That creamy, buttery aroma you sometimes find in a bottle of Chardonnay or other wines is thanks largely to a compound called diacetyl. It’s the star player released during malolactic fermentation (MLF), and whether a winemaker wants to emphasize or eliminate it is one of the biggest stylistic choices they make.

Diacetyl is a temporary byproduct of the bacteria’s metabolism. It is later reduced into less aromatic compounds, which is why winemakers have to move fast to capture the flavor at its peak. To get a wonderfully buttery wine, the goal is simple: make a lot of diacetyl and then immediately halt its degradation. Here are the key strategies winemakers use to ramp up diacetyl production and keep that creamy character in the bottle:

1. Timing is Everything: Inoculate after alcoholic fermentation.

a. Wait until the main alcoholic fermentation is finished or almost finished before adding the bacteria. Why? The yeast that did the primary fermentation and the bacteria themselves are very good at eating up diacetyl and reducing it to non-buttery compounds. Waiting until the yeast is dead or dormant allows the diacetyl to build up.

b. Conversely, co-inoculating the bacteria at the start of alcoholic fermentation is great to minimize diacetyl.

2. Choose the Right Bacteria Strain

Not all bacteria are created equal. Select a commercial Oenococcus oeni strain specifically known for its potential to be a high diacetyl producer, such as Malo-D from ATPGroup. Choosing bacteria to achieve a specific wine style is similar to choosing a high-flavor yeast for primary fermentation: you have to pick the right tool for the job.

3. Stress the Bacteria, But Just a Little

a. A slow, slightly stressed MLF favors the accumulation of diacetyl. It sounds mean, but it works!

b. Keep it Cool: Conduct the MLF at the lower end of the bacteria’s comfort zone (17−18°C or 63−65°F).

c. Low pH: A slightly lower pH (e.g., 3.2−3.4) naturally makes the bacteria work harder, which increases the potential for diacetyl.

4. A Little Oxygen Helps

A little stirring or a small introduction of oxygen during the MLF can actually speed up the chemical reaction that converts diacetyl’s precursor into the final buttery compound. This subtle technique helps ensure more flavor is created and less is lost to enzymatic reduction.

5. The Critical Finish: Stopping the Fermentation Cleanly

a. Regularly analyze or sensory-test the wine as it nears the end of MLF. This is when the diacetyl level will be at its peak. Once the perfect buttery aroma is achieved, immediately stop the metabolism of the bacteria.

b. This is usually done using a sufficient dose of sulfur dioxide (SO₂). However, be advised that sulfur dioxide can bind with diacetyl. Luckily, the reaction is reversible, so even though buttery levels may seem to dip initially, they will return later.

6. Clean Up Promptly

After the fermentation is officially stopped, the wine needs to be racked or filtered off the lees right away. Letting the wine sit on the lees will continue to slowly reduce the diacetyl and diminish the buttery character.

Ultimately, whether you love or hate those buttery notes, their inclusion is up to the winemaker. This stylistic choice can be achieved or avoided by precisely managing strain selection, temperature, timing, and flavor accumulation at the end of MLF to expertly craft a wine that’s either rich, creamy, and decadent or crisp, clean, and fruit-forward.

For questions, troubleshooting, or advice, contact your ATPGroup Enology Products Specialist, call (707) 836-6840, or visit our website at ATPGroup.com.