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 seemingly 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 effectively stirring it whilst you drink. It contributes to mouthfeel and its existence (or lack of) can help to establish a brew in its proper category.

Last week we examined yeast, which can produce carbonation in beer. This week we take a look at the sparky little bubbles themselves.

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.

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 COinto 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 sealed container until the liquid absorbs the gas. When the pressure is relaxed the COseparates 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.

CO2 also plays a role in the dispensing of beer on draught and 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.

Chris White, president of California’s White Laboratories says that a renewed interest in homebrewing has brought new respect and a fresh look at yeast for carbonation. He said that when technologies came into play where beer could be forced carbonated, that yeast continued to do what it does best, but took a back seat to producing bubbles.

But he cited 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.

Understanding the science is crucial and is a point of passion for many brewers, professionals and home brewers who at one point or another in their career have struggled to keep the balance just right.

Carbonation helps lift the aroma from the beer, to the surface and into a drinker’s nostrils. There, the scent of hops, malt and anything else the brewer chose to use in the particular batch come to life and give the drinker a sense of what is to come.

But, does the COitself 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.

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.”