Fermentation rate: affected by the levels of unfermentable sugars?

I'm fermenting an all grain oatmeal stout. I mashed at about 155f therefore expected a less attenuable wort. OG was 1.07. I pitched wyeast 1318 from a one quart starter. Ferment temps 68-70f. Fermentation began quickly (within 6 hours) and while it markedly slowed after 2-3 days, measurable progress continues (Nice Krausen, CO2 production,diminishing SG readings) 2.5 weeks later. Current gravity is 1.02, very close to the target 1.018. Let me add that I routinely swirl the carboy every 2-3 days after the initial rapid fermentation diminishes.

I'm relatively new to this (one year and 8-9 whole grain brews) but this seems like a long slow tail to an otherwise unremarkable fermentation. I don't think anything is wrong and the beer tastes great, malty rich, without significant sweetness. But it made me wonder whether unfermentable sugars could inhibit or slow utilization of fermentable sugars as fermentation progresses and levels of fermentables diminish.

On the other hand, there are any number of uncontrolled variables that could lead to this result as well as my relative inexperience leading me to label what may be a completely normal observation as an abberation.

Your thoughts welcomed.

I've not noticed any slow down with less fermentable worts. Certainly yeast are able to selectively target simpler sugars (They go after the maltose or glucose/sucrose first.). I'd imagine if you had less readily fermentable sugars (e.g. maltotriose) , then those could take longer to ferment out, though.

So one more question: Will a less attenuable wort finish sweeter? In other words, do the residual unfermentables convey a sense of sweetness as well as body?

choosybeggar said:

So one more question: Will a less attenuable wort finish sweeter? In other words, do the residual unfermentables convey a sense of sweetness as well as body?

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I'd imagine that'd depend on the sugars left over. If there are a fair number of long-chain dextrines left, then they wouldn't add much of any sweetness. If you've ever tried malto-dextrine it's not very sweet. This of course assumes that the reason for a high fg is a less attentuative wort, and not a yeast/fermentation problem.

The stout in question is very good and approached the recipe's predicted FG (measure was 1.02, predicted was 1.018). To my palate, it is slightly sweet but I may be confusing sweet with malty. Didn't seem like there were fermentation problems. It's good brew anyway.