One carbon atom, two oxygen atoms, born in a crossfire hurricane.
At normal, room temperatures carbon dioxide is a heavy, colorless gas. It’s a common by-product of organic activity, not just in your fermenter but in your lungs, too. When that army of yeast cells in your carboy ferments sugar into ethanol and carbon dioxide, most of the CO2 bubbles up through the beer and out the airlock to the atmosphere. It’s a gas.
The CO2 that remains in the beer is a key flavor element. Pure CO2 gas is hot, solvent-like; it will trigger a gag reflex. When balanced in beer, it enhances the malt and hop flavors and adds a clean dryness of its own. Then there’s the tactile experience of the frisky little bubbles doing their dance in your mouth, a dance that is definitely part of the magic.
Carbonation levels vary by beer style. A strong Belgian ale or quenching wheat beer often falls at the highly carbonated end of the scale, whereas a mild or Scotch ale might have a subtle sparkle. Tasters who find the carbonation level too high in a beer will disparage it as gassy.
Dom Perignon did it first, and brewers have been doing it ever since: keeping an elevated amount of CO2 in the beverage until we pour our glassful. Dom Perignon was the French winemaking monk who discovered carbonation by accidentally capping a not-quite-fermented wine bottle. He later took a sample and was supposed to have said, “I am drinking stars!” Brewers do it by capturing yeast-produced CO2 and not allowing it to bubble away or by forcing carbon dioxide into the beer by means of pressure.
To measure the amount of CO2 dissolved in a liquid, brewers use the concept of volumes: a pint of beer with one volume of CO2 has one pint of CO2 dissolved in it, at standard temperature (68° F) and pressure (1 atmosphere). If you forced two pints of CO2 into the same pint of beer, you would have beer with two volumes of CO2, and so on.
No liquid will hold an infinite amount of dissolved CO2, and in fact a liquid is said to have reached its saturation point when additional carbon dioxide will no longer dissolve. Two main factors affect the saturation point, however: temperature and pressure. Lowering the temperature of beer raises the saturation point so the beer will hold more dissolved CO2. Likewise, increased pressure raises the saturation point.
All Things Being Equal
One more concept is important to understanding CO2 in beer, especially for kegging beer and force-carbonating it: the inevitability of equilibrium. CO2 takes time to dissolve into liquid, and if the brewer forces CO2 under high pressure into the headspace of a keg, the CO2 will dissolve into the beer. It stops only when the headspace reaches equal CO2 pressure with the beer. If the brewer purges the keg, reducing the headspace pressure, the carbonation level of the beer will drop until equilibrium is achieved again.
This leads to some interesting enlightenment: We really enjoy CO2 most when it is in the act of leaving.
When you pop the top off a carbonated beer, you’re changing the pressure dramatically. Store-bought beer is packaged at as much as 2.8 volumes of CO2. The hiss you hear is the headspace in the bottle giving up high-pressure CO2. Now the beer is exposed to the atmosphere, which has a much lower pressure than did the head space inside the bottle. The CO2 seeks to reach equilibrium with the atmosphere, and those bubbles mystically appear in any freshly opened carbonated beverage. When they are all done rising, the liquid has become flat (a better beer-drinker’s word than equilibrium), but it does take a while because the pace slows as the pressure drops.
So you put a picnic tap into a half-barrel keg and draw off most of it at the company party. By tomorrow the remaining beer will have given up a bunch of CO2 to reach equilibrium with the vast headspace; it will be heading toward flat or already be there, no matter how much air you pumped in. The air in the head space will be reaching equilibrium with the beer and the CO2 in the beer will reach equilibrium with the head space, which uncarbonates the beer.
Head is the tenacity of the bubbles to hang around on top of your beer as you wipe your upper lip and admire. Head results from the surface tension of the gas/liquid interface of the beer. Some factors that affect surface tension are the amount of hops, protein, and alcohol in the beer. These are really separate issues from carbonation.
So how do brewers control carbonation? The classic method is via bottle or cask conditioning, which implies natural carbonation (produced by yeast) in the serving vessel. The brewer allows fermentation to run its course, then adds a measured amount of yeast food (for homebrewers this is often corn sugar primings) to the beer and caps the bottle or bungs the keg, preventing the CO2 produced from entering the atmosphere. The yeast produces more CO2, the pressure in the headspace rises, the saturation point rises, and the CO2 stays dissolved in the beer.
Factors that affect the CO2 level include: amount and variety of primings used, yeast strain characteristics, wort original gravity, conditioning time and temperature, and conditioning technique. And yet most homebrewers have no problem producing beautifully conditioned beer. The very best way to determine how much to prime is to break a batch up and try different amounts of an identical priming, then sample the results after the conditioning period. If you have always used three-quarters cup in five gallons, try three-eighths cup in 2.5 gallons and one-half cup in the other 2.5 gallons. Later, taste the beers as systematically as you are able. With patience and a pocket calculator, you can break your batch into three or more equal lots and try more options.
If you wish to try some other varieties of priming, go for it. But use a wort and yeast that you are familiar with so you have a better chance of isolating the contribution the priming change makes to flavor perception. As a starting point for a five-gallon batch try 1.25 cups dry malt extract, a half-cup of honey, or — if you’re feeling adventurous — a cup of sorghum.
If you have kegs, don’t sweat it. Chill the finished beer, put 30 psi of CO2 into a typical headspace, and relax for one to two days. Dispense at 8 psi. You can reduce the pressure if you want to encourage some CO2 to leave the beer, or turn it up for more fizz. Remember that it takes time for equilibrium to occur, so check it a couple of days before the party.