Possbile reasons for underattenuation? Pressure-fermented pilsner, W-34/70

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Jul 1, 2021
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Hi guys! I just recently brewed a Czech Pale Lager using the famous SafLager W-34/70 yeast and I'm experiencing underattenuation. Here's the grist and specs:

97.6% Pilsner - 4kg (8.82lbs)
2.4% Red Ale (Melanoidin) - 0.1kg (0.22lbs)

Expected OG: 1.040
Expected FG: 1.008

- I ordered the grain with a double crush.
- I heated 30 liters (7.92 US gallons) of pH- and mineral-adjusted distilled water to strike temp, doughed in, hit 68C (154F) and kept it there for 60 minutes.
- I insulated the kettle by wrapping it in a reflective sleeping mat and a sleeping bag.
- I stirred every 15 minutes in hopes of increasing conversion and keeping temps uniform within the kettle. During the 60-minute period, I experienced a 1 Celsius degree drop in the mash temp, which I compensated by pulse heating the kettle back to 68c.
- After 60 minutes, I pulled out the grain bag, rested it on an oven rack that sat on top of the kettle, and I squeezed the bag using a kettle lid.
- After the mash, I boiled for 60 minutes and ended up with an OG of 1.042, two points higher than anticipated.
- I chilled the wort down to 18c in 15 minutes, transferred the wort to my FermZilla, aerated by splashing and pitched two packs of non-rehydrated W-34/70 yeast, so yeah, quite an overpitch.
- I set the fermentation chamber at 19c, moved my FermZilla there and set the spunding valve at 15 PSI.

Now, after four days, the yeast has already completely crashed out and the beer is crystal clear, but my FG is sitting at a whopping 1.015. I've also raised the temp to 22c to do a d-rest, I even swirled the FermZilla a couple of times to scoop the yeast back into suspension. However, the yeast keeps dropping out and the FG stays stable, meaning that the beer is most likely done. The only reasons I can think of are the following:

1. High mash temp - this seems unlikely, as I've had beers mashed higher and still more attenuated than this
2. Yeast stress
3. Closing the spunding valve right after pitching might've inhibited yeast growth

Do you guys have any experiences with the W-34/70 crapping out on you prematurely? I've heard it's a monster attenuator so I was expecting much lower numbers.

Red over White

Well-Known Member
Feb 23, 2022
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Sounds like you ticked all the boxes. To dry out my lagers I like to get the mash to gel temperature and then just let it eat in beta, I may supplement the enzymes with some 6 row in the grist (before or during the mash) up to 10% as well. I don't mind if the mash drops from gel temperature at ~148 to 140 before I go to the next step in the process. I typically mash for 2 hours when shooting for attenuation, but I know many don't like extending the brew day. I live for brew day and will do whatever it takes to make a highly fermentable wort. With this approach 84% AA is achievable with 34/70 for me. Hopefully some of this helps.


wort maker
HBT Supporter
Dec 22, 2015
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To get down to 1.015 in 4 days using dry yeast is quite impressive, imo. I'd be happy. I'd say be a bit more patient and stop interfering with it. Some of my best lagers have been mashed at 68°C and they still attenuated to around 80% within about a week, using repitched freshly harvested yeast. Dry yeast is going to take longer, ime.


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Dec 17, 2014
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West Coast Island in the Bay
Yeast could have been old, compromised. Although two packs should have fixed that theoretically.

Mash temp is bit high, but the reheating might have been the cause. Or just in accurate temp readings.

Head pressure over 7-10 psi will slow yeast growth. Depending on amount of headspace you may have gotten there quickly.

None of these stands out on its own, but in any combination you may find your answer.


Supporting Member
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Jan 18, 2020
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South Bend
If you're using a Tilt, absolute FG can be way off once krausen gets on the device. (The relative readings -- that FG is steady -- remain reliable.) Double-check with a hydrometer or a refractometer ( but use a calculator to correct the refractometer reading.)