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Managing Diacetyl Production During MLF

Last Updated: 6/2021

Applies to: Winemakers looking for ways to maximize or minimize the production of Diacetyl (butter aroma and flavor) during malolactic fermentation.

Diacetyl and Malolactic Fermentation

Malolactic fermentation (MLF) is more than a simple conversion of malic acid to lactic acid. The ability of malolactic bacteria, Oenococcus oeni, to affect wine texture and sensory characteristics in a positive way can be harnessed to influence wine aromas and drive wine style. As seen in the 2021 Scott Labs Handbook and the figure here, we've characterized the sensory impact of the strains we offer to better assist winemakers in achieving their stylistic goals.

One of the sensory impacts, "buttery," can largely be attributed to the compound diacetyl. Controlling the production of diacetyl can be important in achieving a desired wine style. For example, high diacetyl concentrations in Chardonnay are often characterized as distinctly "New World," however; if too high, they can be perceived as overly ‘buttery’ and may be regarded as undesirable by some consumers. Similarly, low diacetyl concentrations can represent an "Old World" style, but may not appeal to certain sectors of the market.

Manipulating Diacetyl During MLF

As diacetyl is largely a byproduct of malolactic fermentation, there are several factors that can be easily manipulated by the winemaker during MLF to achieve their desired diacetyl level including:

Strain Choice

Strains of malolactic bacteria vary in their potential for production of diacetyl. Refer to the figure below to compare strains sold by Scott Labs.

Timing of ML Inoculation

A simultaneous alcoholic/malolactic fermentation (MLF) will tend to favor lower diacetyl levels. As the diacetyl is being produced, yeast and bacteria will break it down. Similarly, inoculating for MLF following the completion of alcoholic fermentation will contribute to higher diacetyl levels in wine. To learn more about this, check out our article: "How Does Timing of ML Inoculation Affect Diacetyl Production?"

Contact Time with Lees

The shorter the contact time with yeast and bacteria, the higher the diacetyl. This is because living yeast and bacteria break down diacetyl irreversibly.

Malolactic Fermentation Length

The longer the malolactic fermentation the higher the diacetyl. This is influenced primarily by wine pH and temperature. Low pH and temperatures lead to longer MLFs and vice versa.

Stirring

Stirring during MLF affects the redox potential of the wine – stirring allows slightly oxidative conditions and lack of stirring allows reductive conditions. Oxidative conditions (higher redox potential) favor the production of Diacetyl while reductive conditions (lower redox potential) favor the production of acetoin and 2,3-butanediol which do not contribute to the overall aroma of wine.

SO2 Addition

SO2 binds to diacetyl in a reversible reaction. Initially, the diacetyl level will seem to disappear, but over time the reaction will reverse and the diacetyl will be released. However, sulfites also inhibit or kill off yeast and bacteria that help break down diacetyl. Adding SO2 immediately after the completion of MLF will favor the highest diacetyl levels in the final wine.

Quick Reference Guide

Consult the table below for specific winemaking recommendations on maximizing and minimizing the production of diacetyl during malolactic fermentation.

MaximizingMinimizing
Strain ChoiceChoose a strain with a high potential for diacetyl production (ENOFERM BETA™ and PN4™).Choose a strain with low potential for diacetyl production (VP41™ or O-MEGA™).
Contact Time with LeesRack off lees or wait a few days for the yeast to transition to a metabolically inactive state after primary fermentation before adding bacteria.Complete MLF on the lees and monitor diacetyl levels. When diacetyl has reached the desired level, add SO2 and rack off lees.
Timing of ML InoculationAdd malolactic bacteria after primary fermentation (sequential fermentation). Filter wine to remove yeast or rack off the lees prior to inoculation with malolactic bacteria.Co-inoculate with yeast and malolactic bacteria (simultaneous fermentation). If bacteria is added after alcoholic fermentation, let the wine stay on the lees until a desired level of diacetyl is reached.
Wine ConditionsAdjust to conditions that will favor a longer MLF - Lower pH and cooler temperature.Adjust to conditions that will favor a quicker MLF - Higher pH and warmer temperature.
Stirring During MLFStir during MLF.Do not stir during MLF.
Addition of SO2Add SO2 immediately following the completion of MLF (when Diacetyl is highest).Complete MLF, then monitor diacetyl and add SO2 when it reaches the desired level.

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