Carbonation, Perceived Bitterness and Carbonic Acid

It's wild how we can keep increasing our vocabulary for and our ability to detect and identify different sensations and flavors in beers. Malt, hops and yeast derived flavors all get a lot of credit in this department, but I think carbonation tends to get missed as a major contributor. Particularly since trying to dial in our carbonation levels for competition (that's a whole separate post for a future date), I've really started to notice how carbonation can alter the flavor of a given beer.

It's pretty common knowledge at this point that a well-poured beer has a nice rocky head on it which laces its way down the glass as you enjoy your beer. It's also fairly clear that one of the important contributions from carbonation is to help carry the subtle, more volatile aromatic compounds up and out of the beer and into your nose. Without that, you wouldn't perceive much of the pungent herbacious, fruity, spicy notes from late hop additions and you'd miss out on many of the chemical compounds produced by the yeast as fermentation by-products. Lastly, it's pretty self-evident that carbonation affects how you "feel" a beer in your mouth. It combines with proteins, residual sugars and other carbohydrates to create the slight to moderate viscosity we associate with a beer. To little carbonation can leave a beer feeling thin and watery, where as the ideal level creates a pleasant tingling sensation and an appropriate perception of body.

However, it can also play some other significant roles. As CO₂ goes into solution, it doesn't simply dissolve into the liquid and magically stay gas. It reacts with the water and reaches equilibrium with some part of the gas having combined with the water to form carbonic acid. (CO₂ + H₂O  H₂CO₃)

The chemistry gets complicated (and to be honest is way beyond what I remember from college chemistry) at this point, but the formation of carbonic acid under pressure contributes in a material way to the acidity of the beer. In beer conditioning, we're always adding CO₂ to our beer under some sort of pressure either by bottle conditioning, by force-carbonating in a keg, or by adding CO₂ to a bright tank in a commercial brewery. As the level of partial pressure goes up, the acidity of the solution does as well. Looking at this table shows that at 1 atmosphere of pressure (about 15 psi), the acidity of a water - CO₂ solution is 3.92. At 2.5 atmospheres (about 37.5 psi, which also happens to be the level of carbonation in soda) it drops to 3.72. The pH of finished beer tends to be around 4, so as we're conditioning and carbonating our beer, we're actually making it slightly more acidic.

[[Quick refresher on pH: It's a logarithmic scale from 0 to 14 with 7 being completely neutral, 0 being massively acidic and 14 being wickedly basic. Logarithmic means that a change in one number on the scale represents a ten-fold change in acidity. So a pH of 5.0 is ten times more acidic than a pH of 6.0.]]

And so what? I'll answer that. On my palate, highly carbonated beers come across as both more bitter and slightly "sharper" on the tongue. I'm not totally sure what drives the latter sensation, but I'm guessing that the latter is from the acidity. I've picked it up in the difference between un-carbonated wort before conditioning and the finished product. Ashley is particularly good at tasting a sample of young beer and letting me know that it needs a couple more weeks or that it's too flat. It's also readily apparent (at least to me) on very highly carbonated beers and particularly so in beers inoculated with a Brettanomyces strain and bottle conditioned.

My buddy Jeremy and I recently had a bottle of Sierra Nevada / Russian River Brux. When we first poured it cold, there was a massive amount of carbonation and the beer was particularly sharp on the tongue. It came across as bitter and rather monodimensional with a mild Brett character. However, as we let the beer warm and degassed it with some aggressively beer-snobbish glass swirling, the more subtle characters than Brett can and will really came through. The bitterness and sharpness faded from the foreground and let notes of lemon, mild pear, pineapple and some of the characteristic barnyard or horse-blanket (nowhere near as nasty as it sounds) come through.

So the next time you're sampling a beer (homebrewed or commercial), pay attention to the level of carbonation you can feel on your tongue and how you're perceiving the beer. If it's too sharp, too bitter or too bubbly, let the glass warm up and give it a few swirls to knock some of the CO₂ out. It might be a little more to your liking. Or if it's a homebrew that's flat and lacking some of the pronounced bitterness you were trying to get, dial up the pressure on your kegerator for a few days and try to taste the difference.


Sources and References:
http://en.wikipedia.org/wiki/Carbonic_acid
http://www.kegerators.com/carbonation-table.php