Diacetyl rest questions

Homebrew Talk - Beer, Wine, Mead, & Cider Brewing Discussion Forum

Help Support Homebrew Talk - Beer, Wine, Mead, & Cider Brewing Discussion Forum:

Ramjet

Member
Joined
Sep 6, 2022
Messages
16
Reaction score
10
Location
Wisconsin
I have a Scottish ale that has been fermenting at 62 degrees for 13 days so far. I have never done a diacetyl rest before and have some questions. It is in a fermentation chamber with temperature control capabilities, both warm and cold.
#1. Is it necessary to do it, it's my understanding that ales don't need it.
#2. If needed when is the correct time to do it.
#3. Also how much to raise the temperature and for how long.
Planning on cold crashing when done.

Thanks in advance
Ramjet
 

VikeMan

It ain't all burritos and strippers, my friend.
Joined
Aug 24, 2010
Messages
4,993
Reaction score
4,530
#1. Is it necessary to do it, it's my understanding that ales don't need it.
Ales generally don't need diacetyl rests. But some strains produce more diacetyl than others.

#2. If needed when is the correct time to do it.

The best time would be when fermentation is almost finished.

#3. Also how much to raise the temperature and for how long.
Planning on cold crashing when done.

There's no hard and fast rule. But 3 days at 68F would provide some nice insurance, for example.

Also, I would point out a bit of diacetyl can be considered a good thing in a Scottish Ale.
 
Last edited:

cactusgarrett

Supporting Member
HBT Supporter
Joined
Apr 8, 2008
Messages
2,281
Reaction score
864
Location
Madison, WI
#2. If needed when is the correct time to do it.
To put some quantitative to Vike's qualitative, I've seen a rule of thumb of 50% fermentation thrown around. I'm a little more cautious and start the rest when it's 75% complete (to avoid any potential off flavors, while still having some fermentation left to actually do the work). For example, if my 1.055 pilsner is expected to go down to 1.012, that's 43 points total drop. 75% of that is a 32 point drop, so I start my rest around 1.023 (55-32).

The catch is that it helps drastically to be familiar with the recipe/yeast in order to get the expected FG target right. A forced ferment test helps with this if you're unfamiliar with the recipe/yeast.
 
OP
OP
R

Ramjet

Member
Joined
Sep 6, 2022
Messages
16
Reaction score
10
Location
Wisconsin
Thanks for your replies guys. I have heard you should do it when you're within 5 points of FG. So if you start the rest after 75% fermentation, you keep it there until you reach your FG?
 

VikeMan

It ain't all burritos and strippers, my friend.
Joined
Aug 24, 2010
Messages
4,993
Reaction score
4,530
Thanks for your replies guys. I have heard you should do it when you're within 5 points of FG. So if you start the rest after 75% fermentation, you keep it there until you reach your FG?

The idea is to keep it at the rest temp until all of the diacetyl's precursor, α-acetolactate, has been oxidized to diacetyl, and the yeast has cleaned it all up (or at least cleaned it up to an acceptable level and/or below taste threshold).

That might happen before you reach FG or it might happen after you reach FG. Chances are that by the time you reach FG, you'll probably be good to go, but FG isn't a magic "crash/package me now" indicator.
 

AZ Maverick

Well-Known Member
Joined
Feb 19, 2021
Messages
124
Reaction score
554
Location
Prescott, AZ
I use a Tilt Hydrometer to follow my fermentation.
With my Lagers, I transition from fermentation to Diacetyl rest just before fermentation is complete.
Here is plot of my last brew which was a Schwarzbier.

Schwarzbier.jpg
 

Pie11

Member
Joined
Nov 21, 2022
Messages
19
Reaction score
12
Location
Lyon (France)
The best starting point for diacetyl rest is different for each yeast.
With w34/70 you can start when 5-6 gravity points are left. I you do the same with lallemand diamond, you have the warranty of diacetyl flavour in the beer.
For diamond lager you must start early, around 33% gravity left. But diamond lager is very quick, and is difficut at home to find the right time to rise temperature.
This is the biggest reason why I prefer w34/70 over diamond.
 

toxdoc49

Well-Known Member
Joined
Mar 8, 2018
Messages
122
Reaction score
8
Location
Maryland
I found this thread and have a related question about diacetyl rest: I have a baltic porter recipe (which they say is essentially a lager) that called for diacetyl rest (and which I did); I have run across other porter recipes that do not have it in their instructions (yes I am still following recipes, I don't have a lot of equipment to measure all these things quantitatively [yet lol].
So does a diacetyl rest depend on the yeast/grain bill for a porter, or is it something that for porters should typically be performed? Thanks in advance.
 

VikeMan

It ain't all burritos and strippers, my friend.
Joined
Aug 24, 2010
Messages
4,993
Reaction score
4,530
D-rests are not typical for most Porters (or most ales, really). The drivers for the need to do a D-rest, IMO:

- Yeast strain that produces a lot of diacetyl
- Low Fermentation Temperatures
- Diacetyl detected in a beer sample
 

toxdoc49

Well-Known Member
Joined
Mar 8, 2018
Messages
122
Reaction score
8
Location
Maryland
Thank you. That makes sense to me; I was not familiar with the factors influencing the need for the diacetyl rest. Maybe the time that I did it following instructions was based on the yeast strain for that recipe. Still learning and thanks for the information!
 

toxdoc49

Well-Known Member
Joined
Mar 8, 2018
Messages
122
Reaction score
8
Location
Maryland
Thank you. That makes sense to me; I was not familiar with the factors influencing the need for the diacetyl rest. Maybe the time that I did it following instructions was based on the yeast strain for that recipe. Still learning and thanks for the information!

D-rests are not typical for most Porters (or most ales, really). The drivers for the need to do a D-rest, IMO:

- Yeast strain that produces a lot of diacetyl
- Low Fermentation Temperatures
- Diacetyl detected in a beer sample
Now I am quite new to the chemistry, and learn little pieces at a time; I have read that heating a sample is one way to detect the presence of diacetyl, through tasting or smell. But there are quantitative methods involving not-so-cheap laboratory equipment (UV spectrometer for example), which would be great actually. I suppose if one had the means, equipment such as this would be one good way of actually measuring the levels of diacetyl, as likely professional brewers do this I assume. Even just following printed recipes one does get curious about the chemistry of beer. Part of the journey right?
 

VikeMan

It ain't all burritos and strippers, my friend.
Joined
Aug 24, 2010
Messages
4,993
Reaction score
4,530
Now I am quite new to the chemistry, and learn little pieces at a time; I have read that heating a sample is one way to detect the presence of diacetyl, through tasting or smell.

Yep, that's called a forced diacetyl test. The increased temperature accelerates the conversion of α-acetolactate to diacetyl.

But there are quantitative methods involving not-so-cheap laboratory equipment (UV spectrometer for example), which would be great actually. I suppose if one had the means, equipment such as this would be one good way of actually measuring the levels of diacetyl, as likely professional brewers do this I assume.

Not-so-cheap is an understatement. I don't know any commercial brewers using test equipment for diacetyl. (Maybe the macros do.) There are also chemical assays available, but I can't say I know anyone using those either. The forced diacetyl test is (IMO) a practical (and very cheap) option .

Even just following printed recipes one does get curious about the chemistry of beer. Part of the journey right?

If you are interested in brewing chemistry, I recommend Roger Barth's "The Chemistry of Beer: The Science in the Suds." It's comprehensive, but I think it's approachable for anyone who has had, say, a high school chemistry course. I would avoid "Brew Chem 101: The Basics of Homebrewing Chemistry." I was so disappointed in that book that I read and annotated the whole thing, and gave it to someone who had also recently purchased it, so that they wouldn't be misled.
 
Top