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I have brewed a number of ales which have all turned out delicious, and recently I attempted my first Pilsner (92% Pilsner malt, 8% flaked wheat) which did not turn out how I expected. There is a prominent off-flavor, it tastes mostly like butter scotch but I can't quite put my finger on it. After the boil I chilled to room temperature and pitched Wyeast Bohemian Lager at room temperature then brought my fermentation chamber down to 55°F.

Fermentation Timeline

9/10/20 Pitch yeast at room temp, slowly bring down to fermentation temp

9/10/20 1.056 55°F

9/13/20 1.027 60°F (diacetyl rest)

9/14/20 1.013 60°F

9/19/20 1.011 35°F

9/29/20 1.011 35°F (transfer to keg and add fining agent)

Looking at the numbers above, where did I more than likely go wrong?

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Looks just like a little bit rushed for the total schedule. It wouldn't have hurt the beer any to let it finish out at the warmer temp of 60F. Instead your data shows you started to chill it back down while it was finishing. In the future, take a sample and taste it before chilling to lagering temp. Take a sample and microwave it for 30 seconds and that will usually amplify any diacetyl if its there; indicating the need to go longer. The diacetyl rest isn't something that happens perfectly by paper techniques. Use your senses as a brewer to determine when to go to the next step.

Give it some time and it might slowly right itself. Pull the keg out of the chill box and let it come to room temp can also help.

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Short Answer:

Oxygenation or Yeast

  • Did you stir it enough to oxygenate the wort?
  • Did you use enough yeast (you must have, sorry answered this one for you)?
  • How did you store the yeast?
  • Yeast age?

Longer Answer:

Diacetyl is made at the start of fermentation by the yeast and is absorbed towards the end of the fermentation. A brew that a weak yeast or insufficient oxygenation will produce more diacetyl than a strong/healthy yeast or well oxygenated must than the yeast can absorb and it can dominate the flavor/smell of the beer. As you said diacetyl can smell like butter or butterscotch flavor.

Diacetyl rest is usually done 2 to 5 specific gravity points away from the target gravity of the brew, the purpose is to allow the yeast to reabsorb the diacetyl that is naturally produced during fermentation.

Yeast will absorb diacetyl at cooler temperatures, the absorption will be quicker at a warmer temperature when the yeast activity is speed up. This diacetyl rest may (although I am not convinced) help to reduce the overall necessary aging time.

With all that said I always taste it (because that is what it is about, right) before final racking/kegging/bottling and you spend any more time and money on something that could have gone 'wrong'. Taste it then if it needs a more aging, off gassing or anything else it can be adjusted easily.

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