Diacetyl Formation

A consequence of malolactic fermentation (MLF) is the formation of diacetyl or butane-2,3-dione, responsible for the buttery flavor found in some wines. The sensory threshold for diacetyl in wine varies with the type of wine and ranges from 0.2 mg/L in Chardonnay, to 0.9 mg/L in Pinot noir, and to 2.8 mg/L in Cabernet Sauvignon wines.

I knew that diacetyl was a by product of malolactic fermentation, but until I read an article written by Brigitte Martineau and Thomas Henick-Kling, I didn't know that diacetyl can be formed during alcoholic fermentation as a result of the oxidative decarboxylation of α-acetolactate, a compound synthesized by Saccharomyces cerevisiae and excreted out of the cell.
Also during the early stages of fermentation by S. cerevisiae, if there is an accumulation of pyruvate, the presence of lactic acid bacteria such as Lactobacillus brevis and fermentable sugars, can contribute to the formation and accumulation of diacetyl.¹
The following diagram shows the pathway that leads from citric acid to the formation of pyruvate and then on to diacetyl.²

Malolactic bacteria, such as Oenococcus oeni, synthesize diacetyl from citrate or other carbohydrates via pyruvate present in the wine at the time of malolactic fermentation inoculation.
Factors that influence diacetyl concentrations include:³

• Presence of lactic acid bacteria during fermentation
• Aeration
• pH
• SO₂
• Citric acid concentration
• Malolactic bacterial strain
• Inoculation rate
• MLF temperature

At one point in the journey of wine attributes, especially in Chardonnay, the buttery flavor that diacetyl contributed was a sought after style, but it seems that it has now fallen out of favor. I am not a fan of diacetyl in my Chardonnay, but I do know that I have had some Chardonnays which have had a touch of diactyl that I appreciated with the food that I was eating. So as far as diactyl is concerned, to each their own palate.
References:
1. Brigitte Martineau and Thomas Henick-Kling, Formation and Degradation of Diacetyl in Wine During Alcoholic Fermentation with Saccharomyces cerevisiae Strain EC1118 and Malolactic Fermentation with Leuconostoc oenos Strain MCW, Am. J. Enol. Vitic., 1995 46:442-448.
2. Kenneth C. Fugelsang and Charles G. Edwards, Wine Microbiology Practical Applications and Procedures, Second Edition, Springer, 2007, page 42.
3. Roman Mink, Ralf Kölling, Stephan Sommer, Hans-Georg Schmarr, and Maren Scharfenberger-Schmeer, Diacetyl Formation by Oenococcus oeni during Winemaking Induced by Exogenous Pyruvate, Am. J. Enol. Vitic., February 2015 66:85-90; published ahead of print October 30, 2014, doi:10.5344/ajev.2014.14056.
4. All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.