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Old 03-08-2010, 05:17 AM   #1
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Default Possibly Stuck Imperial Stout

I brewed an all-grain imperial stout recently that had an OG of 1.097. I racked it to secondary today (spent 19 days in primary) and when I took my gravity reading, I am getting 1.042, which is an apparent attenuation of about 54%. I used a 1.5L starter of Safale S-05. The starter spent 2.5 days on a stir plate prior to pitching. My wort was sufficiently aerated prior to pitching, and the yeast was pitched at 68 degrees. I was expecting to ferment down to somewhere between 1.025 and 1.030.

I racked this beer to secondary today. I moved it into a room where the temperature is a little warmer. The first 19 days in primary was at a temperature of between 61 and 63 degrees. Today, I moved it upstairs into a room where the temperature hovers around 70 degrees. Hopefully the agitation of racking and the warmer temperature will kick start the yeast and give me a little more fermentation. I will check the gravity again in about a week to see if any additional fermentation has happened.

If, after a week, I haven't seen any additional fermentation, should I try to pitch more yeast? In my previous experience, I have never seen any useful benefit of pitching more yeast to a wort that hasn't fermented out completely. I can't really come up with any ideas as to why this beer hasn't fermented out completely. I paid close attention to details with my yeast starter, aeration, and pitching temperature. What have I missed?


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Old 03-08-2010, 05:26 AM   #2
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I'm thinking you were fermenting a bit cool, despite US-05 working at 59-75. It's probably just taking its time.


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Old 03-08-2010, 03:54 PM   #3
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Could be... I hope it ferments down a little more. This entire process with this particular beer has been a learning experience. It's my 11th all-grain batch, but the first with such a large grain bill...
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Old 03-08-2010, 04:27 PM   #4
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Getting oxygen into high gravity worts is tough. It is also possible your yeast was a little old, which would mean more oxygen demand. I believe you that you aerated well, but it is just hard. I would only pitch more yeast if I thought the alcohol content was too high for the original yeast. But the numbers you give should be ok for an ale yeast.

It is possible that the yeast were using trub lipids in place of oxygen in your primary. When you moved it to the secondary, off the trub, this source of energy was lost. If this was the case, moving it was not a mistake, as lots of off flavors can result from trub lipid utilization. There should be some hardy yeast in suspension still that will finish your beer more. Keep us posted.
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Old 03-08-2010, 04:54 PM   #5
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At the very least, it's a great name.

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Old 03-08-2010, 05:36 PM   #6
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I suppose it could be an aeration issue, but even shaking the carboy will yield up to 8ppm. Also according to The Crabtree Effect, if the wort is >0.4% glucose (more likely with a big beer) then you don't really need oxygen for fermentation. You could be right but I still lean toward the temp. I'm sure it will drop a fair amount yet either way, just have to exercise a bit of patience.
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Old 03-15-2010, 01:54 AM   #7
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No additional fermentation after raising the temp to 70 degrees. It's still at 1.042. I pitched another pack of S05 yeast two days ago and there are no signs that it is doing anything. I'll check it again in a few more days, but I think it's done. Maybe I'll pitch some stronger yeast to it and see what happens...
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Old 03-15-2010, 04:14 AM   #8
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maybe try pitching some amylase enzyme. I had it bring a beer from 1.022 to 1.016 (last I checked).
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Old 03-15-2010, 05:24 AM   #9
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if you happened to use lots of dark malts and mashed hot it might account for some of the high FG, but I'd guess that second pitch will get you down more cuz thats just way too high. Probably not what you're going for, but some bretts will get you lower too.
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Old 03-15-2010, 01:21 PM   #10
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+1 to amylase enzyme. My RIS stuck at 1.044 from 1.090 -- added amylase and seven days later I'm down to 1.026. Figure I got a bad batch of Golden Promise as everything I've made with it has under-attenuated.


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