Lagerthon Follow-Up 1: Diacetyl Rest

The Helles and the Schwarzbier are far enough along in their fermentation process I can put them into a diacetyl rest.  Diacetyl is a compound that tastes like the butter flavor in butter flavored popcorn.  Combined with malt it can taste like butterscotch but it’s generally considered a flaw in lagers.
To test for it is relatively simple.  You can’t just taste it, there’s a diacetyl precursor that’s tasteless but converts on storage to diacetyl.  So what you have to do is warm the beer up to about 160 degrees to allow the precursor to convert, then cool and taste for diacetyl.  Not everyone can taste it but it also has a mouthfeel, a slippery, oily feel.
Producing diacetyl in lagers is the result of a compromise we homebrewers make.  If we could pitch our yeast at fermentation temperature, about 50 degrees, lager yeast won’t produce enough diacetyl to be perceptible.  But we homebrewers generally pitch warmer, either because we can’t get the wort that cool or because we don’t want to wait while it cools in our refrigerators, the benefit of which is the longer we wait to pitch, the greater the chance some bug can take hold in our brews and produce something much worse than butter-flavored beer.  Yeast both produce it and, once nutrients start getting scarce, metabolize it so we can accept early production and get rid of it later.
To conduct a diacetyl rest, let the beer warm to about 65 degrees for a couple of days prior to lagering.  This will allow the yeast to become a bit more active and consume the diacetyl.  They will produce some esters due to the warming, also not desirable in a lager, but the amount will be miniscule.
The diacetyl rest illustrates the compromises a brewer has to make but thankfully, yeast are very good at their job.  Give them the proper conditions and care and they will make good beer for us.

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