Question regarding diacetyl rests and final gravities.

[MBasile]

So, I'll start with what I understand of doing diacetyl rests:
•Rest should be done when beer has reached 50%-65% attenuation
•Rest should be about 10˚ higher than fermentation temp
•Rest should last 24-48 hours.
•Rest is only needed it buttery flavor is detected.

(please correct me if any of the above is incorrect).

Now for my question:
•Is my bock supposed to reach it's final gravity during the D-rest or will there be enough yeast activity to drop it any remaining points during the lagering stage (which will be in my keezer around 38&#730?

Thanks!

 

[humann_brewing]

I think the d-rest should be done when 75% of your total attenuation has been reached.

Also, to answer your question, yes the fermention "should" all but finish when raising the temp for the 24-48 hours since they were already in action and they just have a bit more to do and at a faster rate too.

 

[MBasile]

I think the d-rest should be done when 75% of your total attenuation has been reached.

Also, to answer your question, yes the fermention "should" all but finish when raising the temp for the 24-48 hours since they were already in action and they just have a bit more to do and at a faster rate too.

Thanks for the response. The impression I've gotten from the research I've done is that it should be done with the attenuation reaches 50%-65%. So, if I'm expecting 75% attenuation, then 75% of that expected attenuation (as you stated) would be 55%. I think we're both saying the same thing, but stating it differently.

 

[wizardofza]

According to New Brewing Lager Beer by Noonan, you should perform a diacetyl rest when the density of your wort is 1.5plato (1.006 SG) above terminal - 52 degrees or above.

 

[menschmaschine]

According to New Brewing Lager Beer by Noonan, you should perform a diacetyl rest when the density of your wort is 1.5plato (1.006 SG) above terminal - 52 degrees or above.

Yes. This is about 80-85% complete on most beers of middle gravities. Put another way per the OP, this is about at 60-65% attenuation. I don't think starting it at 75% complete (55-60% attenuation) will do any harm. There will be some time (hours) for the beer to rise towards the D-rest temperature. So, starting the temp. rise at 75% complete might be more like starting the D-rest more towards 80% complete.

It's probably a good idea to let the beer reach it's final gravity during the D-rest. Plus, if you slowly cool the beer back down to lagering (~5°F or less per day), you will have a couple days still in the fermentation temp. range for lager yeasts. And since you're not shocking them (temp-wise), you further ensure a complete primary fermentation before lagering. Even during lagering, lager yeasts can break down a few residual sugars, gaining another miniscule amount of attenuation.

The rest is only needed if diacetyl is detected, but doing one won't hurt and helps ensure a more complete primary fermentation. I do them for every lager and I've probably only needed to do it once or twice out of my 12-15 batches (10 gallon) of lagers I've brewed.