Halting Fermentation at a Specific Gravity
Q: Thanks for your feedback on “soured beer” and Guinness (5,6). My question of the day is: How do you stop fermentation at a specific gravity? The reason I ask is that the good people at Hale’s Ales (Kirkland and Spokane, Washington) are helping me make a homebrew similar to their Moss Bay Extra. One of the brewers suggested I stop the fermentation at a gravity of 1.014 or 1.015 (3.57–3.83 °P). They skim the yeast off the top and artificially carbonate their beer. If I were to stop this fermentation early, prime the beer, then bottle, would I create a bunch of time bombs in my basement, ready to explode?
The recipe calls for 0.25 lb cara pils dextrin, 1.5 lb crystal 35 °L, 1.5 lb crystal 140 °L, 6 lb pale malt extract, 1.5 oz Centennial hops (7.8%) for 60 minutes and 1 oz for 3 minutes. According to my Suds report, I should start off with a gravity of 1.060 (14.67 °P). Any comments would be appreciated!
A: The best way I know of to stop an ale fermentation is to crash cool the beer; that is, chill it to 32 °F (0 °C) as quickly as possible. This method will stop most ale yeast in their tracks, and it usually works on lager yeast too, if you do it quickly enough. I suspect that’s how the folks at Hale’s do it. The only other practical method would be to physically remove the yeast by filtration or centrifugation. Fining to precipitate the yeast might work, but usually finings are not 100% effective.
In any case, I don’t think arresting the fermentation is a good idea if you intend to bottle the beer. To bottle condition it, you will have to reintroduce yeast. This will do two things. First, it will turn your bottles into time bombs, as you say. Second, the yeast will consume the residual sugars, thus diminishing the malty sweetness which is (I assume) the reason the Hale’s brewers arrest their fermentation.
To get a higher terminal gravity in your ale, the simplest tactic would be to use a less attenuating strain of ale yeast. Check “The Yeast Directory” in BrewingTechniques’ 1996 Brewers’ Market Guide (7) for likely candidates. Of course, changing yeasts will also change the flavor of your beer, so you won’t be duplicating your model.
If your heart is set on brewing this beer at home, using the same yeast that Hale’s is using, you will have to invest in two soda kegs and a used refrigerator. Transfer the beer from the primary fermentor to the first keg when the gravity is down to about 1.020 (5.08 °P). Monitor the attenuation closely. As soon as the gravity hits 1.015 (3.83 °P), put the keg in the refrigerator and set the thermostat for maximum cold. Let the keg lie on its side if possible so that the yeast won’t get pulled into the draw tube when you rack the beer again. After a few days in the cold, move the beer under counterpressure to the other keg. Carbonate it and serve it as draft beer.
Nothing is foolproof, but even if by some dreadful chance the finished beer resumes its fermentation, the soda keg will take the pressure (soda kegs are rated to 130 psi). You’ll be a lot safer than you would be with bottles.
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