Putting It In

Today, commercial beers generally contain a fairly consistent amount of carbonation. Among brewers, the extent of carbonation is usually specified in "volumes of CO₂." Brewers would say that the average American beer contains about 2.5 volumes of CO₂. Now here's what that means to the beer drinker: the CO₂ dissolved in one pint of beer would fill two and one-half pint glasses. Seen another way, the beer in a standard 12-ounce bottle contains enough CO₂ to fill a 16-inch softball.

That seems like a lot of gas, doesn't it?

Just remember that those gas volumes are based on what's present in the beer before it is poured into the glass. The process of pouring, foam formation, etc., reduces the amount of gas that is actually present in the beer when you drink it.

Now, even though yeast produce a lot of CO₂ during fermentation, most of it is released from the beer. As a result, fermented beer nearly always requires an additional dose of carbonation before it is packaged. To do this, brewers have two choices.

On the one hand, they can put the beer in a closed pressure tank, hook up a bottle of carbon dioxide, and pump a bunch in. Using pressure gauges and perhaps some simple testing equipment, they can determine exactly when the beer contains the desired level of carbonation. Most breweries use this technique because it is cheap, easy and fast.

The second choice relies on more traditional methods. Rather than adding carbon dioxide directly, brewers can add a measured amount of unfermented beer (called wort) along with some yeast. The yeast ferments the wort to produce additional CO₂. As long as the beer is stored in a pressure-tight container (keg, bottle or serving tank), during this phase, the newly produced gas will dissolve in the beer.

This more traditional carbonation technique is used by a number of brewers, both in the United States and Europe. When actively fermenting beer is added instead of separate doses of wort and yeast, it's called "kreusening." Because there is a German word for it, this simple variation makes the marketing folks go into hyperbole overdrive.

When natural carbonation is done in a bottle, the resulting beer is said to be "bottle conditioned." Craft brewers like this approach because it helps to protect the beer from oxidation. Just look at a bottle of Sierra Nevada Pale Ale. On the bottom of each bottle, you will see a thin white or tan layer of sediment. That's the yeast that produced the carbonation in the beer.

Cask ales contain less carbonation than regular draft beers, but they show how yeast is used to carbonate beer in a keg. Here, the beer/yeast/wort mixture is sealed in an English-style cask then kept at cellar temperatures while the carbonation develops. Before serving, the cask must be set in one place and allowed to settle. This drops all the yeast to the bottom of the cask so that the resulting beer is clear and bright.

A properly produced cask ale can help to illustrate the impact of carbonation on beer flavor. Look for a brewpub or beer bar that offers the same beer in cask and draft forms. Order a pint of each and compare them side by side. With the carbonic bite stripped away, cask ale usually displays a very different balance of hop and malt flavors. The resulting beer is quite different from the traditionally carbonated pint—and that's a key reason for the rising popularity of cask ale.

It's in the Tank

So now we know how CO₂ gets into the beer. But we still haven't talked about those big tanks of gas found at restaurants and bars. Here's the scoop.

Keg beer is usually stored in a cooler that is some distance from the tap where it is served. Getting the beer from the cooler to the tap takes some sort of energy. In the old days, the bartender provided this energy by walking to the keg and carrying the beer to the bar. Now, we let gas pressure do the work. Carbon dioxide is applied to the top of the keg. A line to the tap is drawn from the bottom of the keg. Thus, anytime the tap is opened, gas pressure drives beer out of the keg, through the line and into your glass or pitcher. Viola!

The trick in this system is setting the proper gas pressure. Too much and the beer becomes over-carbonated and foams excessively. But guess what? Too little pressure has the same effect. The correct pressure is different for every set-up since it depends on many variables, from the diameter of the hose and length of the run to the temperature of the beer and the height of the tap. Too often, well-intentioned bartenders lower gas pressure to prevent foaming of one beer, only to discover that all of the other beers start to foam as a result.

These days, another gas—nitrogen—is all the rage with certain brewers. Guinness took the lead in this area, developing a nitrogen draft system nearly 40 years ago. Now the brewer puts nitrogen in the canned "Guinness Pub Draft" as well. This is done with a plastic wafer called a "widget." This nitrogen-bloated toy lurks at the bottom of the can so it can spew a shot of gas into the beer at the very instant the beer is opened.

Both the draft and can systems infuse the beer with a nitrogen/carbon dioxide mixture that leads to a firm, creamy head while leaving a relatively light carbonation in the beer itself. This system has become popular in Britain because it mimics some properties of cask ale. Following this lead, many US brewers have adopted nitrogen draft for some of their British-style beers.

Other gasses have yet to be used commercially for "carbonating" beer, although some experimentation has been done by homebrewers. Helium seems promising (imagine the squeaky-falsetto burps!) as does nitrous oxide (although one pint might knock you out). Despite widespread distribution on the Internet, the story of Japanese beer carbonated with hydrogen and used for fire-breathing karaoke displays was nothing more than a spoof.

back to the top

This story originally appeared in All About Beer Magazine in January 2000.