Predicting When to Start a Diacetyl Rest

[Easycreeper]

TL;DR - How does one go about predicting when to start a diacetyl rest?

According to White Labs, one should start the rest when the beer is 5 to 6 points away from finishing fermentation.

That’s all fine and good, but if you’re not using a tried and true recipe, it seems as though it’s a guessing game that could have unintended consequences, namely esters from improper fermentation temperature. One NHC judge dinged me for some fruity esters in my doppelbock submission while the other didn’t have much issue with it. I’ll admit that it had some fruitiness which I didn’t find unpleasant, but they judge to style and by gosh, I’m going to brew to style for competition submissions (and make the beer I want to drink any other time).

My current situation involves a doppelbock with an OG of 1.103. I started the rest (free rise to 67F) when the Tilt was reading right around 1.030 and it’s currently sitting around 1.022 and still going. This says I started the rest too early, at least according to White Labs. I can’t imagine trying to guess without knowing approximate gravity real time…

The yeast is WLP833 that has an approximate maximum apparent attenuation of 76%. The beer is sitting at nearly 79% AA right now and the last time I used this yeast, it hit right at 83%. Fresh yeast from appropriately sized starters was pitched each time.

So, how does one make sure the rest isn't started too early or late? Making the same recipe each time? Just get lucky? Wait until FG is stable and then raise the temp?

Some parameters:
Mash Temp/Time: 154F/60 minutes
Total Boil Time: ~3 hours (this has bitten me in the butt before, but not this time…)
Ferm Temp: started at 50F and gradually made it to 55F over 1.5 weeks

 

[MaxStout]

The most certain way is to take a few gravity readings along the way and when it has dropped about 3/4 of the way to FG, that is a good time to start the rest. The uncertainty is knowing just what your FG is going to be. But starting the rest a few points away from what you think should be FG will be fine. It doesn't have to be an exact thing. Close to the end of primary fermentation is good enough. Most of the yeast's work is done well before then, so raising the temp at that point shouldn't throw off-flavors.

 

[BlueHouseBrewhaus]

I don't think you need that much precision. As long as the vast majority (75-80%) of the fermentation is accomplished at lager yeast temps, the D-rest just let's the yeast finish up without changing the flavor profile. A few points early or late shouldn't make a noticeable difference, especially if you're fermenting down from 1.100+.

 

[Easycreeper]

Thanks to both @MaxStout and @BlueHouseBrewhaus.

I don't brew many lagers and worry a bit about the outcome considering my lack of experience and feedback from the NHC judges. Just trying to avoid the esters this time around and end up with a more textbook example of the style.

As an aside: with it being this late in fermentation, its pretty amazing to see so much yeast activity. It keeps moving the Tilt around the carboy... Never in the same place twice.

 

[Northern_Brewer]

One of the notable things about Cloudwater recently publishing details of both their original DIPA v3 and a "how we would brew it now" version (follow link to the spreadsheet) is how much of their process centres around reducing diacetyl. They're adding ALDC enzyme, valine, zinc and have changed from starting a 22C diacetyl rest at 1.030 to starting it at 1.045 (on a 1.076 beer). They then keep it there until it tests clean for VDK.

So don't sweat getting within a couple of points of FG, sooner is (more than) fine.

 

[RM-MN]

TL;DR - How does one go about predicting when to start a diacetyl rest?

According to White Labs, one should start the rest when the beer is 5 to 6 points away from finishing fermentation.

That’s all fine and good, but if you’re not using a tried and true recipe, it seems as though it’s a guessing game that could have unintended consequences, namely esters from improper fermentation temperature. One NHC judge dinged me for some fruity esters in my doppelbock submission while the other didn’t have much issue with it. I’ll admit that it had some fruitiness which I didn’t find unpleasant, but they judge to style and by gosh, I’m going to brew to style for competition submissions (and make the beer I want to drink any other time).

My current situation involves a doppelbock with an OG of 1.103. I started the rest (free rise to 67F) when the Tilt was reading right around 1.030 and it’s currently sitting around 1.022 and still going. This says I started the rest too early, at least according to White Labs. I can’t imagine trying to guess without knowing approximate gravity real time…

The yeast is WLP833 that has an approximate maximum apparent attenuation of 76%. The beer is sitting at nearly 79% AA right now and the last time I used this yeast, it hit right at 83%. Fresh yeast from appropriately sized starters was pitched each time.

So, how does one make sure the rest isn't started too early or late? Making the same recipe each time? Just get lucky? Wait until FG is stable and then raise the temp?

Some parameters:
Mash Temp/Time: 154F/60 minutes
Total Boil Time: ~3 hours (this has bitten me in the butt before, but not this time…)
Ferm Temp: started at 50F and gradually made it to 55F over 1.5 weeks

That's an easy one. Just let the beer go to final gravity, then invent a time machine so you can go back to when it was 5 to 6 points away from that. Otherwise you simply cannot know exactly when it will be that far from final gravity because you aren't in charge of the final gravity, the yeast are and they don't always follow the rules.

Did your competition beer really have fruity esters or did a judge taste something that wasn't there. We all have different palates and judges can make mistakes too.

 

[Easycreeper]

That's an easy one. Just let the beer go to final gravity, then invent a time machine so you can go back to when it was 5 to 6 points away from that. Otherwise you simply cannot know exactly when it will be that far from final gravity because you aren't in charge of the final gravity, the yeast are and they don't always follow the rules.

Did your competition beer really have fruity esters or did a judge taste something that wasn't there. We all have different palates and judges can make mistakes too.

It's so simple! Why did I not see it before now?? Time machine! Of course!

Yeah, it had some fruitiness to it, but it wasn't overpowering. Apparently, low malt derived fruitiness is OK per style, but there was also a little alcohol hotness as well, which points to a problem with temperature control. The judge's score was originally 29 and the other 38, but to maintain a 5 point difference they were adjusted to 30 and 35 respectively (I see you eraser marks...)

I started the rest 10 points later this time, but it's still going. Since free rising from 1.030, it's down to 1.014 (according to the Tilt - it's always a couple points low compared to a calibrated hydrometer). I'm tempted to run it back down to 55F and hold there for another week before lagering.

I thought WLP833 wasn't supposed to attenuate this much. Guess I'll mash even higher next time.

 

[waldoar15]

If you can spare some of your wort and yeast you can do a fast ferment in a flask at room temp with it so you can see what the FG will be before the main batch gets there.

 

[Easycreeper]

If you can spare some of your wort and yeast you can do a fast ferment in a flask at room temp with it so you can see what the FG will be before the main batch gets there.

You know, I always forget to do this. It would work perfectly with a doppelbock fermentation timeline. At the very worst, it would get me in the ballpark of correct rest timing even if it doesn't go quite as far as the forced ferment.

 

[leonardoreges]

Most of ester production happens in the first three days of fermentation, specially day one.
Of course in a big beer things tend to last a little longer, but be sure you're alright.