I will have some butter with my Chardonnay, please

If you are tempted to address the title statement to your host after tasting the un-oaked Chardonnay served at the party, then you are enamored with diacetyl (butanedione, butane-2,3-dione), an organic compound which donates buttery, butterscotch, nutty, and toasty aromas to wine and, at the appropriate levels, a desirable complexity.

Diacetyl is produced at low levels (< 5 mg/L) during alcoholic fermentation (especially at high fermentation temperatures) but is absorbed by the yeast and reduced to acetoin and 2,3-butanediol (Jackson). Its production, however, is most strongly associated with malolactic fermentation (MLF) and the metabolism of citric acid by lactic acid bacteria (LAB), a process wherein its maximum concentration occurs simultaneously with the exhaustion of L-malate. This chemically unstable compound is reduced to acetoin and 2,3-butanediol (sensorially insignificant compounds) by the actions of the LAB Oenoccus oeni and yeasts. The accumulation of diacetyl is a function of the MLF rate, with lower levels produced at higher rates and vice versa (Lern, et al.). 

Bauer and Dicks describe diacetyl as one of the most important flavors produced during MLF. The threshold levels for its detection varies by variety and wine type. The threshold for detection in Chardonnay is 0.2 mg/L, is 0.9 mg/L in Pinot Noir, and is 2.8 mg/L in Cabernet Sauvignon. In general, diacetyl levels in young red wines range between 0.2 and 1.84 mg/L while aged reds have levels that range between 1.25 and 3.39 mg/L (Lern, et al.). At concentrations between 1 and 4 mg/L, the compound contributes the aforementioned aromas and complexity to the wine but at levels above 4 mg/L the aromas are overpowering and undesirable (Lern, et al.). Jackson refers to the resulting aroma as lactic off odor and notes that it is generally associated with spoilage induced by certain strains of LAB.

Knowing the qualities imparted to the wine by diacetyl, the enterprising winemaker can take action to influence the levels in his/her final product. The table below identifies the factors which can impact on the final diacetyl levels in the finished wine.

Influencing Factors	Effect on diacetyl concentration/sensory perception
LAB strain	Varying production levels
Wine type	Higher levels of diacetyl in red wine production than in white
MLF inoculation rate	Lower inoculation rate favors dactyl production
Yeast and lees contact	Reduces diacetyl content
Aeration during MLF	Increases dactyl production
Sulfur dioxide content	Sulfur binds with diacetyl, rendering it sensorially inactive but this is reversible.  Sulfur dioxide addition inhibits yeast/LAB activity, stabilizing diacetyl content at time of addition
Citric acid concentration	Favors diacetyl production but acetic acid is also produced
MLF temperature	Diacetyl production enhanced between 18 and 25℃
Wine pH at MLF	Lower pH favors diacetyl production
Wine stabilization	Immediate stabilization post malic and citric acid metabolism will increase dactyl content

Source: Lern, et al.

References:
Bauer and Dicks, Control of Malolactic Fermentation in Wine: A Review, S. Afr. J. Enol. Vitic., 25(2), 2004.
Jackson, Wine Science.
Lern, Engelbrecht, and du Toit, Malolactic Fermentation: The ABCs of MLF, S. Afr. J. Enol. Vitic., 31(2), 2010.

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