Stalled Imperial Stout at 1.046
You brew on a 10 bbl system. An imperial stout, 1.082 OG, 70 IBU, is mashed at 154°F (68°C) for 60 minutes, single infusion, mash pH 5.5. It is pitched with a 1,500 mL stir-plate starter of an English ale yeast, lab count showing 8% viability, oxygenated for 60 seconds with pure O2 through a 2-micron stone, and fermented at 66°F (19°C). Four days in, gravity is 1.046 and has not moved in 72 hours. The fermenter has drifted to 73°F (23°C) and the airlock is still. Samples are clean - no phenolic, solvent, or sour notes. The brewer proposes adding a fresh packet of the same strain and waiting.
1. Rank the probable causes with your reasoning. Show the numbers.
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1. Severe underpitching (primary cause). A 1,500 mL starter is roughly 150–200 billion viable cells at best - and at 8% viability that is an effective pitch of maybe 12–16 billion cells into 10 bbl (≈ 310 gallons, ≈ 1,170 L). Healthy ale pitching for a high-gravity wort is 10–20 million cells/mL; 10 bbl needs on the order of 1.5–2.5 × 10^13 cells, i.e. trillions. This is off by orders of magnitude, not by a percentage. Underpitched yeast stalls after the easy sugars are gone.
2. Under-oxygenation for the gravity. 60 seconds through a stone with no DO meter is a guess. A 1.082 wort wants 10–15 ppm dissolved oxygen at pitch; a short burst on a 10 bbl batch commonly delivers 5–8 ppm. Yeast needs sterol synthesis for the first division cycles - and the same sterols let it tolerate alcohol later.
3. Mash temperature compounding it. 154°F produces a dextrin-rich wort. Even with healthy yeast a 1.082/154°F stout may finish near 1.022–1.028 - 1.046 is far above that, so mash profile is not the root cause, but it sets a floor you should expect rather than chase 1.018.
4. Ethanol and osmotic stress. At ~4.7% ABV in a sugar-rich, oxygen-poor, warm-but-not-hot fermenter, the remaining yeast are inhibited - which is why the airlock has stopped while temperature drifted up.
Ruled out: contamination (samples clean) and mash pH (5.5 is correct).
2. Why is the brewer's plan - a fresh dry packet of the same strain - likely to fail?
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Because the fermenter is now hostile.
- Alcohol and pressure. The beer already contains roughly 4.5–4.7% ABV, low residual oxygen, and high dissolved CO2. Sprinkling dry yeast into that is not the same as pitching into oxygenated wort: viability drops sharply, and the survivors must build sterols from nothing.
- Dry yeast has no reserve for this. A single packet (~200 billion cells, and a fraction viable after rehydration in an alcoholic, acidic, hop-bittered medium at 70 IBU) into 310 gallons is another massive underpitch - and dry yeast is the wrong tool for a stuck high-gravity ferment.
- The remaining sugars may be unfermentable. If the stall is at the limit of what the yeast can attenuate and not a true stuck ferment, no amount of new yeast converts dextrins. That is why you need a forced ferment test first: pull a sample, dose it heavily with healthy yeast at 70°F, and see where it lands. If the forced ferment also stops at ~1.046, the problem is the wort, not the pitch.
- Temperature drift to 73°F suggests the yeast already worked hard and quit; adding more at 73°F risks fusels and esters if it does take off.
3. Give a corrective plan, in order, with the decision point that tells you which path to take.
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Step 1 - Run a forced ferment test (do this before anything else). Pull 500 mL, pitch a heavy dose of fresh, healthy, actively fermenting yeast, hold at 70–72°F, and check gravity daily for 3 days. This tells you whether the sugar is there.
Step 2 - Depending on that result:
If the forced ferment drops below 1.030: the wort is fine and the ferment is genuinely stuck. Build a proper propagation: step a fresh, high-viability culture up to at least 1–2% of batch volume as an active starter at high krausen, and pitch it into beer warmed to 68–70°F. Add yeast nutrient (zinc and FAN) and rouse gently with CO2 from the bottom. Never oxygenate at this stage - it will oxidize the beer and drive staling.
If the forced ferment also stalls near 1.046: you have a dextrin wall. Options are (a) accept it and adjust the packaging blend, (b) add glucoamylase/amylase enzyme to convert dextrins - an irreversible intervention that will keep chewing in the bottle, so use it only for keg/draft or if you can arrest it, or (c) blend down with a drier beer.
Step 3 - Document and correct the root causes for next time: target 1 million cells/mL/°Plato, oxygenate to 10–15 ppm with a meter, and run a diacetyl rest before crashing. On a 1.082 beer, a healthy pitch finishes this in 5–7 days; the current pitch was never going to.