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How Does the Timing of ML Inoculation affect Diacetyl Production?

Last updated: 6/2021

Applies to: Winemakers wanting to understand the impact of co-inoculation vs. sequential inoculation of ML bacteria on diacetyl production.

As discussed in our piece on "Managing Diacetyl Production," malolactic fermentation (MLF) is more than a simple conversion of malic acid to lactic acid as the metabolism of Oenococcus oeni can affect wine texture and aroma. One byproduct of note is diacetyl, which is associated with nutty and buttery characters. Many factors can influence final diacetyl levels in wine and manipulating them can help winemakers to achieve their desired styles. Here, we delve specifically into the influence of timing of malolactic inoculation.

The timing of inoculation has a strong impact on diacetyl levels. In citric acid metabolism, pyruvic acid is decarboxylated to diacetyl via α-acetolactate (see Figure 1). Diacetyl is chemically unstable and can be reduced further by active O. oeni and yeast to less flavor-active products (acetoin and 2,3-butanediol). When ML bacteria is added 24-48 hours after yeast inoculation and alcoholic and malolactic fermentations occur together (co-inoculation), diacetyl is converted to the less buttery compounds resulting in fruitier wines.

Using a co-inoculation strategy in white, rosé, or red wines is a powerful tool to enhance fresh, fruit-driven styles and to avoid the production of diacetyl, even with bacteria known for high diacetyl production (see Figures 2 and 3).

If high diacetyl concentrations are desired, a sequential inoculation of O. oeni after alcoholic fermentation will promote the retention of diacetyl. The potential of diacetyl production is strain dependent (see Figure 2).

For more ways to control the production of diacetyl in your wine, check out our article, "Managing Diacetyl Production."