Diacetyl Sources and Metabolism

In the post Diacetyl Basics I described the structure, flavor and aroma of diacetyl.  The next article will rely an online article by George de Piro and a review by Krogerus and GibsonKrogeris and Gibson, DOI 10.1002/jib.84 .  Another article you may wish to read is from Brew Your Own.

It is desirable that if diacetyl is being made, that only your brewer's yeast culture is doing so.  Most of the time, any diacetyl produced will be reduced to acetoin (by either enzymes that generally reduce any small ketone to an aldehyde, or enzymes specific to the task).  This is the process labeled 1.  In biochemistry, reduction is commonly done by using adding two H atoms via NADH + aqueous proton + 2 electrons.

A second reduction will form 2,3-butanediol (not shown, that reduction reaction is indicated by the unlabeled single down arrow below acetoin)  The carbon with the -OH group is chiral.  In 2,3-butanediol the second alcohol group is where the remaining ketone (C=O) group was, and that carbon also is chiral after the second -OH is made.

A large source of problems with diacetyl in the final product, is high levels of alpha-acetolactate which can convert to diacetyl in the oxidative decarboxylation process labeled 2.  Generally, you can reduce formation of diacetyl in finished beer by decreasing process 4, increasing process 3, or increasing process 5.  You can also lower the amount of diacetyl in the final product by making more diacetyl (should be a line in Alanis Morissette's song Ironic, don'tcha think?).  In other words increase the rate at which reaction 2 occurs, as long as reaction 1 removes the diacetyl so produced.  The latter approach (reaction 2 followed by reaction 1) is done intentionally when doing the diacetyl rest  in lagers(raising the lager to room temperature for a couple of days after fermentation is complete).  If you judge a lager with detectable diacetyl, absent any other off-flavors that indicate an infection, drop a hint that their diacetyl rest may not have produced the results desired.  Don't say "You fergot you're diassetyl rest, dumbass!".

What is not obvious is that if the yeast is not provided with enough valine, the pathway which makes valine from pyruvate (top of figure straight down to bottom) will be upregulated, producing higher levels of alpha-acetolactate.

Of course, different strains of yeast will vary in the amount of diacetyl they tend to produce even under optimal fermentation conditions.  Highly flocculent English ale yeasts tend to produce higher levels of diacetyl.  This is why English Ale styles in the BJCP guidelines says that some diacetyl is acceptable.

A last note on removing diacetyl or preventing its formation: adding enzymes such as SEBmature L or Alphalase Advance 4000 or alpha acetolactate decarboxylase (an enzyme that accelerates process 3.)  I think you can google that for yourselves if you got this far in the post.

Alpha Acids - The Bitter Truth

(Thanks to Mike Jablonsky for the title).  A lot of brewers know that alpha acids cause bitterness somehow.  In this article I will discuss just a little bit of their structure, names, and use.

First of all, why are these called acids?  There are no carboxylic acid groups in the molecule (see the structures below and note the absence of a carboxylic acid group).  The reason they are called acids is that alpha acids are more soluble in basic pH.

Humulone is just one of the three common examples of the class of molecules called humulones. One step in the biosynthesis by the hop plant (Humulus lupus) is the transfer of the side chain from an amino acid to the humulone precursor.  I have aligned the structures of the amino acid above the humulone, so that you can see where the side chain went onto the humulone molecule.  The other two humulone structures are cohumulone and adhumulone:

Looking at the bottom of the molecular structure, we see a hydroxyl group, and an isoprenyl group. Isoprenylation (commonly called prenylation) is a common building process in plant biochemistry.  If you replace the bottom hydroxyl of humulones with an isoprenyl group, you get the beta acids (lupulones). Lupulones are even less soluble in water, due to the switching out of a -OH group which can hydrogen bond with water, for a "hydrophobic chain" which cannot favorably interact with water.

Neither the alpha acids nor beta acids dissolve at a high enough concentration to impart any perceivable bitterness.  You must change these structures through isomerization.  That will be a separate topic for another day.

Diacetyl Basics

First things first: you may have heard the word diacetyl pronounced many different ways.  A couple of them are common, and largely depend on which country you are in.  The problem lies in the different ways that acetic ( \ə-ˈsē-tik-\) acid and acetate (\ˈa-sə-ˌtāt\) are usually pronounced. The common pronunciations of diacetyl are \ˌdī-ə-ˈsēt-əl, dī-ˈas-ət-əl\, and are based in turn how the acetyl group is pronounced.  I think more UK speakers pronounce with the long e, Americans with the second pronunciation.  You could finesse this issue entirely by saying 2,3-butanedione, which is the systematic nomenclature for this compound.  Here it is:

Any chemical structures which I post are generated by me, feel free to download to your collection under a noncommercial Creative Commons license. https://en.wikipedia.org/wiki/Creative_Commons_license

Diacetyl is a flaw in almost every style of beer if it is noticeable.  There are variations in how sensitive tasters are to the flavor and aroma.  At high levels the aroma is strongly artificial butter (which IS diacetyl).  At lower levels, diacetyl mixes with other flavors and aromas and is perceived more as butterscotch.  This level is acceptable in some beer styles, but even then moderation is desirable.  Even if a taster does not perceive any of these flavors and aromas, the taste will definitely seem off to the consumer.  The consumer would likely finish the beer, but not order another.  Diacetyl coats the tongue, and diminishes perception of hop bittering and hop flavor compounds.

There are several sources and sinks for diacetyl.  Diacetyl is produced during fermentation, then normally reduced by yeast enzymes to acetoin and 2,3-butanediol.  However, things can go awry with yeast.  If the fermentation is problematic, high diacetyl levels will be just part of a whole suite of symptoms, and the beer would not be released for public consumption.  The consumer would be more likely to experience high diacetyl levels from bacterial contamination, either in the package or through dirty beer lines.

A future post on diacetyl will get more into the weeds of chemistry.  Any beer judges want to hop in with their experience in detecting diacetyl, or its effects on the beers they judged?

Credit to https://www.merriam-webster.com/ for the pronunciations.