Unless it asserts itself by, say, fizzing up your nose or aggressively dancing on your tongue, it can be easy to forget about carbonation in beer. Sure, when light filters through a glass and highlights the tiny bubbles as they appear at the bottom it offers a visual moment of enjoyment as they flitter to the surface. Carbonation is the spark of beer; it delivers aroma while effectively stirring it as you drink. It contributes to mouthfeel and its existence (or lack of) can help to establish a brew in its proper category.
Carbonation can be traced back through the centuries. There is evidence that the ancient Sumerians had foam in their beer, which would indicate the existence of carbonation. It would be a few more centuries before airtight commercial bottles allowed drinkers to have stronger carbonation one pop at a time and then a few years more before advances in molecular sciences allowed brewers to force carbonation into the beer itself.
To just think of carbonation as merely bubbles in a glass is to dismiss the science, passion and countless hours brewers and researchers have put into making sure the effervescence is all it should be.
More Than Bubbles
Carbonation occurs after carbon dioxide (CO2) is dissolved inside a liquid. In beer natural carbonation first occurs during the fermentation process when yeast absorbs the sugar in the wort creating both alcohol and carbon dioxide. Some brewers will also add additional sugars to unpasteurized bottles of beer, allowing yeast to feast a second time, thus allowing additional CO2 into the beer.
Another way carbonation is created inside beer is by inserting the gas under high pressure—referred to as pounds per square inch or PSI—in a sealed container (a keg for instance) until the liquid absorbs the gas. When the pressure is relaxed the CO2 separates as small bubbles, causing the liquid—in this case beer—to fizz, form foam atop the beer and release the pleasant malt and hops aroma from the key ingredients.
Carbonation is measured by the volume of CO2 in liquid. The volumes vary by beer style, but the majority fall in the 2.1 to 2.8 ranges with cask ales offering less and some German varieties, like a weissbier or a Belgian lambic, offering more.
The ubiquitous American light lagers, as a point of reference, carry a volume of about 2.6 volumes.
CO2 also plays a role in the dispensing of beer on draught. Currently there is a concentrated effort on behalf of America’s craft brewers to make sure that when a pint is poured the carbonation levels are what they should be. This ensures existing customers and newcomers alike get the full experience of an expertly made commercial brew.
Understanding the science is crucial and is a point of passion for many brewers, professionals and homebrewers who at one point or another in their career have struggled to keep the balance just right.
“The knowledge has changed in the last fifteen years,” says Chris Graham, the owner and partner at the Concord, CA-based homebrewing supply shop More Beer. He recalled his early days of homebrewing and how a pamphlet would say to set the CO2 to 20 pounds per square inch (psi) and then shake the keg.
“There was a lot of bad information on how to carbonate a beer,” he recalls. “I quickly learned that [technique] didn’t work.”
Now, there are a number of well-respected manuals, easy to use equipment and a greater respect overall to making sure carbonation is how it should be.
Can You Taste It?
Carbonation helps lift the aroma from the beer to the surface and into a drinker’s nostrils. There, the scent of hops, malt and any other ingredient the brewer chose to use in the particular batch comes to life and gives the drinker a sense of what is to come.
But does the CO2 itself have a taste?
“We tend to think of beer as being basically hops on one hand and malt on the other hand. It is more complicated that than. Carbonation puts an acidic quality into beer,” says Randy Mosher, a Chicago-based author and beer consultant. “Beer is mildly acidic anyway, but carbonic acid adds more.”
Carbonation does play a crucial role in a beer’s mouthfeel and in some cases that is the strongest thing a brew has going for it. The acid can lend a bite to the beer and even a slightly sour taste.
Looking at the so-called mainstream pilsner beers, carbonation is the most intense sensory experience they offer. “There are almost no hops, there is not a great deal of malts, even the yeast is weak,” says Mosher. “So, what you’re left with is carbonation.”
But in other cases a mild tingle of carbonation is enough to stimulate the taste buds and give the drinker a chance to feel bubbles on the tongue while getting a kick of hops and tang of malt.
“It just makes the beer much more alive, more animated,” says Mosher. “Everyone one of those bubbles are full of aroma as well and they create that head on the surface. It does a couple of pretty important things in the overall experience of beer.”
But, again, it is important to brewers to make sure that the carbonation volume is where it should be when served.
“Higher carbonation can suppress flavors,” says Jurado. “If beer is 85 percent aroma, the more carbonated the product is the less flavor you will perceive.
Not long after yeast is added to chilled wort inside a fermentation tank, the fungi gets down to work. The single cell microorganisms begin to ingest the fermentable sugars converting the beverage to alcohol and also releasing CO2 in the process.
Drinkers owe this bit of information to a French inventor and scientist named Cagniard de Latour who, around 1840, discovered that it was in fact yeast that was adding carbonation to beer. A few decades later Louis Pasteur released a book that studied fermentation and beer and, of course, pasteurization methods.
Over the centuries brewers have been able to cultivate yeast strains to get them to a consistency that is most suitable for beer, says Chris White, president of California’s White Laboratories.
Brewer’s yeast does not produce a phenolic flavor, where as most other strains will. Brewer’s yeast does keep some other enjoyable flavors that White says were selected over the years as brewers worked to perfect batches.
The brewer’s yeast also ferments the right kinds of sugars, where as some strains, White says, will consume all sugars. The yeast also settles to the bottom, which is unique and does not happen in nature. That last point is also a bonus for bottled-conditioned beer.
“They were domesticated by brewers, which is unusual for a microorganism,” he says.
White says that a renewed interest in homebrewing has brought new respect and a fresh look at yeast for carbonation. He explains that when technologies came into play where beer could be forcefully carbonated, that yeast continued to do what it does best, but took a back seat to producing bubbles.
“Except maybe in Belgium and in some odd recipes, people would rely on forced carbonation,” he says.
But he cites anecdotal evidences that more people are using just yeast to carbonate beers and in many cases, when done properly, naturally carbonated beer can produce just as much CO2 as a forced carbonated beer. If the beers use yeast over CO2 they tend to have a creamier flavor, White says.
Overall, White calls yeast “amazing” because it produces alcohol but “it has all these flavor compounds, we get something that tastes good and that carbonates as well, it’s pretty amazing.” (need to mention the flavors)
Forcing In The CO2
Once a year Graham, of More Beer, teaches a homebrewing course at the Siebel Institute in Chicago. Without fail, he says, the majority of questions at the end of the week focus on carbonation issues.
Recipes call for carbonation levels of various pressures and the equipment available to brewers today help to regulate the CO2 intake to make sure the gas is dissolved into beer. Also, CO2 works best when it is working with a beer below 60 degrees.
This can be accomplished with a CO2 tank and regulator where the carbon dioxide is slowly introduced to a keg, allowing the liquid to absorb the gas over time and achieve perfect serving carbonation. Carbonating slowly over the course of five to seven days not only gives the beer a chance to age, but also increases the likelihood that the beer will not over carbonate.
However, Graham says that more often than not beginning brewers will cheat a bit and “do double carbonation in half the time.” The problem is that unless you are constantly checking the beer along the way and able to know when it has reached the desired level by look and taste, you are likely to wind up with an overly carbonated beer.
Graham admitted to hedging his bets in the past, but says there is no proper substitute—in this method—to taking ones time. “In a way, you are screwed when you over carbonate. It will foam like crazy, and the only thing you can do is release pressure from a keg over the course of several days, tasting along the way. Still, there are no guarantees it will come out OK.”
Another way to force the CO2 into beer is by using a carbonation or diffusion stone. These slightly resemble spark plugs because of their long shape and fat head. Popular with brewpubs, they are attached to tubing connected to CO2 tanks and lowered into the fermentation tanks prior to serving. CO2 is forced into the stone (actually made of stainless steel) with increasing pressure and creates tiny bubbles that are immediately absorbed into the water. They are incredibly porous and should never be touched by hand prior to use, as skin oils can clog it and cause less than perfect results.
Larry Chase, the brewer at Standing Stone Brewery, a brewpub in Ashland, OR, remembers one particular batch early during his career with the brewery where he hooked up a carbonation stone.
“I hadn’t tested this particular one well enough before I stuck it in the tank,” recalls Chase. “So, we started carbonating and we can hear that the bubbles were really big. After 24 hours of trying, testing it, we couldn’t get this beer carbonated, it was still pretty flat.”
Knowing the beer was not yet ready to be served, Chase made the bold-but-necessary decision to switch out the stone. He and another worker at the brewery waited until the restaurant was closed, and climbed up to the mezzanine level above the bar where the fermentation tanks rest.
“So, we have a new stone ready to go, and we’re going to do a quick switch out,” explains Chase. “We know beer is going to come swooshing out of the hold, so one person has to open it and the other has to ram the new stone into the tank. We got soaked, the bar below did as well, but that second stone did the trick.
“I’d rather not serve beer and wait and then serve the beer when it is ready,” he says finishing the story.
Chase then offered this by-sight perspective: “When I pour samples from the Zwickel (a valve on the fermentation tank used to pour samples for testing purposes), and I have an eighth of an inch of beer on the bottom and the rest is foam. to me—in practice—that tells me my beer is carbonated to where it should be. Anything more than an eighth of an inch and it’s not where I want it and I need to get some CO2 in there.”
On the bottling front, many commercial brewers use a machine that first evacuates air, purges the bottle (growlers too) with CO2 and then fills from the bottom up with counter-pressure.
Getting Serious About Draft And Bottles
An overwhelming amount of craft beer is sold on draft and there exists a battle between breweries, bar owners and manufacturers to make sure that each pint served has the desired level of carbonation when it is placed before a customer.
The dread, of course, is a flat brew. Without the pop and fizz, a beer can be practically undrinkable. An overly carbonated beer, where a glass of foam is forced to settle into a full glass can alter the flavor of the beer, and leave it undrinkable to discerning customers.
“Draft beer is always a challenge in the United States,” says Jaime Jurado, director of brewing operations at the Gambrinus Company. “But we’re getting smarter.”
In the grand scheme of beer draft is fairly new. It was not until the late 1700s that the beer engine was introduced to pull beer from a cask into a glass. Before that, the fermented beverage was just poured directly into a mug, quickly losing what natural carbonation existed. Advances were made to mechanical CO2 systems about 50 years ago and drinkers were able to get carbonated beer on draft in a way that could mimic a bottle, which offered a carbonated beer as far back as the 1800s.
There are three general types from draft systems and each offer their own pros and cons. There they fall into three categories: temporary, direct-draw and long draw. All have a number of components from hoses, to connection points, to regulators that each play a crucial role in making sure the beer is presented the way the brewer intends.
The temporary system is the preferred choice for the casual backyard BBQ or even some beer festivals where drinkers using a picnic pump force compressed air into a keg, then dispensing the beer from a thumb-operated tap handle. There are also models that can accommodate single use CO2 canisters. More sophisticated brewers or breweries will use a jockey box where the CO2 pressurizes the system, and the beer is brought to the desired temperature by flowing through an ice chest before being served.
To educate and encourage proper pours, the Colorado-based Brewers Association has released its Draught Beer Quality Manual which offers a step-by-step look at every component and factor that goes into making sure a beer is drinkable. They warn that the temporary systems are good for one day at a time and should not be regularly incorporated into regular beer dispensing.
Then there are systems that are likely more familiar to bar patrons; the short and direct pour systems that use pure CO2 to push beer from kegs through lines, into taps and into a glass. These systems can include kegerators or kegged beer held relatively close to the tap handles.
There is the less desirable long-draw draft system where the beer travels a longer distance from the keg to glass. There is not only increased potential for contamination, but also cooling issues and increased beer loss that bar owners must contend with in using the systems.
It is not just CO2 that is used to dispense beer. Guinness, for example, uses nitrogen along with CO2 to get that signature effect in the glass and creamy taste. However, that mixture does not work as well with other beers and bar owners who cut corners trying to use the Guinness system on other beers quickly find themselves with under-carbonated lagers and ales.
“Put Bud on a Guinness system and within days the beer is flat,” says Jurado.
In The Glass
In conversations with brewers, industry professionals and even customers about carbonation, most brought up the Samuel Adams Boston Lager Glass, the result of two years of design and thought by the Boston Brewing Co. and Rastal Glass. With its bulb head and bottom curve for a person’s hand, the glass is nice to look at. It is the laser etching at the bottom of the glass, however, that was of particular interest to those interested in carbonation and overall flavor.
The laser etching is actually called a nucleation site. On a smooth glass surface, like the bottom of a shaker pint, there is no place for the carbonation to be released.
“We intentionally created the small imperfections on the bottom of the glass to allow the carbon dioxide to be released in the form of tiny bubbles,” explains Michelle Sullivan, a spokeswoman for the Boston Beer Co. “The act of pouring the beer into the glass releases some of the CO2 and the sites continue to release the gas. A slight decrease in the gas content makes the beer a little smoother on the palate. The constant stream of bubbles rising from the bottom also releases aromas within the beer. This allows the more volatile notes from the hops, malt and fermentation notes from the yeast to be collected in the top of the glass for the drinker to enjoy.”
Other brewers and glass manufacturers have followed Sam Adams’ lead in recent years.
A Bit About Cask Ale
When most think of cask ale, they will cite “lack of carbonation” or how the beer is flat, and when it comes to carbonic acid, Alex Hall, a brewer, cask ale expert and beer entrepreneur, is quick to give a lesson.
“If kept correctly, there is a subtle, natural carbonation which imparts a gentle gas prickle on the tongue, as opposed to a harsh one from CO2-pushed beer. The implication that cask beer is meant to be flat is incorrect, and is damaging to the growth of beer as such in its most natural form of dispense.”
Hall joined others in saying that aggressive, extraneous gas in a beer can take away from the full mouthfeel. However, cask beer does not fall on the other extreme of the spectrum by being considered flat or near flat.
Also linked with this issue is the serving temperature of modern kegged beer. To taste cask ale to the full flavor spectrum possible, traditional cellar temperature (i.e. 54 to 56 degrees) is optimum. The temperature is, yes, warmer than the freezing mark that some American breweries say is necessary to drink their beers. Serving a beer at the proper cellar temperature allows for a greater aroma from the beer, a more accurate mouthfeel and a still thirst-quenching beverage.
“Chilled any further, nuances on the palate are lost,” said Hall. “Cask beer is ideally served at that temperature, so the joint issue of this and the gentle carbonation from the secondary fermentation in the cask are jointly vital for the full flavor spectrum to be enjoyed.”
The common fallacy, Hall concluded, that cask beer is “warm and flat” must not be continued.
“Any beer that is truly warm and flat would be undrinkable,” says Hall. “Cask beer in good condition with a gentle, naturally produced carbonation is many miles away from that description.”
Critical Yet Understated
When drinking a beer we focus on the malts, the hops, the yeast strain—all vitally important—and so long as the beer is not overly carbonated or woefully flat, it will not get noticed. That’s a good thing for the brewers who want drinkers to focus on the ingredients and not just the science behind things.
Carbonation is a critical part of beer. Even before a drinker takes that first satisfying swallow, they see the beer itself. A proper thick head atop a beer and those rising CO2 bubbles facilitate the anticipation. No matter how they arrived into the beer, be in natural carbonation or forcing a gas into a liquid, it would be nearly impossible these days to imagine a beer being sold without it.
“Bubbles in general are just fascinating,” said Mosher.
A journalist since 1996, John Holl writes about beer and the culture of drinking. His first book, Indiana Breweries is now available in stores and online. He lives in New Jersey and blogs at his website beerbriefing.com