Beer brewing is all about beer flavor. There are endless techniques, gadgets and opinions about the best way to make beer, but at the end of the day, a brewer is judged by how his beer tastes. And the last step in the brewing process that can influence flavor is carbonation.
Carbonation stimulates a nerve in the mouth called the trigeminal nerve. This nerve senses hot, cold and irritants, like the capsicums in spicy peppers. Highly carbonated beers are said to have a "CO2 burn." This burn is a sensation that is felt throughout the mouth.
Carbonation also influences beer flavor by carrying volatile aroma compounds out of the beer, making these compounds easier to detect with your nose. In addition to carrying flavor compounds, carbon dioxide has a dramatic influence on beer foam and the creamy mouthfeel associated with good beer foam. In a nutshell, beer carbonation is important and should be controlled as carefully as original gravity, bitterness and color.
Measurement and Mechanics
Carbonation level is usually expressed either in terms of "volumes" or grams of carbon dioxide per liter of beer. The latter term is much more common in countries that embrace the metric system (everywhere in the world but the United States!). Most beers in the United States contain roughly 2.5 volumes of carbon dioxide, or about 5 grams per liter. This means that if all the carbon dioxide in one liter of beer were expanded at 0° C and at one atmosphere of pressure, its volume would be 2.5 liters.
The volume is a fairly obtuse unit, but the key thing to understand is that CO2 volumes increase in beer as top pressure increases at a fixed temperature. CO2 volumes also increase as temperature decreases at a fixed top pressure.
For example, a beer equilibrated with carbon dioxide at 38° F with a top pressure of 12 pounds per square inch (psi) will have a lower CO2 content than a beer equilibrated with carbon dioxide at 38° F with a top pressure of 15 psi. The CO2 content would increase further if the beer were cooled to 34° F.
The word "equilibrated" is very important when it comes to understanding carbonation. Suppose you take a keg of uncarbonated beer at 38° F, hook a CO2 supply to the keg and set the gas regulator to 12 psi. If you refer to a standard gas chart, it will say that beer contains 2.57 volumes of CO2. But that's not exactly right: While the beer in the keg will eventually contain 2.57 volumes, it begins flat and picks up carbon dioxide from the headspace over time. It's a dynamic condition.
A gas chart shows equilibrium carbonation levels in beer at various temperatures and pressures. (For a complete chart, see "Carbonate with Your Keg" in the May 2000 issue of BYO.) But beer that's already in equilibrium is far different than the dynamic condition in the example above.
The rate of carbonation, or the time it takes for beer and headspace pressure to equilibrate, depends on three primary factors: (1) the ratio of gas surface to beer volume, (2) the difference in carbon-dioxide concentration between the gas and beer phase and (3) time. The topic of gas diffusion can be made very complicated by mathematicians and engineers, but the basic concepts are fairly simple.
If the surface area of gas is increased for a given beer volume, the carbonation rate will increase. Increasing the difference between the concentration of carbon dioxide in the gas and beer phase will increase the carbonation rate. CO2 concentration in the gas phase is a function of pressure - as the head pressure increases, so does CO2 concentration.
Rate is by definition dependent on time. A system will equilibrate given enough time. In relation to carbonation, beer will eventually equilibrate with headspace pressure. Fortunately for homebrewers, batch size is small enough for time to be on our sides.
Head Pressure and Time
One of the most reliable methods of carbonating beer to a desired level is to establish a constant pressure and temperature and wait. Suppose you want to carbonate the pilsner you just kegged to 2.6 volumes. Also suppose that you plan to serve the beer from your beer refrigerator at 40° F. According to the gas chart the equilibrium pressure associated with 2.6 volumes and 40° F is about 13.5 psi. This method is so simple! Place your keg in the refrigerator, connect your carbon dioxide source to the gas inlet of the keg, set the regulator to 13.5 psi and wait. A 5-gallon keg of beer usually takes 5 to 7 days to equilibrate. Although this is not the fastest method, the risk of over-carbonation is minimized and the beer is carbonated in a reasonable amount of time.
Head Pressure and Shake
Many brewers are anxious to taste their new batch and don't want to wait a week. A faster way to equilibrium is to set the system as described above and shake the keg. This technique is made easier if you use 3 to 4 feet of gas hose between the CO2 bottle and the keg. The principle involved in this method is surface area - shaking the keg increases the exposure of gas and beer. This method works best if the keg is shaken, allowed to rest and shaken again. After several shake-and-rest cycles you'll notice a difference in the sound of the carbon-dioxide flow from the gas regulator. When the beer is equilibrated with the headspace pressure the sound of carbon dioxide flowing through the regulator will stop.
Some brewers take this method a step further and increase the headspace pressure (the carbon dioxide concentration) above the desired equilibrium pressure. This is the second principle I mentioned in the three principles of carbonation rate. Although this method increases carbonation rate, it is not easy to control. Since the pressure applied to the system is higher than the desired equilibrium pressure, the beer can be easily over-carbonated. I personally do not like the technique, but many brewers use it and it's worthy of consideration.
One tip to remember when using the "pressurize and shake" method is to wait a couple hours between the last shake and the first pour!
Another technique used to cut down on carbonation time is to inject the carbon dioxide into the beer from the bottom of the keg. This method increases the gas surface area and also increases the time of gas exposure to the beer as the bubbles rise. Some brewers simply inject gas through the dip tube on the keg and slowly bleed gas from the headspace of the keg.
Bleeding off the pressure is important to keep the gas flowing through the beer. The idea is to bleed the headspace so slowly that the headspace agrees with your target equilibrium pressure and the beer does not foam from the top of the keg. This can be accomplished by barely unscrewing the pressure relief valve on the top of most Cornelius kegs. Kegs without the screw-in pressure relief can be manually bled. This method is greatly improved when the gas is introduced to beer through a carbonation stone. The stone diffuses the gas into a stream of very small bubbles. Small bubbles are preferred because they have a very high surface-to-volume ratio and small bubbles rise slower than large bubbles. The combined effect is both greater surface area and increased contact time.
This is the fastest method of bulk carbonation in a tank or keg. ("In-line carbonation," which is accomplished by diffusing CO2 into beer as it travels though an injection device in a pipe, is the very fastest method and is used by larger commercial brewers.) When using a stone, it is very important not to exceed your target equilibrium pressure because your beer is likely to get over-carbonated. Carbonation stones cost about $25.
Control of beer carbonation is as important as weighing out malt and hops. Carbon dioxide is a vital component of beer; its level influences aroma, mouthfeel and foam. The scientific principles may be a bit tough to master, but the methods are a snap. So control your bubbles and make better beer!
Ashton Lewis is BYO's technical editor and Master Brewer at the Springfield Brewing Company in Springfield, Missouri.