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Working with a 1.060 original gravity wort, mashed at 152F, and using the WLP565 Dupont yeast (that is known to stall), when should the brewer investigate if adding a secondary strain of yeast will be required? Is there a rule of thumb or accepted practice that suggests if X% of expected attenuation is not complete by Y days, then attenuation will very likely require a very long time?

Of course there are various options for the brewer, such as raising the fermentation temperature, removing back pressure, or simply waiting for the fermentation rate to start again. But this question is about the earliest point where the brewer might know with reasonable certainty that action is going to be required to get the beer to quickly completely attenuate, and what to do at that point.

The default process is to take multiple gravity readings, and when the slope doesn't change from one measurement to the next, then it's time to take action. But is there a way to "know", right away, by taking just one, properly timed measurement?

With this yeast, there are many brewers reporting "stuck at 1.030" or "stalled at 1.035", which are roughly only 50% or 60% of expected attenuation, but it's usually after a few weeks of frustration and multiple measurements.

Given that in a typical fermentation curve, the majority of the attenuation occurs in the first six days, when should I take the measurement that will indicate a stall, and above what value would indicate a stall?

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The Dupont strain is a kinda special beast. We found in an experiment on Experimental Brewing that you need to open ferment it to prevent the stall. Whether it's pressure or CO2 toxicity hasn't been determined. Assuming you're using an airlock, remove it and use a piece if foil loosely over your fermenter. That should fix it.

https://www.experimentalbrew.com/experiments/saison-yeast-airlock-vs-open-ferment-does-it-prevent-stall

https://www.experimentalbrew.com/podcast/episode-18-saison-under-pressure

To answer your question more directly, though, no, there is no formula for how much attenuation to expect by a certain time. If you want to have an idea of how far the wort might possibly ferment, a forced ferment test is very useful.

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    Off topic to the question, but one inch of water causes an additional 0.036 PSI. So instead of 14.70 PSI with foil, the bubbler is 14.74 PSI, an increase of 0.24% in pressure. That would mean that someone on the 6th floor would have a stall, whereas the ground floor wouldn't. Back pressure is off the hook! It MUST be CO2 toxicity! – Dale Jul 14 '17 at 19:38
  • Well, you're free to make that conclusion. Chemists I've talked to aren't certain. – Denny Conn Jul 15 '17 at 15:26
  • CO2 in solution can increase the acidity of the wort - maybe it is not CO2 but the increase in acidity that it causes in solution. Some microbes are quite acid sensitive and maybe that small change in concentration brings the acidity down slightly? – GrainMother Jul 18 '17 at 8:41
  • Do we have reports of higher altitudes having fewer stalls? Boulder, CO is 12.2 PSI vs 14.3 in Charlotte, NC (the same as a blowoff tube at the bottom of five FEET of water). Personally, I think it must be the air getting in, mixing with the CO2, giving the yeast a bit of oxygen. The next experiment should include closed fermentation with an added puff of air every so often. – Dale Jul 18 '17 at 18:35
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In my experience yeast gives up early when I see it drop out of high krausen / exponential growth / feeding phases in 1-2 days. It's the first sign for me to be diligent in taking readings and making adjustments to keep it going well. There's many reasons why this effects final gravity.

Over all its speculated that when the yeast works too fast in growth phase that cell division leaves too many bud scars for the yeast to take up complex large molecule sugars easily.

Also the abv increases faster that the yeast can strengthen its cell walls, and basically poisons its self.

Now I've had very fast primary fermentation without any issues, but this is always when I've done a full pitch of yeast and there's no growth needed to complete fermentation.

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