Yeast Attenuation

[cubbie]

A couple of weeks back I put down a Wee Heavy (Scottish Strong Ale).

It was about 98% TF Marris Otter and 2% Roasted Barley, Fuggles 34g 60min to 24IBU

Mash was 69deg 60min, Mash out and Sparge 75deg

120min Boil

I fermented at 18c for 14 days with Wyeast 1084 (Irish Ale).

My OG was 1.084 as measured by refractometer, on Tuesday I measured the gravity and it was 1.030 (measure by refractometer - used Promash to estimate gravity during fermentation)

Anyway that gave me an apparent attenuation of 62.6% (ADF) and an Alc% of 7.13%

My question is would you expect to get a greater attenuation considering the mash temp and alc% The rated attenuation of the yeast is 71-75.

All signs of fermentation appeared to be finished although I must admit I have taken only the one SG reading. The other point of note is as the fermentation slowed the overnight temps did start to drop the temp of the fermentation (even though I have a fermentation fridge).

Having never brewed a beer with such a high finishing gravity I would like to determine if my fermentation has finished or has it just stalled?

 

[reviled]

It may well have stalled, ive been having similair issues lately and im not quite sure what it is... Im thinking maybe the batteries in my thermometer may be flat causing incorrect readings?

But in saying that, ive also read that 1084 is quite a slow attenuator, so maybe try to bring the temp up a bit, swirl the yeast back into suspension and see what happens...

 

[PostModern]

98% TF Marris Otter and 2% Roasted Barley, Fuggles 34g 60min to 24IBU

Mash was 69deg 60min, Mash out and Sparge 75deg
<snip>
My question is would you expect to get a greater attenuation considering the mash temp and alc%

With a hot mash like that, you expect lower attenuation, not higher. Still 64% is pretty low. I generally mash 65-ish and 63-64 for big beers, or else they become sticky high-FG things.

Not necessarily saying this is all that's going on. How much yeast did you pitch? Aeration?

 

[cubbie]

With a hot mash like that, you expect lower attenuation, not higher. Still 64% is pretty low. I generally mash 65-ish and 63-64 for big beers, or else they become sticky high-FG things.

Not necessarily saying this is all that's going on. How much yeast did you pitch? Aeration?

I mashed at 69 as I thought it was appropriate for the style. I usually make a 1.5 litre starter (must admit this starter was a bit slow to get going). My aeration is primarily dropping the wort from the kettle to the fermenter.

 

[Swinging Beef]

Ferment temps ok?
Big beers take ages to finish off.
Dropping one point per day is not unusual when the yeast has had to run down from 1.090 or so.

 

[PostModern]

I mashed at 69 as I thought it was appropriate for the style. I usually make a 1.5 litre starter (must admit this starter was a bit slow to get going). My aeration is primarily dropping the wort from the kettle to the fermenter.

Big beers need big healthy bunches of yeast. I'll often brew a sacrificial pale ale and pitch a cup or two of the slurry when I make a strong beer. Not made a wee heavy, so not sure how Scottish ale yeasts react. Definitely will get a sweet, high FG beer from the high mash temp, but again, I'm not a brewer of Scottish beers, so it could be right :huh:

 

[THE DRUNK ARAB]

Big beers need big healthy bunches of yeast. I'll often brew a sacrificial pale ale and pitch a cup or two of the slurry when I make a strong beer. Not made a wee heavy, so not sure how Scottish ale yeasts react. Definitely will get a sweet, high FG beer from the high mash temp, but again, I'm not a brewer of Scottish beers, so it could be right :huh:

Umm, cubbie used the Irish Ale Yeast PoMo :lol:

i've used the 1084 a few times but not in high gravity beers. Never had a problem with low attenuation either and even though you mashed high I reckon you might have got better attenuation with said yeast.
So I would say PoMo's point is valid and a larger than normal ale starter would have seen your beer ferment out a lot better.