Stuck fermentation???

[Divenglide]

Hi all - here’s hoping someone could advise me.
I recently made a one gallon test batch export stout with 25% speciality malts and 75%?MO which had an OG of 1.084. I mashed at 154f and I pitched a starter of London Ale yeast. Fermentation took of in a brew fridge really quickly (<12 hours) but then stopped at 1.034 - so about 60% attenuation. I have since tried to restart it with half a pack of US-05 straight in to the fermenter (no luck) and then tried again using the other half which I allowed to hydrate in some of the semi-fermented wort - I was being lazy and could not be bothered to make a starter but I seemed to get a small krausen before pitching. Is it possible that my mash temperatures and the 25% speciality malts have let to such poor attenuation? Thanks I advance

 

[kev211]

I have a hard time believing that 25% specialty malts and mash temps could leave you that far off. Just out of curiosity... What were you using to measure the FG?

 

[Divenglide]

I used a refractometer and then an online calculator to account for the alcohol. I’ve since checked the calibration of the refractometer and also tested it on the commercial stout that I have been trying to emulate and it seems to be fine. I’m now doing a forced fermentation on a sample of the ‘partially’ fermented beer to see what happens.

 

[Gnomebrewer]

Which specialty malts did you use? Some are less fermentable than others, and 25% in a beer that big is a lot of specialty malt.

 

[Divenglide]

This has certainly been a good lesson in the fermentability of speciality malts. Those that I used were as follows:

Brown Malt 8%
Cara Munich 2%
Roasted Barley 4.2%
D Crystal 2%
Special B 2%
Chocolate 3.7%
Amber 2%
Black Malt 1.1%

The forced fermentation suggested that this did indeed ferment as much as it was able to therefore I either have a mash temperature issue (I have checked the mash thermometer and believe it to be ok) or a recipe issue. I think the latter is likely and will probably look to tone it back to about 22% speciality malts and lower the mash temperature - what are some thoughts on this?

 

[Lefou]

When I see 8 specialty malts in one beer it makes me suspect fermentabilty and efficiency could've been affected by an out-of-balance mash pH.

 

[Divenglide]

That’s a good point. However, I effectively have a London Thames Water profile and checked with bru’n water to confirm that the mash PH would be ok. I had no idea what mash efficiency to expect from this one (I use BIAB) and I conservatively went for 65% but achieved something closer to 75%. I will be brewing this again this coming weekend and will check the pH this time to be sure. Having read a bit more on the subject, I think mashing really low will make a huge difference but i’m still surprised by this batch.

 

[RM-MN]

I used a refractometer and then an online calculator to account for the alcohol. I’ve since checked the calibration of the refractometer and also tested it on the commercial stout that I have been trying to emulate and it seems to be fine. I’m now doing a forced fermentation on a sample of the ‘partially’ fermented beer to see what happens.

When using a refractometer on fermented beer where you have alcohol present, the calculated gravity is an approximation and may vary depending on the specific beer recipe. Hydrometers are accurate.

 

[Divenglide]

Thanks - I was trying to get a handle on this but it seems that the algorithms are normally about 2 points out which might therefore have contributed to my low FG to a point. This is why I tested the refractometer on a commercial beer of the same style.