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.