Really confused by this stuck fermentation

[othellomcbane]

I'm working on a Galaxy single-hop IPA that was supposed to finish out very dry... and has become stuck at very sweet instead.

Here's the grain bill. This is a mostly-grain partial mash — I'm in the process of going all grain but I still have some DME to use up.

29.9 % extra light DME
63.4 % Maris Otter
2.2 % honey malt
1.5 % aromatic malt

I mashed at 149. I had just calibrated my digital thermometer before starting this batch, and cross-checked it with my dial thermometer. So I'm pretty sure the mash couldn't have been off by more than a few degrees. 149 should have resulted in a highly fermentable wort, right? Original Gravity was 1.063, which is right in line with what Beer Smith predicted. Beer Smith suggests that my Final Gravity should be 1.016. And after three weeks, the gravity has been sitting at 1.020 for a full week.

Onto the yeast: I pitched a good 3/4 inches of healthy-look yeast slurry that I had harvested 2 days before from an ESB. The yeast was Safale S-04. The ESB was lower gravity than the IPA. I harvested the yeast, washed it, kept it in a large pickle jar in the fridge, and pitched it into the IPA. Didn't make a starter because I had so much slurry (and this was only a 3.5 gallon batch, I should point out... not even a full five gallons!), although obviously in retrospect, I should have made a starter anyway. Fermented at 65 degrees to start out with, moved up to 67 degrees after a few days, and then after two weeks moved it up to about 69/70. It's remained stuck at 1.020 despite warming up and shaking the bucket to stir up the yeast.

Finally, I resorted to pitching a totally fresh packet of S-04 to bring it down, while still at 70 degrees. No effect, bizarrely. Boiled and added a few ounces of sugar to see if that would accomplish anything. The yeast ate the sugar and brought it back to 1.020. How can a beer with this grain bill, OG, yeast and mash-temperature possibly be stuck at 1.020?

This makes no sense whatsoever to me. Can any of you wise internet people alleviate my confusion?

 

[Yooper]

It's not uncommon for many extract beers to finish at 1.020. I'm not a honey malt fan, but 2% seems to be a bit high, so I would expect a sweet finish. The beer is finished.

 

[othellomcbane]

It's not uncommon for many extract beers to finish at 1.020. I'm not a honey malt fan, but 2% seems to be a bit high, so I would expect a sweet finish. The beer is finished.

Hmm. All my beers so far have had at least some extract in them, and it's the only time I've had this issue. Every other beer I've done has finished out pretty much exactly where Beer Smith suggested it would.

I'm not so worried about the fact that it tastes sweet, I'd just like to understand why this particular beer stalled out where it did.

 

[thanantos]

I'm having the same issue with an IPA using Safale S-04.
https://www.homebrewtalk.com/f164/second-batch-trying-good-everyday-beer-327710/

Did you ever figure this out?

 

[duboman]

I agree with Yooper, especially after you re-pitched and swirled and still got nothing more.

 

[othellomcbane]

I don't think I figured it out, but I have some ideas. After this batch I switched to all grain and then I started to have this exact issue with almost every batch... so I really, really don't think it was extract. None of my other extract beers ever finished high at all.

I know that mashing high results in a less fermentable wort... pretty common knowledge. But apparently mashing really thin does the same thing, which I don't hear many people discuss. (I encountered this info randomly on some unrelated BYO Magazine article.) I was mashing all my partial mash / AG beers very thin (1.5 to 1.6 qts per lb.), not for any particular reason, just seemed like it would be easier than a thick mash. All those batches finished with a much higher FG than Beer Smith calculated or I would have expected.

I've done two batches since this occurred to me, mashed both of them at the standard 1.25 qts per lb., and both have hit the predicted target FG. Could just be coincidence, I can't claim to have experimented enough to have actually figured this out, but I think this might have something to do with it in my cases.