The most reliable way to get rid of diacetyl in beer is a diacetyl rest: raising the temperature near the end of fermentation so yeast can reabsorb the compound and convert it into flavorless byproducts. Diacetyl tastes like butter or butterscotch, and most people can detect it at concentrations as low as 0.1 to 0.2 parts per million in lager. But the diacetyl rest is just one tool. Preventing diacetyl in the first place, through yeast health, fermentation management, and sanitation, is equally important.
Why Beer Produces Diacetyl
Diacetyl is a natural byproduct of amino acid production inside yeast cells. When yeast builds valine and isoleucine (two amino acids it needs to grow), it creates a precursor compound that leaks out of the cell and spontaneously converts into diacetyl in the beer. Under normal conditions, healthy yeast then reabsorbs that diacetyl and reduces it to acetoin and eventually to a compound called 2,3-butanediol, which has a flavor threshold so high you’ll never taste it.
The problem arises when yeast can’t finish that cleanup job. Cold temperatures slow the process. Removing yeast too early (by crashing or filtering) stops it entirely. And if bacteria are the source, no amount of yeast activity will fix it.
The Diacetyl Rest
A diacetyl rest works by warming the beer slightly so yeast becomes more active and reabsorbs diacetyl faster. This is standard practice for lagers but can benefit any beer style.
For lagers, the typical approach is to raise the temperature by 2 to 5°F once fermentation reaches about 50% attenuation (roughly halfway through the gravity drop). If you’re fermenting at 54°F, you’d bump it to 56°F and hold it there. At Wyeast’s recommended schedule, the rest takes about six days at 56°F (13°C) after the temperature increase. Some brewers go higher, up to 65°F, to speed things along, especially for homebrewing volumes where a few extra degrees won’t cause off-flavors.
For ales fermenting at warmer temperatures (64 to 72°F), diacetyl reduction happens more naturally during primary fermentation. Still, letting the beer sit on the yeast for two to three days after reaching final gravity gives the yeast time to clean up. Rushing to cold crash or transfer too early is one of the most common causes of lingering diacetyl in homebrew.
Testing Before You Cool Down
Tasting alone isn’t always reliable, especially if you’re near your personal detection threshold. A forced diacetyl test lets you check whether diacetyl precursors are still lurking in the beer, waiting to convert after you package.
Pull two small samples of beer and remove as much yeast sediment as possible (centrifuge or cold crash the sample). Place one sample in a hot water bath at 140 to 160°F for 10 to 30 minutes while keeping the other at room temperature. Then cool the heated sample to match the control and smell both side by side. The heat accelerates the conversion of the precursor into diacetyl, so if the heated sample smells buttery and the control doesn’t, your beer still has precursors that will become diacetyl over time. Keep conditioning until both samples taste clean.
Yeast Health and Pitching Practices
Healthy yeast produces less diacetyl and cleans it up faster. Three factors matter most: pitching rate, nutrition, and strain selection.
Underpitching forces yeast to grow more aggressively, which means more amino acid synthesis and more diacetyl precursors. Using a yeast calculator and making an appropriately sized starter (or pitching enough dry yeast) reduces the problem at the source. Adequate oxygenation of your wort before pitching also supports healthy cell growth.
Wort nutrition plays a surprisingly direct role. Research published in the Journal of the Institute of Brewing found that supplementing wort with valine at 100 to 300 mg/L reduced peak diacetyl concentrations by up to 37% and end-of-fermentation levels by up to 33%. The mechanism is feedback inhibition: when yeast has plenty of valine available in the wort, it doesn’t need to manufacture as much internally, so fewer diacetyl precursors are produced. For practical purposes, this means using well-modified malt and ensuring adequate free amino nitrogen in your wort. All-malt worts with a proper mash generally provide enough, but high-adjunct recipes can fall short.
Yeast strain matters too. Production of diacetyl precursors varies significantly between strains. Research at University College Cork showed that adapted lager yeast isolates produced over 60% less diacetyl at the end of primary fermentation compared to the parent strain, with no negative effects on fermentation speed or alcohol yield. While you can’t replicate that in a homebrew lab, you can choose strains known for clean fermentation profiles and fast diacetyl reduction. Check your yeast supplier’s flavor descriptions, and if a strain is flagged as a high diacetyl producer, plan for a longer rest.
Dry Hopping and Hop Creep
If you dry hop after terminal gravity, you may get a diacetyl spike you didn’t expect. Hops contain enzymes that can break down unfermentable sugars in beer into fermentable ones, restarting fermentation in a process known as hop creep. The problem is that yeast at this stage has low vitality. It ferments the new sugars but lacks the energy to reduce diacetyl back into flavorless compounds.
To avoid this, dry hop while fermentation is still active (a few gravity points above terminal) so yeast is vigorous enough to handle any extra sugars. If you dry hop post-fermentation, keep the beer warm enough for yeast activity for a few extra days and run a forced diacetyl test before packaging.
Enzyme Additions
Commercial breweries sometimes use an enzyme called alpha-acetolactate decarboxylase (ALDC) to prevent diacetyl formation entirely. This enzyme converts the diacetyl precursor directly into acetoin before it ever becomes diacetyl, bypassing the problem. It’s added to cold wort at the start of fermentation at a rate of 1 to 5 grams per hectoliter (10 to 50 ppm). For post-fermentation treatment, lower doses of 0.4 to 1.0 grams per hectoliter are used, mixed into the bottom of the vessel with CO2 back-flushing.
ALDC is available to homebrewers through some specialty suppliers. It’s particularly useful for high-gravity lagers or beers where a traditional diacetyl rest is difficult to manage. It won’t fix diacetyl that’s already formed, though. It only prevents the precursor from converting.
Bacterial Contamination
Sometimes diacetyl isn’t coming from your yeast at all. Lactic acid bacteria, particularly Pediococcus and Lactobacillus, produce diacetyl through citrate metabolism. These two genera are responsible for roughly 70% of microbial beer spoilage events. If your beer has a buttery flavor along with haze, sourness, or a ropey texture, contamination is the likely cause.
No diacetyl rest will fix bacteria-sourced diacetyl because the organisms keep producing it. The only solution is prevention: thorough cleaning and sanitizing of all equipment, replacing scratched plastic parts that harbor bacteria, and keeping post-boil wort protected from exposure. If you brew sour beers alongside clean ones, dedicate separate soft equipment (tubing, gaskets, plastic fermenters) to each program.
You can distinguish bacterial diacetyl from fermentation diacetyl by timing. If diacetyl appears weeks after packaging and gets worse over time, bacteria are almost certainly involved. Fermentation-derived diacetyl is present at the end of primary and either gets cleaned up during conditioning or stays constant.
Quick Reference for Common Causes
- Crashed or transferred too early: Return beer to a warmer temperature with yeast in suspension for several days.
- Underpitched yeast: Increase pitch rate in future batches; for current beer, rouse the yeast and warm slightly.
- Lager fermented too cold without a rest: Raise to 56 to 65°F and hold for three to seven days.
- Dry hop creep: Keep beer warm after dry hopping and test before packaging.
- Bacterial infection: Discard the batch, deep clean all equipment, and replace porous materials.