How to calculate safe amount of sugar when bottling brew that might still be fermenting?

[Yirg]

I'm brewing American Amber Ale (extract with specialty grains) and 3 weeks into fermentation, there's still pressure in the airlock and occasional bubbles. A few days ago it was a bubble every 2 minutes, now it's slower, though I haven't timed it yet. Current gravity is 1.018 (OG was 1.051). Would it be safe to bottle the beer and just aim for lower carbonation?

According to homebrewanswers.com, suitable carbonation range for this style is 2.3-2.8 vols. If the brew is at 1.018 and I aim for lower carbonation (2.3 vols) I should add (according to BeerSmith) 112g (0.33lb) of sugar. That's in comparison to the 150g (0.33lb) required for higher carbonation (2.8 vols). Is this approach safe enough if, say, final gravity (excluding the sugar) would continue to drop in the bottle to 1.010?

I know I could just wait until fermentation is for sure over, but 3 weeks in I'm beginning to worry about off flavors. I can transfer to secondary, but prefer to avoid it. If there's a way to calculate the amount of "safe" sugar this would be my preferred approach.

Thanks!

P.S.
This is the first and last time I use BRY-97 yeast. Way too slow for my liking.

 

[Yooper]

The short answer is that if it's still fermenting, you better not bottle it as there is no "safe" way to guess where it might stop.

A better answer is to check the gravity again in a couple of days. If it's still 1.018, it's fine to bottle.

 

[bucketnative]

Just wait for fermentation to finish. For every 1 gp (0.001 SG) that the gravity drops, you will gain approximately 0.5 vol CO2. So if you bottle your beer at 1.018 and it drops to final gravity of 1.010 in the bottle, you will have at least 4.0 vol CO2 (likely close to 5.0 when you consider the CO2 already in the beer). That's a big safety risk.

 

[flars]

There is no safety factor for bottling a beer that may still be fermenting since final gravity cannot be determined. I've never used that yeast but 1.018 is high for an amber ale. May depend upon how fermentable the ingredients were, pitch rate, and fermentation temperature. Racking to a secondary won't improve on the problem. Don't worry about yeast off flavors for another 2 months.

I would rehydrate a pack of US-05. Pitch the rehydrated yeast into 2 liters of very well aerated 1.040 wort. Pitch the entire liter when it is at high krausen in about 12 to 18 hours. US-05 will take care of remaining fermentable sugars and is a great neutral yeast for an amber ale.

 

[kh54s10]

What they said. Make sure it is not still fermenting before you bottle it. It can be done allowing the remaining fermentation to carbonate the bottles but it is a guess and is not what I would call safe.

 

[Yirg]

Very helpful answers. Thanks everyone!

I'll check gravity again in 2 days and will definitely not take any risks.

 

[Lefou]

I've played this game with cider before using 750ml PET bottles, but not with bottled beer requiring a dose of priming sugar.
In order to get a slightly petillant apple cider, I waited until the gravity was nearing 1.010, knowing the cider would ferment nearly "dry" down to .999 or below.
Instead of using glass, I opted for PET bottles. Although it wasn't the first time this particular cider recipe had been done, I didn't want to risk breaking my bottles.
It worked out fairly well. The cider carbed just fine and no bottles burst or leaked. It's not something I'd recommend with bottled lager or ale, but might be an interesting trick if performed on fresh cask ales in a smaller homebrew-sized barrel.

 

[Ninoid]

1.018 is too high for FG!
Try to stir it lightly or pour little dissolved sugar to wake up yeast.

Or, if you think that is enough for FG, pour gelatin, wait for two days and bottle. The gelatin will pick up most yeast so there should be no problem.

But, 1.018 is too high...

 

[kh54s10]

1.018 is too high for FG!
Try to stir it lightly or pour little dissolved sugar to wake up yeast.

Or, if you think that is enough for FG, pour gelatin, wait for two days and bottle. The gelatin will pick up most yeast so there should be no problem.

But, 1.018 is too high...

Gelatin will clear out proteins etc and some of the yeast, but it would not make it safe to bottle a beer that might continue to ferment some. If needed the yeast still in solution would propagate and continue fermentation and possibly cause bottle bombs. Otherwise, if gelatin took out all the yeast, you could not carbonate the beer by using priming sugar.

 

[Ninoid]

Gelatin will clear out proteins etc and some of the yeast, but it would not make it safe to bottle a beer that might continue to ferment some. If needed the yeast still in solution would propagate and continue fermentation and possibly cause bottle bombs. Otherwise, if gelatin took out all the yeast, you could not carbonate the beer by using priming sugar.

This is true, but gelatin pick up most of yeast.
I know from my own experience.

I am relative new homebrew and never have beer with that high FG (1.018). My beers end with 1.007 to 1.010 .
Maybe the yeast has a lower attenuation.