Over Attenuating Yeast - How is it Possible?

Ingredients
1

Confused … :confused:

I brewed a Rogue Double Dead Guy clone, mashed at 152F - it started at 1.084 and finished at 1.010, fermented with Wyeast Pacman. No mash out.

Beersmith calculates that as a 82.7% attenuation. OG measured with a refractometer and FG measured with two hydrometers.

Wyeast’s stats are 72-78% attenuation for this strain.

The yeast was about 11 months old, smelled and tasted good, and pretty creamy white. I made a 2L starter with the decanted slurry and it took off in about 4 to 6 hours.

I hit the wort with O2 and pitched the decanted starter. Within 4 to 8 hours it began to show signs of fermentation.

The beer fermented @ 65F in a refrigerator (measured by a thermowell) for about 3 weeks. I let it sit on the yeast cake @ 45F for another three weeks. Approximately 6 weeks total.

This recipe should have finished around 1.018 - 1.020, mine however continued to 1.010. As a result, it is much drier with an alcohol bite. At 6 weeks old, it’s not so green anymore.

With that being said, what in your opinion might account for the higher attenuation?

If anything, I suspect old/aged yeast would be less attenuating, unless it mutated.

What about sitting on the yeast cake for an extended period? After three weeks @ 65F, the yeast should have been entirely done, and at 45F, the yeast should be dormant and inactive.

Would over pitching push it lower?

Would no mash out increase attenuation?

Any ideas?

2

I’ve only used pacman a few times and IIRC, its always been in the 80%+ attenuation range.  Its named pacman for a reason.

3

I got the DDGA recipe from Rogue, John himself, and used Pacman, as Rogue does. Theirs doesn’t drop to 1.010 and they hit the target of 5 Plato.

??

4

I would be very reluctant to compare numbers directly with a big brewer.  I would not be surprised if the yeast in my carboy acts much different than that in a large fermenter:  much different pressure on the yeast, much different temperature gradient, unless you got the yeast from the brewery, the strains are probably evolved differently, etc.  I also wonder what variability Rogue gets batch-to-batch.  I presume they would blend batches together to average out variability.

5

well, your lack of mashout could have created a more fermentable wort, or growing up from an 11month old slurry could have naturally selected the most resilient yeast cells which in turn attenuated more.

I recently did something similar with 007 - grew up a pitchable amount from a few tablespoons of old slurry and pitched in black IPA. took it from 1.085 to 1.015.  That’s a bit high from what I was expecting, and it has made me reconsider using old slurry, even if growing up from a small amount.  But then again i mashed at 147 for more fermentability.

6

The yeast attenuation rating is only good for comparing one yeast to another using a standardized wort.  It’s not necessarily an indication of the attenuation you’ll get.  That depends on the fermentability of the wort.  You can get a huge variation in attenuation by varying mash temp and recipe.

7

I think people under-estimate this effect. IME a 5 gal fermentation will finish about 0.5-1.0°P lower than a 7-15 bbl batch using the same recipe.

8

Wouldn’t this also be a factor in determining pitch rates, i.e. say 3 barrels vs 15 barrels of the same recipe. Or do you use the same rate regardless of the size of the batch?

9

Pitching rates don’t have a significant effect on attenuation, at least within reason. I suppose that massive over-pitching could reduce the FG in some circumstances, but I don’t think it would be a good solution. You’d be much better off adjusting the mash parameters and keeping the fermentation flavor profile as similar as possible, IMHO.

10

Yeah, sorry I was concerned with overall fermentation. So I guess my question really is , would(do) you adjust your rate for the same recipe if the volume change were large(2x)? It just seems that the pressure difference would affect all aspects of fermentation and therefore require more yeast to compensate.

11

I don’t, and I don’t know of anyone who does. I’ve worked with one mid-size brewery that has fermenter volumes ranging from 2-8 brew lengths, and their pitching rate is constant across all of them (for a given product). Their consistency is also very good, which probably isn’t surprising since they have a full-time QA tech managing their yeast strains.

What I do adjust in an effort to maintain consistency is fermentation temperature. My very approximate, entirely anecdotal approach is to add about 1-2°C per 10 bbl of fermenter volume. So where I start most 5 gal batches of ale at 18°C, I start 15 bbl at 20°C and 30 bbl at 21°C. Bear in mind that my atmospheric pressure is already ~4 psi lower, so a 15 bbl batch here is like a 5 gal batch at sea level, at least as far as hydrostatic pressure goes.

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Thanks for the info, very interesting.

13

80+% attenuation is normal with Pacman, IME. It also doesn’t poop out in high-gravity beers like a lot of other strains and it will ferment vigorously in much colder environments than most other ale strains (I fermented a 1.062 Black IPA at 52º F with it, and prefer it over the German strains for Altbier). I would try raising your mash temperature by 2-4 degrees if you want a little less attenuation, or brewing it again with 051/1272.

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Doesn’t Rogue use malt from house-grown barley? Not sure if they use it in all of their beers.

If so, substituting a highly-modified base malt could make a big difference.

15

They use it in some of the beers.  Some are 100%, some are only a bit.  Most beers at Rogue are made from Great Western malt.

16

Checked after I posted - website said DDG uses GW pale malt.