirty drill wobbling about above your precious hard work. It also requires at least 15 minutes work and possibly more in a few passes over a number of days. That seems like a lot of hard work and possible exposure to oxygen and contamination though that can certainly be minimised with preparation and sanitising.

The other method is via a vacuum created with an electric or hand pump. With no exposure to oxygen there is no chance of oxidation. There is sadly the change of explosion.. implosion… well implosion then explosion and much crying I imagine. The first consideration is your demijohn. They need to be strong enough to undergo the stresses of a vacuum – do not risk it with a demijohn that has air bubbles as it was hand blown, with differing thicknesses of glass in the walls or with any faults like a damaged neck. You should never de-gas a half filled demijohn ever. The pressure difference of the liquid and the air cavity will stress the glass and could cause implosion. Although glass is strong in compression it is weak if it is deformed – the differential between uncompress-able liquid and compress-able glass will cause the glass to flex then deform and shatter. Boom! You have not only lost your wine but have exploding glass everywhere. If the demijohn is not fill to the thickened glass reinforced neck do not vacuum de-gas!

Degas set up side
L to R – wine, vacuum chamber, pump

To create a vacuum you need a a pump and a chamber. The resulting pressure differential means the wine pushes the gas from itself into the lower pressured chamber. The better the vacuum the quicker the de-gassing though the minimum seems to be -12inHg (inches of mercury) The sweet spot is about -18inHg and any higher than -22inHg has the potential to destroy a glass demijohn as that is approaching more than the container can take. I’m sure you have worked it out but a plastic PET bottle will simply collapse.

inHg – Inches of mercury… obviously!?

A motorised vacuum pump is probably the only way to create anything above -22 inches of mercury while a hand pump like I have used is more gentle and unlikely to exceed the maximum tolerance. The hand pump I use is a car mechanics break bleeder – brand new rather than covered in oil and from reports Mitivac models have been used to great effect. Another ingenious version used large syringes and one way aquarium valves. I imagine it is similar to this on the Instructables site: The stronger the vacuum the better it is at de-gassing and if it is quite gentle it can be left over night to work. As a home wine maker it is nigh on impossible to de-gas too much though in an industrial setting an over de-gassed wine will have a flatter taste.

Brake bleeder
Mechanic’s break bleeding vacuum pump.

Once a high vacuum is maintained and bubbling has virtually stopped your work is done. A sight test is best to monitor the progress. Younger wines will bubble more vigorously with larger and more numerous bubbles. A wine that is seven or eight months old may just have a few small bubbles as it de-gasses. Some people choose to give the wine a shake test to see if more bubbles are produced or see if pressure builds against their hand once the wine is off the de-gassing kit. If so there is still carbon dioxide present.

Pig nose
Two hole bung – This little piggy was the hardest kit to source!

Always allow the pressure to stabilise back to normal before removing the bungs and tubes to stop any splashing from the pressure change or cracking of the demijohn if catastrophic. After de-gassing the wine is ready to either bottle straight away if it is a dry wine or it can be stabilised and eventually back sweetened if desired. If you want to de-gas and stabilise a wine always de-gas first. A de-gassed wine allows any fine sediment and the sediment from campden and stabilisers to be settle easily after being stirred in as fewer carbon dioxide bubbles cause less agitation keeping particles in suspension.

Errors, omissions or poems about de-gassing please throw it my way, but remember I’m not a scientist so no long words!

The Wine Makers Companion – B C Turner and C J J Berry
Big shout out to “TheLoneCabbage” who wrote about the syringe vacuum!


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