Fermentation puzzle

[talkin]

I've been brewing AG for about a year with the help of threads on this forum. I now have a dozen or so batches under my belt (literally). My last two batches have thrown up a sluggish fermentation puzzle that has me stumped, so I figured I'd hazard a first post.

After a few SMASH batches aimed at hop flavour, my last two batches have been aimed at malt flavour. The grain bill for both was roughly 80% Maris Otter pale, 10% Caramunich II and 10% melanoidin. Way overboard on the speciality malts I know, but I like to learn from my own experiments!

I mashed both batches at 152F to produce a starting gravity of 1.046. The first I fermented with WLP005 (Ringwood), the second with Wyeast 2112 (California lager). In both cases I oxygenated the wort for 60 seconds using oxygen and an aquarium stone and pitched a starter made to Mr Malty's instructions. I fermented both at 65F from the off. Assuming 70% attenuation (which I have achieved comfortably with both these yeasts before), my target FG was 1.014.

Now here is the puzzle. In both cases I had really sluggish fermentations that gave up at the 1.020 mark, even after I roused the yeast by gentle stirring and upping the temperature to 68F. By my calculation, that is a lacklustre 56% attenuation.

I appreciate that this is not the worst home brewing disaster in the world. Both batches taste OKish, if a little sickly. But I am puzzled that both behaved the same way despite the very different yeasts. I am wondering whether this might be the 20% speciality malts. Could this be the explanation? And if so why?

Thanks in advance for any thoughts.

 

[h-bar]

It might be due to the specialty malts. Those have more unfermentable sugars. Use fewer specialty malts, and see if that helps.

 

[JimRausch]

Welcome to your 1st post- it's a conundrum. It reads like you have covered the usual suspects and are doing things properly.
So, what I would do next is double check my mash thermometer. Is it accurate or could it have gone out of whack and be running 5 or 6 degrees higher?
The 2nd Q I have is how you are measuring your fermentation temp. Environmental or actual beer temp? And how accurate might that thermometer be?

 

[ol-hazza]

Were your initial readings taken at the correct temperature for your hydrometer?

Im guessing you know about this already or im sure you would have come across problems earlier.

 

[McKnuckle]

That is odd, and I don't think the grain bill is the culprit. I will say that if you're trying to rouse sluggish yeast to finish up, that 68F is not a big kick in the pants. Try 75F at that point - fermentation is mostly done; you're not going to get unpleasant flavors due to the high temp. It has certainly worked for me a couple of times to finish up a beer.

I second the recommendation to calibrate your thermometer(s).

 

[talkin]

Thanks for all the input.

I spotted during the second batch that my thermometer has drifted to 101C in boiling wort, so I guess (if anything) the mash temperature was slightly lower than reported.

With the hydrometer, I always let hot wort cool a bit, measure the temperature, take a reading, then use an online tool to do the temperature correction (after believing I was getting efficiency in the 40s on my first few efforts!). For the samples during fermentation I tend not to bother because a few degrees from 20C seems to make no difference.

For fermentation temperature, I strap the probe to the carboy behind a couple of thicknesses of closed cell foam (one of the many things I learned on here). But I haven't calibrated the probe since I got the controller, so that could be the problem.

Again, thanks for pondering this. I'll definitely calibrate my thermometers before my next effort and, if it is still happening, have a go at a higher temperature to rouse the yeast. And next time I won't be using 20% specialty malts come what may!