Carbonation is a key trait in beer. Scientists have recently determined that it is a taste just like sweet, salty and bitter. You don’t just get the physical sensation of the gas but you actually get a flavor of a mineral-like drying effect on top of the overall flavor of the beer. If you want to see this in action, try a few Belgian Trappist ales. Many come highly carbonated but if you let them sit for 15 minutes you’ll find the carbonic flavor has diminished and the malty and yeast-derived fruity flavors have become more pronounced. But not all beers have the same amount of carbonation. In fact, at many bars you’ll find some beers with four times as much carbonation as others.
In the United States, we measure carbonation levels in “volumes of CO2.” A common level of carbonation for a beer would be 2.5 volumes of CO2. That means that if we took all of the CO2 out of a keg of beer and put it into unpressurized containers the size of kegs, the CO2 alone would fill up two and a half kegs of space.
So how dowe put a keg full of beer plus 2.5 kegs of CO2 into one actual keg? Pressure, of course. Beer is kept under pressure and that both compresses the gas so that it takes up less room and dissolves the CO2 into the beer.
While most American-made beers contain 2.3 to 2.8 volumes of CO2, in most good beer bars you’ll find examples that are both lower and higher. At the low end are so called “nitro” beers like draft Guinness Stout. They are typically carbonated to as low as 1.0 to 1.5 volumes of CO2. At the high end, you’ll find both Belgian styles like Trippel and the German wheat beers called hefeweizens. These can be carbonated to at levels of 4.0 to 5.0 volumes of CO2.
As you can see, carbonation levels vary widely. The difference changes what you taste and it also dramatically changes how the bar serves the beer on draft. If the pressure applied to the keg doesn’t match the carbonation of the beer, flat or foaming beer can easily result.