Can CO2 Form a “Blanket?”

708px-Carbon_dioxide_3D_spacefill

A space filling model of carbon dioxide (CO2). The black center portion of the molecule represent carbon and the two oxygens are depicted in red.

Carbon dioxide (CO2) is an important component of almost all beers. Unless the beer is still (uncarbonated), carbon dioxide gas gives it its fizz. This fizz lifts the beer’s aromas up to the beer drinker’s nose and helps form and sustain the foam. A small percentage of the gaseous CO2 in beer reacts to form carbonic acid (H2CO3), which is a dissolved solid that lowers beer pH slightly.

Carbon dioxide (CO2) is a simple molecule, just a central carbon atom with two oxygen atoms bound to it. Still, some misunderstandings about this molecule exist in the homebrewing and home winemaking community and correcting these could help brewers both improve their brewing process and understand the safety related aspects of dealing with CO2.

 

CO2 “Blanket”

One idea that exists in both the homebrewing and home winemaking hobbies is that CO2 can form a “blanket” that will sit atop a fermenting or conditioning beverage and protect it from oxygen (O2). Sometimes, the idea is explained as such — CO2 is heavier than air and will sit at the bottom of an air-filled space and form a barrier to oxygen getting through.

Some folks, however, have objected to this idea, claiming that CO2 is fully miscible in air. In support of this idea, one of the ideal gas laws (relating to the partial pressure of gases in a closed system) is sometimes cited as evidence that CO2 cannot pool or form a protective “blanket” over a batch of beer.

In reality, both ideas have elements that are true. If you pumped CO2 and oxygen (O2) into a sealed container and waited, they would eventually be evenly mixed throughout the space. The key to that sentence — and the whole “CO2 blanket” idea — is the word “eventually.”

If you put CO2 and O2 (or nitrogen (N2), or a blend of three) in a sealed space and waited until any initial temperature differences between the gases disappeared and the gas molecules had time to bounce around in the container, they would eventually be distributed evenly throughout the container. CO2 would not sink below the O2 (or N2), even though molecules of CO2 weighs more than molecules of O2 or N2. However, depending on how the gases were delivered to the vessel, temperature differences and how much time had elapsed, it is not at all hard to get gases to form temporary “layers” (or “be stratified,” in the lingo).

If it’s been awhile since you’ve had high school chemistry — or you cant figure out why the ideal gas laws don’t tell the whole story — see the sidebar for three quick thought experiments that will give you everything you need to know about the CO2 blanket question. Or, just keep reading for the take home message.

In actual homebrewing conditions, there are times that CO2 can pool, excluding oxygen — to the some degree — from the liquid below it. The length of time a “CO2 blanket” can exist, and the extent to which is impedes oxygen ingress, depends on a number of variables.

 

Fermentation

During an active fermentation, CO2 molecules are continually being expelled from the surface of the fermenting liquid. In a closed fermentation, oxygen molecules in the headspace will eventually be displaced and the headspace will soon (for all practical purposes) be devoid of oxygen. Until the vessel is opened, the trapped CO2 will protect the liquid from oxygen (except for any backflow of oxygen through the airlock or stopper).

In an open fermentation, especially one in which the level of liquid is well below the upper lip of the container, the pressure of carbon dioxide gas leaving the solution will lower the amount of oxygen reaching the surface, compared to if the liquid was not fermenting. Essentially, the “crowd” of CO2 molecules emanating from the fermenting liquid would knock back most of the oxygen molecules headed towards the surface. In a vigorous fermentation, the column of CO2 moving up from the liquid is sufficient to protect it from any harmful amount of oxidation. In a beer fermentation, the layer of kräusen and yeast atop the beer also protects it from oxygen.

In an open fermentation, the slowing of fermentation, higher temperatures and air currents moving across surface of the fermentation are all going to contribute to the breakdown of the CO2 blanket. If you practice open fermentation, you should rack to a closed container when fermentation has slowed dramatically.

The blanket of CO2 atop an active fermentation is not a solid barrier. Gas diffusion is occurring constantly as CO2 diffuses into the air above it and oxygen (and nitrogen) dissolve into the region of high CO2 concentration. And, of course, a strong air current can blow this blanket away. It is the fact that CO2 is constantly being replaced that keeps the “CO2 blanket” intact during fermentation and able to protect the fermenting liquid from the vast majority of the oxygen exposure it would have suffered, if it had not been fermenting.

 

Gas From A Cylinder

In the short term, you can add CO2 gas from a CO2 cylinder to a vessel and it will form a temporary pool or blanket. However, unless you continually refresh it, this CO2 will fairly rapidly diffuse into the air around it (and vice versa). When CO2 comes from a gas cylinder, it is colder than its surroundings. If you direct the flow of CO2 into the bottom of a vessel, it will flow to the bottom and pool there. Because the CO2 gas is in the form of a stream, the collective mass of CO2 molecules will be heavier than the surrounding gases and sink. Although the pool of CO2 will eventually diffuse away, it will remain long enough to provide some measure of protection if you rack beer (or wine) into the vessel, assuming you work quickly.

In commercial breweries, the ability of CO2 to pool can lead to a suffocation hazard. For example, when workers enter tanks to clean them, they make sure both the top opening and lower manway are open and enough time has elapsed for CO2 to flow out. In home breweries, the odds of hazardous amounts of CO2 accumulating somewhere are slim. If you do perform a fermentation (open or closed) within a mostly sealed chamber, for example, a converted chest freezer, CO2 levels can become elevated inside the chamber. If you lean into the chamber, you will definitely feel lightheaded. Also, if CO2 cylinder fails in an enclosed area (for example if the overpressure valves blows when the cylinder is in a car), this can lead to problems. Although the potential is there, I have never heard of a single homebrewer suffocating from CO2 gas.

See the sidebar on Lake Nyos for information on how carbon dioxide gas can be deadly in real world situations.

So, to make a long story short, pockets or pools of CO2 gas can remain intact long enough to be at least a partial barrier to oxygen in homebrewing situations. The ultimate fate of any mass of CO2 gas in contact with surrounding air, however, is to diffuse evenly into it.

Comments

  1. As a spelunker I’ve known for a long time that CO2 does not form a permanent pool or blanket. In mines/caves it requires continuous rejeneration through breakdown of carbon bearing rocks or rotting vegetation (such as wood pilings). It’s nice to see the topic get some air-play in the brewing arena as well. Putting aluminum foil over your fermenter top works while CO2 is being generated by active fermentation. However, once that has slowed down the CO2 will slowly leak away and you will end up with just standard air (21% Oxygen) under your foil.

  2. Hi,
    Nice post!
    I have a question:
    When you use a 5 Gallons glass carboy fermentor, with a blow-off, the CO2 comes up from the beer during fermentation, right? But, at the end of fermentation (post-krauesen phase) you need to take a sample from the top to check SG. To do this, you need to take off the blow-off and insert a wine-thieve to take the sample. Is this procedure introduzcing O2 to the beer?

    Cheers
    Luis

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