Diacetyl Control
Ripped off from George de Piro <gdepiro@mindspring.com>

At the May meeting in Dr. Bob's lab, George Perrin put on another in a series of beer flavor evaluation sessions. He doctored Miller Genuine Draft (used as bland, and tasteless base beer) with various substances so that we could learn to detect and recognize flavors that might be encountered in our real-life beer tasting experiences. One flavor that we explored was diacetyl. A fairly good number of people identified this one correctly.

The following is a post to the HBD from George de Piro, a homebrewer from NY who has recently become a professional brewer at a brewpub called the Albany Pump Station. George is a regular contributor to the HBD, though not so regular since he turned pro. Anyhow, he has some interesting things to say on the subject of diacetyl that I thought I'd pass along to you.

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Date: Thu, 4 May 2000 15:48:08 -0400
From: "George de Piro" <gdepiro@mindspring.com>
Subject: diacetyl control

Hi all,

Ryan writes regarding Ringwood yeast and diacetyl.  He mentions that Wyeast
recommends a diacetyl rest after fermentation.  The following is some
general buttery babble I recently wrote for the IBS Brewers Forum.  It
should answer Ryan's question:

Ringwood yeast is a notorious diacetyl producer, but the brewer can do
things to keep it at reasonable levels.  Of course, "reasonable levels" is a
personal thing, but be aware that diacetyl is something that only gets
bigger with time, so if you like the level of butteriness in your beer 3
weeks from brewday, drink it fast and store it cold.

Diacetyl production is proportional to yeast growth, so pitching a lot of
yeast may help reduce the final level of diacetyl.  The yeast need to be
healthy as well:  diacetyl isn't actually made by yeast (see below), and the
precursor is converted to diacetyl at a relatively slow rate.  If your yeast
have crapped out (techie term), they won't be able to absorb the diacetyl.

How does one know if their beer is going to end up with high diacetyl
levels?  There is a very simple test requiring no lab equipment at all that
can answer this question for us.  Before we get to that test, I will quickly
review how diacetyl is formed by yeast (a proper understanding of how
diacetyl is formed and reduced will lend clarity to the test procedure).

Yeast do not actually make diacetyl, they make a compound called alpha
acetolactate (AAL).  It is this chemical that is oxidized to diacetyl.  This
oxidation is relatively slow at fermentation and lagering temperatures, but
is conveniently accelerated by higher temperatures.  That is why a diacetyl
rest works for lager brewers:  the fermentation is warmed to the low 60's F
for several days to allow the faster conversion of AAL to diacetyl.   The
yeast will then consume the diacetyl, thus eliminating it from the beer.  If
an appreciable amount of AAL makes it into the final package and there is no
active yeast to consume it, the beer will become buttery with time as the
AAL oxidizes.  The thermal abuse a beer experiences in the distribution
chain will accelerate this.

The following procedure will tell you if you have excess AAL in your young
beer:

After fermentation is complete, remove 2 small samples of beer from the
fermentor.  Aseptic technique is not necessary (other than not contaminating
the fermentor).  Put each sample into a jar that you can fit with a lid (a
Mason jar would work well).  Put one sample in a hot water bath and heat to
about 140-150F for 20-30 minutes.  Keep the other sample at room temperature
(or less) during this time.

Cool the heated sample down to about the same temperature as the room
temperature aliquot by putting the jar in an ice bath.  Remove the lids and
smell them both.  You will find one of the following results:

A.  If the cool sample does not smell of diacetyl, but the heated one does,
then you have an excess of AAL in the young beer that will be converted to
diacetyl in your packaged product.

B.  If both samples smell clean, yet the product develops a buttery note
over time, it is likely that pediococcus is the culprit.  (Note:  If both
samples are devoid of diacetyl and you are a lager brewer, you need not
perform a diacetyl rest with this batch.  Why waste time?)

C.  If both samples smell of diacetyl, and your fermentation is fairly warm,
it could be that the yeast flocculated so early that it could not absorb the
diacetyl that was produced toward the end of fermentation.  It could also be
that bacteria or respiratory deficient mutant yeast are at work.  Plating
the beer on differential media will help you to solve such microbial
quandaries.

If result "A" is obtained, you are not doomed to a buttery packaged product.
If you are brewing a lager, now is the time to do that diacetyl rest.  If
you are brewing ales, simply allowing the yeast more contact time with the
young beer will reduce the diacetyl as it is formed.

You absolutely need active yeast in contact with the beer to reduce the
diacetyl.  If you are using a very flocculent yeast, like the Ringwood
strain, there may not be enough yeast left in suspension to reduce the
diacetyl.  In this case, you need to either use a less flocculent strain, or
kraeusen with fresh, active Ringwood yeast.  You could try rousing the
yeast, but without fresh wort to consume, the yeast may be pretty inactive
and uninterested in deflocculating.  The oxidation of the young beer that
can be caused by rousing the yeast is, of course, detrimental to beer
quality.

Some of you might be wondering why I suggest taking two separate samples of
beer, rather than just smelling the fresh sample before heating it.  This is
done because if the amount of diacetyl is not obvious to you, you are much
more likely to tell the difference between the two samples if memory is not
involved.  You can also conveniently invite others to help you as "taste
panelists."

Have fun!

George de Piro

C.H. Evans Brewing Company
at the Albany Pump Station
(518)447-9000
http://evansale.com (under construction)