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Cistercian · 11-08-2008, 06:20 PM

I just experienced my first stuck fermentation.

It was a Guiness clone. My OG was 1.044 (a little high).

After three weeks in the primary it stayed at 1.022.

I used Wyeast Irish Ale (1084).

I read through some of the previous posts on the subject and most seem to blame the yeasts. However, I wonder whether low attenuation doesn't have more to do with not enough beta enzymes gained through lower mashing temps (below 149).

Could this be the case?

Edcculus · 11-08-2008, 06:50 PM

enzymes won't have much to do with a stuck fermentation. It might affect the fermentability of the wort, but not enough to leave it at 1.044. Lower mash temperatures actually will make a more fermentable wort. I'm guessing your yeast are getting lazy.

I used 1084 in my sweet stout with lactose added. It pooped out a little higher than I would have liked, even accounting for the extra gravity the lactose gives.

menschmaschine · 11-08-2008, 07:14 PM

Higher mash temps can result in fermentation pooping out in the low 1.020s. But it's usually accompanied by less fermentable malts. I had an oatmeal stout poop out around 1.021 and nothing I did lowered it anymore. It was still good, but the thicker body was definitely noticable. What was your mash temp and are you confident in having not overshot it?

Cistercian · 11-08-2008, 07:47 PM

I mash in a kettle with a false bottom over a burner.

I mashed at 151 F. I turn the burner on low when it drops to 150 and bring at back up to 151 F.

I'm pretty good at keeping the temp at the right place...however, at one point I forgot to turn off the low burner. Next thing I knew it was up at 160 F!

I brought the mash temp back down by taking off the lid and turning off the heat, and continued as usual. Within a few minutes or so I had back down to 151 and kept it there until I mashed out at 170 F.

Nevertheless, it did get up to 160 F in the middle of the mash.

Might this be the answer to my question?

menschmaschine · 11-08-2008, 09:29 PM

Yeah, that may have done it. Same thing happened to me before and the results were similar. Unless I'm step mashing, I've learned to leave the burner off during the mash (insulated keggle MLT). Losing a degree or two in an hour is no big deal.

You could maybe pitch some Nottingham to get another point or two, but no guarantees. Also, you could dilute it with water at bottling/kegging time to get it down another point or two. Yeah, it will lose a little ABV and IBUs, but I did it to the oatmeal stout I mentioned earlier and it still came out tasting great. I wouldn't dilute it too much though.

Cistercian · 11-08-2008, 09:36 PM

My ABV is at 3%.

What about pouring a little vodka in it?...

menschmaschine · 11-08-2008, 09:47 PM

Quote:

Originally Posted by Cistercian

My ABV is at 3%.

What about pouring a little vodka in it?...

I don't see why not. I wouldn't get too crazy with it though.

Cistercian · 11-09-2008, 01:01 PM

Coming back to the title of this post then, a high mash temp would lessen Beta enzymes and that would be the reason for the stuck fermentation, i.e. the sugars aren't fully fermentable.

Did I get that correct?

menschmaschine · 11-09-2008, 01:21 PM

In a nutshell, yes. Each group of enzymes have their own optimal range and it isn't black and white. Beta amylase can break down starches into sugars in the 130s and up through about 160. The ideal temp (and therefore, the highest proportion of maltose being produced) is in the mid 140s. But if you kept your mash temp at 145, you'd have an extremely dry beer. With mash temps in the 150s, you start to favor alpha amylase. But mash thickness can play a role too. For example, if your mash temp was in the upper 150s and you had a thick mash (less than 1 qt/lb), your maltose production would be seriously diminished.

Piotr · 11-09-2008, 01:54 PM

Quote:

Originally Posted by Cistercian

Coming back to the title of this post then, a high mash temp would lessen Beta enzymes and that would be the reason for the stuck fermentation, i.e. the sugars aren't fully fermentable.

Technically, such fermentation is not stuck since yeast fermented properly all fermentable sugars and cleaned all fermentation leftovers.

It is good to make fast fermentation test (FFT) - using some yeast left on the bottom of the jar and some 100ml of wort you can check how low the farmentation can go, for this very wort and yeast. Just leave it in warm place, shaking from time to time. After 2-3 days check FG.