I have an Irish Stout and an American Pale Ale in secondary. I’m sure that I don’t need to do a diacetyl rest on the stout, but do I need to do one on the APA? In general, is this rest needed for ales or jusr for lagers? If so, at what point in the color/style scale is it no longer needed? Finally, if I need to do a rest for the APA, what temperature should I get to? The primary and secondary temps for the APA were ~66 and 62 respectively. :?:
Thanks in advance,
jr
Typically a D-rest is done for lagers. All beer yeast produce AAL, the precurser for diacetyl, which slowly converts into diacetyl at higher temps (60-70F) which is then consumed by the yeast. The temp range for a D-rest is typically in the 65-70F range which is very close if not at the temp that most ales ferment so the yeast will eventually consume most of the diacetyl over time. The time factor depends on how much diacetyl, available active yeast and temperature.
You only need a diacetyl rest for lagers, and only sometimes but not always. If the lager doesn’t smell or taste buttery, you can safely skip the rest. Two or three days at ~65 F in the last half of fermentation is helpful to the yeast to be able to digest the diacetyl in lagers.
With ales, it’s only a potential issue if you ferment really cold, like down in the 50s (which is cold for an ale yeast). But if you ferment in the 60s, you’ll very rarely if ever get a lot of diacetyl in an ale, except maybe for English ales where a low level of diacetyl is actually acceptable and expected for the style. And again, this is probably due at least in part to the fact that some English beers are traditionally fermented cool around 60-62 F, so the yeast has probably evolved to not care so much about eating diacetyl or whatever.
