Stuck Imperial Porter

What's the gravity now? And are you using a refractometer?

 

Probably not the source of your problem, but your grain bill adds up to 91.1%.

Is there any reason to think the gravity hasn't fallen to a reasonable FG by now? When did you last measure it?

 

In that case you had over 58% MO and over 14.5% Munich to start, which certainly seems as though it would provide plenty of diastatic power for that malt bill. I also don't think a 159°F mash would elevate your FG this much. So I'm a little puzzled, although I'm curious to know what the gravity is at now.

 

Have you tried rousing the yeast? Sounds like they got tired. They'll probably finish eventually.
But maybe give them a swirlie.

 

Patience. Also....... if you dry hop, that can reinvigorate the fermentation as well, since hops contain enzymes. It's worked for me.

 

The highest regarded robust porter finishes at 1.030 for 7.5%... just sayin

 

• I know many people do not rehydrate dry yeast and typically get good results. I think Fermentis may now even recommend against it. (I always rehydrate)
• I know many people do not oxygenate when using dry yeast. (I typically do not, but have sometimes for higher gravity batches, especially if I only have one pack of yeast)
• I know many people do not make starters with dry yeast (I don't, except see below).
  
• I know many people would not hesitate to use one pack of dry yeast for a 1.079 OG beer. (I use 2 packs of dry yeast when I get out of the 1.060s)

Not trying to say "holier than thou," but I do think you are exposed to risks. I think the risks all point towards a lower than ideal population of yeast, which is suggestive of the sluggish fermentation.

What I would do is get some US-05, rehydrate, add it to a 1-L starter wort. Let the starter go for about 18 hours or so, to get the yeast active, and then pitch the whole thing into your beer. I wouldn't worry about the 1-L of starter wort messing with your beer flavor. I would disregard the people who say don't use a starter with dry yeast. You are pitching into hostile territory; you want them active at the time of pitching.

 

Id also up your temp to 68-69. Helps sometimes

 

I just kegged a 9% stout that I mashed at 162. thought the yeast had given up at 1.035 but it got down to 1.017 by week 3. Honestly I wish it had given up a little sooner. I’m guessing it’s something about my water I can’t seem to keep my beers from drying up. Even when I mash way high

 

Checked your thermometer? I don’t think there’s anything about water that would have a dramatic affect on attenuation. Kai did tests on Ca content I believe?

 

What would affect attenuation that much? Hydrometer read same as refractometer

 

Mash length. Was your grand total mash length for 60 minutes, or 90 minutes, or longer? Personally I only mash for 45 minutes. This achieves acceptable efficiency and attenuation every time. There's no need to mash longer, and as you are seeing it can increase conversion of complex sugars to simpler ones and thus increase attenuation to a point where it's ridiculous in some cases.

 

I’ll have to try a shorter mash I’ve always done 60 minutes

 

I've mostly gone to 45-minute mashes myself, partly to avoid over-attenuation, which I have also struggled with. It wasn't the only variable I changed, so it's hard to say how much effect it's had. I've also started doing a mashout step to try to stop the enzymes at the end of the mash, have you tried that?

 

Keep in mind that mash length is only one of four major factors affecting attenuation...
- Grain Bill
- Mash Temp
- Mash Time
- Yeast Strain

Whether 60 minutes is too long or too short depends on your goals and also the other factors.

 

I bet that’s it. I never do a mashout