When to Perform a Diacetyl Rest?

I've run across conflicting advice as to when to perform a diacetyl rest:
1. Right after the Krausen falls.
2. When you're 75% of the way to FG.
3. When you're a couple of points above FG.
4. After fermentation is complete.

So when's the "right" time to do a d-rest? Or does the timing just not make that much of a difference?

Like so much in brewing, if you ask 10 brewers, you will get 12 different answers, and all will be correct. You'll ultimately have to choose one and make up your own mind. I do it after fermentation is complete, and 72 hours before I am going to rack to secondary and lager. The yeast will still warm up and get excited and swim around cleaning their messes no matter when you do it.

This thread covers the topic really well.

https://www.homebrewtalk.com/f13/preventing-diacetyl-hold-butter-please-70438/

ToledoDave said:

I've run across conflicting advice as to when to perform a diacetyl rest:
1. Right after the Krausen falls.
2. When you're 75% of the way to FG.
3. When you're a couple of points above FG.
4. After fermentation is complete.

So when's the "right" time to do a d-rest? Or does the timing just not make that much of a difference?

Click to expand...

3 and 4 are substantially similar, 1 is effectively a visual proxy for determing when 2 has been reached, so you really have two methods.

1. Raise the temperature after fermentation, aka diacetyl rest. This is popular with home brewers and some American lager brewers.

2. Raise the temperature for the later portion of the fermentation, aka the Narziss fermentation. This is more common with German brewers.

It is possible that neither is necessary if yeast pitching is adequate and done cold and oxygenation is adequate.

FWIW, Greg Noonan stated in New Brewing Lager Beer to do it 6 points shy of FG. That may be a little exacting and difficult to realistically gauge in the homebrew environment, but it's a good starting point for a standard. At any rate, to get the most out of the rest, it's best to do it before fermentation is finished.

Revvy said:

Like so much in brewing, if you ask 10 brewers, you will get 12 different answers, and all will be correct. You'll ultimately have to choose one and make up your own mind. I do it after fermentation is complete, and 72 hours before I am going to rack to secondary and lager. The yeast will still warm up and get excited and swim around cleaning their messes no matter when you do it.

This thread covers the topic really well.

https://www.homebrewtalk.com/f13/preventing-diacetyl-hold-butter-please-70438/

Click to expand...

Thanks for the reply, Revvy. I've heard in other places that waiting until after fermentation is finished can be a problem, since the yeast won't be active enough to clean up all the diacetyl. But it makes sense that raising the temps will wake them up enough to do the trick. I suppose that at the higher temps they might eat another point or two of sugar as well; have you noticed that in your experience?

remilard said:

3 and 4 are substantially similar, 1 is effectively a visual proxy for determing when 2 has been reached, so you really have two methods.

1. Raise the temperature after fermentation, aka diacetyl rest. This is popular with home brewers and some American lager brewers.

2. Raise the temperature for the later portion of the fermentation, aka the Narziss fermentation. This is more common with German brewers.

It is possible that neither is necessary if yeast pitching is adequate and done cold and oxygenation is adequate.

Click to expand...

Thanks for your reply; I see your point that there are really only two methods. So what's the difference between the two? I'm going to guess that doing an earlier rest poses the risk of picking up off flavors from the yeast, as they're fermenting a significant amount of sugars at high temps; whereas
doing a later rest risks not having the yeast active enough to clean up all the diacetyl. Have I missed anything?

Do you have an opinion on how much of a concern either of these risks are; are they mostly theoretical, or do they regularly create problems?

menschmaschine said:

FWIW, Greg Noonan stated in New Brewing Lager Beer to do it 6 points shy of FG. That may be a little exacting and difficult to realistically gauge in the homebrew environment, but it's a good starting point for a standard. At any rate, to get the most out of the rest, it's best to do it before fermentation is finished.

Click to expand...

Thanks MM. I understand the point that doing it prior to fermentation finishing ensures that the yeast are active and will effectively clean up the diacetyl. But people report (like Revvy above) that doing the rest after hitting FG doesn't pose a problem, since warming the beer will activate the yeast enough to do the job.

I suppose that waiting until afterward puts you at a little more risk of incomplete clean up, but the risk is probably not all that high. Likewise doing the rest earlier puts you at a greater risk of off flavors; but again, not much more.

Your advice seems like a very safe method; late enough that the yeast are winding down and won't produce many off flavors, but not so late that they're completely dormant.

I don't test the gravity to figure it out, I just watch for the activity to slow almost to nothing. It's definitely not when the krausen falls, but more when it thins out and the airlock is burping very infrequently. Right, I know it's inexact.

My lagers go 2.5 wks fermenting, 3 days at 65 (d. rest), then 1 wk. ice cold. I now rack the beer off the yeast when it is still ice cold. I always follow this exact procedure for all lagers.

Honestly, those 3 days for the d. rest are really just assurance for me that the fermentation is complete before I turn my chamber into Antarctica.