Part 11 - Diacetyl, Acetoin & Butanediol

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.

               

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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 ?