Does 1056 Dislike Dark Wort?

[blonberg]

My house yeast is WYeast American Ale 1056, and for the most part, I love the strain. I don't always love it when I brew dark beers, however, or rather, it seems to not like the dark wort.

Here's my problem: For my Cream Ale, Grätzer, Pale Ale, IPA, Red Ale, among many others, this strain does great. I often get even better attenuation than I'm supposed to. When I brew darker beers, though, I always have an issue with beers finishing too high and at a lower percentage than this strain should be able to accomplish.

Just FYI, the predicted attenuation for this strain is 73-77%.

Here are some examples from my most recent batches:
Cream Ale - 12 Plato, 83% attenuation
Grätzer - 16.5 Plato, 83% attenuation
Pale Ale - 15 Plato, 77% attenuation
IPA - 16 Plato, 80% attenuation
Red Ale - 14.5 Plato, 79% attenuation
---Things are different when I brew darker beers---
Brown Ale - 14 Plato, 71% attenuation
Porter - 16 Plato, 70% attenuation
Oatmeal Stout - 19 Plato, 68% attenuation

My darker beers aren't overly sweet, and I've never felt they tasted under-attenuated, and I've never gotten a customer complaint, but it mystifies me. Does specific gravity get effected by some chemical compounds in darker malt? I can't think of anything else that would have any effect on the yeast itself. I use the same mashing temps for nearly all of my beers, I'm hitting my brewhouse efficiency numbers very close to the same levels from batch to batch, and use a 60 minute mash for all of my beers except for the Cream Ale, which I let go for 90. I have a standard 90 minute boil, except for the Cream Ale and the Brown Ale, which only boil for 60 minutes. It is the same water, same oxygenation rate, same fermentation temperatures, same pitch rates, et al. And it clearly isn't an issue of wort strength, as you can see from the above examples.

Can anyone tell me what's going on here? I'm stumped.

[nickfl]

How much caramel/crystal malt are you using in these darker recipes? Caramelized malt tends to make a wort less fermentable regardless of how you mash it. Other dark grains may have a similar effect because they have also had some of their starch converted into compounds which can no longer be broken down into fermentable sugars but which still increase the measured density of the wort. Even so, I would look to caramel malt as the most likely culprit. BTW, your attenuation numbers are in the correct range for those styles, an oatmeal stout really shouldn't be much dryer than 68%AA, maybe 73-75% at the most, so I wouldn't worry about it.

[barleyfreak]

from here,

Dark caramel and roasted malts like Crystal 80, Crystal 120, Special B, Chocolate Malt, and Roast Barley have a high proportion of unfermentable sugars due to the high degree of caramelization (or charring). The total soluble extract (percent by weight) of these malts is close to that of base malt, but just because it's soluble does not mean it is fermentable. These sugars are only partially fermentable and contribute both a residual sweetness and higher FG to the finished beer.

[blonberg]

How much caramel/crystal malt are you using in these darker recipes?

My brown is only about 6.5% dark malt, but it does have some buckwheat flour in it, so who knows what that makes, aside from being a flavor addition. The Porter has a larger percentage, about 11%, and the O.S. has 12.5%, and another 7% of C-60, if that is dark enough to make a difference. I knew that these malts had a higher percentage of unfermentables, but I didn't know it was enough to make a significant difference.

Thank you both for your reassurance.

[blonberg]

I will also mention that I've never seen this difference to quite this degree with either the 2206 Bavarian Lager or the 3538 Leuven Pale Ale, my preferred Lager and Belgian strains, respectively. Thoughts?

[spetrovits]

The other thing that might be affecting the yeast is the tendency for dark malts to lower the pH of the wort. Acidic worts are less hospitable to yeast. That said, nothing about your grist bills seems out of the ordinary, and I personally haven't seen a reduction in attenuation for our darker beers. YMMV, of course. Just a thought.

[MSBCAKbrewer]

The other thing that might be affecting the yeast is the tendency for dark malts to lower the pH of the wort. Acidic worts are less hospitable to yeast. That said, nothing about your grist bills seems out of the ordinary, and I personally haven't seen a reduction in attenuation for our darker beers. YMMV, of course. Just a thought.

I would tend to disagree with this assessment. Yeast can perform at very low pH levels. We just fermented a kettle sour with a pH of 3.4 and achieved 81% attenuation with San Diego Super Yeast, which is fairly standard in our experience. I think the issue is simply that malts with higher kiln temps simply do not offer the same amount fermentability, so higher quantities of dark malts = less attenuation.

[Junkyard]

I would tend to disagree with this assessment. Yeast can perform at very low pH levels. We just fermented a kettle sour with a pH of 3.4 and achieved 81% attenuation with San Diego Super Yeast, which is fairly standard in our experience. I think the issue is simply that malts with higher kiln temps simply do not offer the same amount fermentability, so higher quantities of dark malts = less attenuation.

Agreed! If anything darker malts lowering the Ph of the wort would be preferred by yeast, since they need to lower it anyways to 4.0-4.6 ph finished, from 5.0 (dark wort) or 5.3 (light wort)