Part 11 – Diacetyl, Acetoin & Butanediol
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Diacetyl or 2.3-butanedione, (butane = 4xC, di =2,
one = ketone groups C=
0, 2.3 = position of ‘one’, see molecular structure in the image
underneath) gives off a buttery smell
Acetoin also
gives off a buttery smell, but to a lesser extent
2.3-Butanediol (diol = 2 alcohol groups –OH) is virtually
odorless.
1. Produced
by yeast from pyruvate in alcoholic fermentation
These compounds are produced by yeasts during alcoholic
fermentation from two pyruvates or one pyruvate and one ethanal. The
acetolactic acid is turned into diacetyl in one or more steps (see image underneath).
Normally, the amount produced here is not enough to have a real impact on wine
aroma.
2. Produced by LAB from citric acid
in malolactic fermentation
Considerable more diacetyl can be produced by lactic acid
bacteria during malolactic fermentation from citric acid. Citric acid is first
divided into acetic acid and oxaloacetic acid. The oxaloacetic acid is
decarboxylated to pyruvate. The pyruvate binds with ethanal to become
acetolactic acid (see image underneath). The acetolactic acid is turned to diacetyl
in one or more steps ( see image above)
The production of diacetyl, and hence the strength of the
buttery flavor, has to do with, among other things, the pH of the wine. The
higher the pH (= the less acid the wine), the greater the increase of lactic
acid bacteria and the higher the potential of diacetyl forming. It's one reason
why a mature Chardonnay from Napa usually has a much stronger buttery flavor
than a Chalis.
v ‘Malo’ during or right
after alcoholic fermentation
Diacetyl and acetoin can displace the fruity aroma. That
buttery smell can be limited by the malo to take place during or immediately
after the alcoholic fermentation and allow the wine to ripen ‘on lees’ (‘sur
lie’). The lees will then reduce the buttery diacetyl to the less fragrant
acetoin and the virtually odorless 2,3-butanediol.
P.S.
Next post, the malolactic fermentation, with 2 interesting questions :
What is the most significant difference between yeasts
and bacteria?
Why do malolactic bacteria convert malic acid into lactic
acid ?
