Any way to calculate carbonation factoring in unfinished wort fermentation?

[Yirg]

Some background: I have this porter in fermentation since Aug 15. After 3 days it seemed like fermentation finished, since there was no activity in the airlock. However, FG of 1.027 seemed a bit high, even taking into consideration the low attenuation of the S-33 yeast, so I decided to take it out of the temperature controlled fridge, and see if room temperature and some agitation would make it pick up again, and it did.

A day later, there was a bubble in the airlock every 30 seconds. Then it seemed to stall again at 1.025 and after some more agitation it picked up to a bubble every 40 seconds. Two days later, it was still at 1.025 with some activity. Then yesterday night it picked up to a bubble every 10 seconds, and today every 3 seconds (!), as if fermentation just started today. The odd thing (if it's not odd enough by now), is that gravity almost hasn't changed. With all this brisk fermentation, it's at 1.024.

I can wait until fermentation finishes, but this unpredictability (S-33 related?) makes me think that I should just bottle it, and calculate whatever remaining wort fermentation is left as part of the carbonation profile. The question is - how? Suppose I assume that FG will eventually end up at 1.017 (as BeerSmith suggests) and that I want to end up with Carbonation of 2.5 vols. How much corn sugar should I add (for a batch of 25L / 6.6 gallons)?

Thanks!

 

[agrazela]

I've had some experience with over-carbing due to continued fermentation in the bottles myself.

Using data from gravity at bottling vs. gravity after many weeks' room temp storage vs. target vol CO2 (based on how much priming sugar I added) vs. actual vol CO2 (as measured by Zahm-Nagel carb tester)--and having 6 overcarbonated batches worth of data--I calculated that every gravity point the brew lost between bottling and actual (i.e., long-term stored) final gravity came out to be about 0.6 vol CO2.

(This jibes with the theoretical, since 1 grav pt of sugar in 5 gal is 45 grams IIRC, and that DOES equal about 0.6 vol in the calculators)

Problem is, you don't REALLY know how many more pts. of gravity you're going to lose. So much depends on the fermentability of your extract, your partial-mash parameters, etc. Maybe it's not too late to do an FFT test to see how much you'll really drop.

One option is to go ahead and bottle with the correctly calculated amount of priming sugar, store at room-ish temp, then once they get to the carb level you want (maybe test a bottle weekly for 2-3 weeks--AFTER putting that bottle in the fridge a few days), fridge them all (or pasteurize them all if you are brave). This is what I've started doing with pale / hoppy "drink young" brews.

Another option--especially if you don't have that kind of fridge space--is to let the brew sit in the fermenter longer (at least 3-4 weeks, ideally more like 6-8 weeks) before bottling, to see if it really is going to drop any more. I know it is said that 2 same gravity readings over 2-3 days means you're done, but I'd say stretch that to no change over 2-3 weeks (at least) to be sure. I think that porter recipe can take that kind of bulk aging no problem, it WILL help more junk come out of suspension and lead to a better, more stable brew on bottle storage, and this is what I do now with strong and/or malty "age them" type brews.

 

[Yirg]

Thank you so much for the detailed reply. It's helpful and interesting.

The gravity of my brew has finally started dropping again (to 1.018) and fermentation has slowed down (bubble every 20 seconds or so), so I think I'll let it finish and just be patient. I don't think that at this level of gravity it's too risky to wait the typical two days of no activity before bottling, so this is what I'll do. I don't think it's going to drop a lot more below the current gravity.

BTW, have you done a comparison of the results you're getting when bottling after 3-8 weeks of no active fermentation (I assume in a secondary container) vs. 2 days? If fermentation really stopped in both cases, then in theory there shouldn't be much of a difference in carbonation, no?

 

[agrazela]

Thank you so much for the detailed reply. It's helpful and interesting.

The gravity of my brew has finally started dropping again (to 1.018) and fermentation has slowed down (bubble every 20 seconds or so), so I think I'll let it finish and just be patient. I don't think that at this level of gravity it's too risky to wait the typical two days of no activity before bottling, so this is what I'll do. I don't think it's going to drop a lot more below the current gravity.

BTW, have you done a comparison of the results you're getting when bottling after 3-8 weeks of no active fermentation (I assume in a secondary container) vs. 2 days? If fermentation really stopped in both cases, then in theory there shouldn't be much of a difference in carbonation, no?

There ya go, see. Following the standard mantra of "stable gravity over 2-3 days," you might have bottled at SG 1.025, and, if you stored these bottles for an extended time at room temp, you'd have set yourself up for something like (at least!) 4.2 vol CO2 of carbonation over and above your added priming sugar target, and very likely many bottle bombs.

I've only been at this for 1 year, but of the 20 or so batches I've bottled, only four did NOT become overcarbed upon extended room temperature bottle storage:
1. A big barleywine that I let sit in secondary for 8 weeks before bottling; and it continued to slowly drop in gravity for the first 5 of those 8 weeks.
2. A sweet stout that sat in secondary for 6 weeks, and it continued to drop for the first 4 of those weeks.
3. and 4. A saison that got 3 weeks' primary only and a Belgian-ish IPA-ish brew that got 1 week primary and 2 weeks secondary, but I attribute those batches' proper carbonation success to the Belle Saison yeast (and the purposely high primary fermentation temperature) simply eating up everything from the wort that it was going to very, very quickly (as compared to a typical American or British Ale yeast in a cool primary).

I still have the barleywine (now 6 mos old), the saison (about 3 mos old) and the Belgian-ish IPA (about 8 weeks old); I do not have carb-tester data at ~3 weeks in the bottle on the first two. I should get some current carbonation data on all of those to see how they've changed, but I've cracked at least one of each over the last 2 weeks and they seem just right on carbonation.