Can CO2 Form a “Blanket?”

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 and gives at a bit of “zing.”

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 this way — CO₂ is heavier than air and will sit at the bottom of an air-filled space and form a barrier to oxygen getting through.

Other folks 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 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 between the gases, 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 a while since you’ve had high school chemistry — or you can’t 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 some degree — from the liquid below it. The length of time a “CO₂ blanket” can exist, and the extent to which it 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 will 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. And, 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. (As a consequene, 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 the fermenting liquid 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, the cool, dense mass of gas 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 valve 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.