So I am brewing lots of bitters using WLP002. Typically, I mash a grist with around 10-15% specialty malts at 66 C and ferment at 20 C. (I use BIAB and dunk sparging.) The typical situation is that after 2 weeks the SG is around 1.013 with all activity stopped, a huge cake of cheesy WLP002 on the bottom of the fermenter. Samples taste great.

Then, I bottle the beer, and sometimes fermentation kicks in again, the finished beer reaching SG 1.008, or even lower! Needless to say, I am not too happy about these over-carbonated and dry beers.

These bitters are probably not infected, I am very meticulous in my process, and it is only 002 that behaves like this for me.

Why is this? The closest plausible explanation I have is that the usual low attenuation is due to extreme flocculation. Rousing the yeast just a little bit at bottling time starts the fermentation again.

  • I've never brewed with that yeast, so I can only speculate. That strain flocculates well. It could be that when bottling, you put a small amount of yeast back into suspension -- just enough to kick off fermentation again. If this is what's happening, you could rouse the yeast in the fermenter when activity has apparently stopped. This should have the same effect as what I'm suspecting happens in the bottle. Commented Sep 9, 2014 at 23:42
  • Thank you, this is a good idea. I actually think early flocculation may be part of the problem. However, I do not understand 83 % attenuation at mash temp of 66 C. I think I should check the calibration of my thermometer.
    – Nemis L.
    Commented Sep 10, 2014 at 17:40

2 Answers 2


It can be for a few reasons: 1. Underpitching (not enough yeast), 2. Insufficient wort aeration before pitching, 3. Cold shock, 4. Old yeast that you bought in a store -- well the last one is actually underpitching again. What to do: 1. Make a good 1.5-2 l starter, start it 2-4 days before the brew day, 2. Aerate wort well, not just swirl it in a fermenter for 5 sec, 3. Some say, slightly rocking the fermenter helps to rouse dormnant yeast (never helped me). 4. Krauzening. The latter made miracles for me. The idea is to preserve 1-2 liters of wort (put in a sterile bottle and in a fridge) at brew day, then ignite it with new yeast when your main fermentation is 90% through. When that small portion seems to be on fermentation peak, pour it into the main beer (I tried also "injecting" it into the yeast cake using a (sterile) pipe and a funnel, can't say it makes difference though). You can certainly do 1,2,4 altogether.

  • 2
    ok, a couple more sources point to this: if WLP002 is underpitched, it may flocculate before it finishes its work, it seems to be a known feature of that strain. I personally use WY1496 as my home strain for any English style, and am quite happy with results.
    – Codehopper
    Commented Sep 10, 2014 at 2:33
  • Thanks for the answer! However, please explain to me: How can underpitching or underaeration lead to overattenuated beer? True, it may be that I underaerated, and that the fermentation atcually got stuck before bottling. However, WLP002 attenuating to more thatn 80 % at 66 C mash temperature? I find it hard to believe.
    – Nemis L.
    Commented Sep 10, 2014 at 8:54
  • Ah sorry, I thought your main issue was stuck fermentation. Well, then I can only speculate on: 1. Stability of your mash schedule (if you get below 65 for long time, you can get a rather fermentable wort. Btw your thermometer might need checking; mine shows 3 degrees below the real temperature, for example, and I discovered it only when got to boiling), 2. Do you use the new vial of yeast or reseed the previous cake/starter's culture - I myself do that, but there is a danger that yeast will change its character.
    – Codehopper
    Commented Sep 10, 2014 at 11:26
  • I would like to read more about underpitching 002, if you have explicit sources. In fact, I may have underpitched slightly but not much; about 0.5 mill/ml/deg plato. However, the temperature may have been slightly low during the second week, about 17 C. May explain early flocculation. Still, I think 83 % attenuation is much on this beer.
    – Nemis L.
    Commented Sep 10, 2014 at 17:38
  • 1
    No wait. Two different points here: 1. Underpitching often leads to underattenuation, which may be exacerbated by premature flocculation. 2. Attenuation numbers are not "absolute truth", your mileage may vary depending on grain bill, mash conditions and individual character of your yeast sample (e.g. as I mentioned, my house belgian yeast is wy 3787, but because I've recycled it maybe 10 times alread I shouldn't expect it to behave exactly like 3787 off the shelf).
    – Codehopper
    Commented Sep 12, 2014 at 7:19

My 2nd most popular beer uses 002, we have this issue too. It just flocculates like no other yeast. We bought a stopper with no holes. On Days 2, 4, and 6 of primary we pick up the carboy and shake it into a fury.. ~5 minutes of real agitation. We do 14 days of primary so it's always clear by transfer to secondary time.

It does the trick for us, although I want try the krauzening technique Codehopper mentions.

I also thought about making a custom stir plate we could set a carboy in just for our one beer. It wouldn't aerate but it would stop settling. We tried other yeasts but nothing brings out the flavors we're looking for like 002.

  • Thanks for your answer! Pardon me for asking, but is not shaking the carboy bad for the beer, as in O2? On the other hand, the amount of O2 left in the carboy at that time is probabliy very small. At what temperature do you ferment?
    – Nemis L.
    Commented Nov 17, 2014 at 8:47
  • Well, unlike when your mashing or boiling, you want to get a ton of O2 in the fermentor, so we run our hose from the boil kettle to a screened strainer inside a funnel to aerate it as much as possible. When we agitate it we remove the airlock, put on the stopper and shake it, so it should be nearly all CO2 inside being shaken. We haven;'t gotten any off flavors. We try to ferment at 66F, but we do it passively so sometimes it is +/- 2 degrees depending on the weather.
    – dongemus
    Commented Nov 18, 2014 at 1:51

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