Birallee Beer & Brewing: The Myth of the CO2 Blanket in Homebrewing: Fact or Fiction?

In the world of homebrewing, there are many techniques and tips that brewers swear by to ensure their beer turns out perfect. One such technique is the idea of a "CO2 blanket" protecting the beer from oxygen ingress and subsequent oxidation. But what exactly is a CO2 blanket, and does the science support this line of thinking?

What is a CO2 Blanket?

The concept of a CO2 blanket is based on the idea that carbon dioxide (CO2), being denser than air, will form a protective layer over the beer in a fermenter or keg. This layer is believed to prevent oxygen from coming into contact with the beer, thereby protecting it from oxidation, which can lead to off-flavors and spoilage.

The idea is sound, in theory, but we've always wondered whether this is one of those old wives tales, or if it could be backed up by science. We've dived down the proverbial rabbit hole so you don't have to, in order to try and get an answer to this question.

The Science Behind CO2 and Gas Mixing

To understand whether a CO2 blanket can effectively protect beer, we need to delve into the behavior of gases. CO2 is indeed denser than the oxygen and nitrogen that make up the majority of atmospheric air. When CO2 is introduced into a vessel containing air, it will initially sink to the bottom due to its higher density. This is why some brewers believe that a CO2 blanket can form a protective layer over their beer.

CO2 tanks like this one are commonly used in homebrewing for serving and transferring

However, gases are inherently miscible, meaning they can mix together to form a homogeneous mixture over time. This mixing occurs through molecular diffusion, where gas molecules move from areas of higher concentration to areas of lower concentration due to their random thermal motion. In a closed vessel, CO2 and atmospheric air will eventually mix, leading to a uniform distribution of gases throughout the vessel.

How Long Does Mixing Take?

The time it takes for CO2 to mix with atmospheric air depends on several factors, including the size of the vessel, the initial concentration of CO2, temperature, and whether there is any forced mixing (such as stirring or convection). Without forced mixing, molecular diffusion can be relatively slow, taking from several minutes to hours to achieve a uniform mixture. With forced mixing, such as through mechanical stirring or natural convection currents, the process can be significantly accelerated, potentially taking only a few minutes.

Implications for Homebrewing

Given the science of gas mixing, the idea of a stable CO2 blanket protecting beer from oxygen ingress is more myth than reality. While CO2 may initially form a layer over the beer, it will eventually mix with the surrounding air, reducing its effectiveness as a protective barrier.

From a homebrewing perspective, there's two scenarios where this would apply.

1. Fermenter Headspace

During active fermentation, yeast consume sugars within the wort and convert it to alcohol and carbon dioxide (along with some other compounds). The amount of carbon dioxide generated during fermentation is significant, and would be more than enough to displace any air/oxygen within the headspace of the fermenter and force it out through the air lock or spunding valve.

Removing the lid from your pressure fermenter after fermentation is completed could be considered risky

In the event of the fermenter being opened after fermentation is completed for any reason, such as adding dry hops, CO2 gas is released, and atmospheric air is introduced into the fermenter. Exactly how much depends on numerous factors, such as the size of the opening, but based on this it would certainly be advisable to flush the headspace with CO2 after re-sealing by pressurising, purging, and repeating this process at least a couple of times.

2. Filling Kegs or Bottles

With regards to keg or bottle filling, if you haven't purged your receiving vessel sufficiently with CO2 prior to filling it with beer, any residual atmospheric air (containing oxygen) will eventually mix with the CO2 within the headspace during/after filling.

It's believed that levels as low as 0.1 parts per million (ppm) of oxygen are enough to cause the staling effects of oxidation, so even trace amounts of air left in a vessel prior to filling would likely contain enough oxygen to turn a good batch bad, if it's absorbed into the beer.

Based on this we think it's safe to say that solely relying on a CO2 blanket to prevent oxidation may not be sufficient.

Practical Tips for Homebrewers

To better protect your beer from oxidation, consider the following practical tips:

Minimize Headspace: Reducing the amount of headspace in your fermenter or keg can limit the amount of oxygen that can come into contact with your beer. When bottling or canning, it's ideal to "cap on foam" as the foam takes up the headspace instead of air

Purge with CO2: Purging the headspace with CO2 before sealing your fermenter or keg can help displace oxygen and reduce the risk of oxidation.

Use CO2 Pressure Transfers: Performing closed pressure transfers using CO2 can ensure that minimum oxygen pickup occurs when transferring the finished beer from the fermenter to the keg.

CO2 pressure transfers are a great way to reduce oxygen pickup

Use Oxygen-Absorbing Caps: For bottling, consider using oxygen-absorbing caps to further protect your beer from oxygen ingress.

Use Counter-Pressure Bottle Fillers: Counter pressure bottle fillers are essentially an oxygen-free pressure transfer from a pressure fermenter or keg to a bottle. Check out our review of the Ultrafill counter-pressure bottle filler for more detail on these.

Practice Good Sealing: Ensure that your fermenter or keg is properly sealed to prevent air from entering. Silicone seals are known to leak gas over time, so consider swapping these out for more reliable materials like KegLand's Low2 seals.

Oxygen-Free Dry Hopping: Use devices like the Hop Bong so you can perform oxygen-free dry hopping into your pressure fermenter to reduce the risk of oxygen ingress, and remove the need to open your fermenter and expose it to atmospheric air.

Conclusion

While the idea of a CO2 blanket is an appealing concept, the science of gas mixing suggests that it may not be as effective as some homebrewers believe. By understanding the behavior of gases and taking practical steps to minimize oxygen exposure, you can better protect your beer from oxidation and enjoy your beer remaining fresher for longer.

We still kind of like the CO2 blanket analogy - perhaps the best way to think of it is there is a CO2 blanket covering your beer, but in order to be effective the blanket needs to be made entirely of CO2, as any other gases being present, such as oxygen, will get absorbed into the blanket and then potentially into your beer as well.