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What Is Diacetyl and How Does It Affect Beer?

Though diacetyl is a natural part of fermentation, it produces an off-flavor in beer often that tastes like buttered popcorn. Just what causes it?

What Is Diacetyl and How Does It Affect Beer?

Diacetyl is a diketone molecule that also happens to be one of the world’s most popular – and controversial – flavoring agents. Diacetyl has a pronounced buttery taste. It’s the substance that makes margarine taste like butter. It’s also what makes low-fat microwave popcorn taste buttery. Even many of the world’s creameries are wise to diacetyl. In the process of making cultured butter, cream is inoculated with a lactic acid culture that releases – you guessed it! – diacetyl. The diacetyl produced during the culturing process is a powerful flavor enhancer that makes the butter even tastier.

Though diacetyl is a natural product of fermentation, it’s undesirable – and is even considered a defect – in many types of beer. Diacetyl is often found in ales and porters in small concentrations, but a lager should generally contain no diacetyl whatsoever.

In this article, we’re going to explore diacetyl and its relation to beer. If you’re a homebrewer, we’ll also provide some tips that can help you minimize and control the amount of diacetyl in your product. First, though, why is diacetyl such a controversial flavoring agent?

Why Is Diacetyl Controversial?

Diacetyl is a controversial flavoring agent because it is believed that inhaling high concentrations of diacetyl can, over time, can cause an irreversible lung condition called bronchiolitis obliterans – a condition known colloquially as “popcorn worker’s lung.” The condition has no cure short of a lung transplant – and that procedure has a poor success rate.

Workers in popcorn factories have contracted the condition, as has at least one person following regular daily consumption of microwave popcorn.

So, while diacetyl is generally considered safe to consume in reasonable amounts, inhalation of diacetyl appears to be unsafe. As a result, many popcorn companies have removed diacetyl from their products. Some nations – such as the United Kingdom and the nations of the European Union – have banned diacetyl as a flavoring in products such as e-liquids for vaping. Platinum E-Liquids from V2 Cigs UK, for instance, contain no diacetyl.

What Is Diacetyl’s Effect on Beer?

People can taste diacetyl in extremely low amounts. In California chardonnay wine, for instance, the formation of diacetyl is often encouraged during fermentation, and people can taste it in concentrations as low as 0.2 parts per million. In a stronger-tasting wine such as cabernet sauvignon, the diacetyl concentration needs to be much higher – about 2.8 ppm – before people can taste it.

The reason why some wineries encourage the formation of diacetyl in chardonnay is because it imparts a buttery flavor and mouthfeel that many people consider characteristic of the varietal.

In beer, you’ll taste diacetyl in concentrations as low as 0.1 ppm. Diacetyl contributes a buttery flavor to beer; some may also interpret it as a butterscotch flavor. In addition, diacetyl gives beer a slippery or oily mouthfeel. Some people find that diacetyl makes beer seem more filling.

As mentioned above, small amounts of diacetyl are often encouraged in certain types of beer such as ale and stout. In lager, diacetyl usually isn’t desirable – and if the diacetyl concentration is high enough, it’s considered a flavor defect in any type of beer because it’s likely there due to a bacterial contamination.

As you’ll soon learn, diacetyl can be present in beer due to improper sanitation during the brewing or bottling process. That’s one reason why people sometimes react so strongly when they taste it in beer. These days, though, some microbreweries are beginning to experiment with diacetyl as a means of creating interesting new flavor profiles. As a result, people are beginning to think about diacetyl with more of an open mind.

Why Would Diacetyl Be Present in a Beer?

Diacetyl can be present in a beer for two reasons. The first reason is because the yeast used in the brewing process introduces diacetyl during fermentation. Some yeast strains introduce more diacetyl than others, but diacetyl is going to be present in fermenting beer regardless of the strain used. If you’re a homebrewer, it’s possible to control the fermentation-introduced diacetyl in your product using a technique we’ll explain shortly.

The other reason why diacetyl can be present in a beer – and in this case, it’s always considered a defect – is due to poor sanitation. Pediococcus and lactobacillus cultures are lactic acid bacteria strains that can reproduce in anaerobic and alcoholic environments, and low heat doesn’t kill them. Those bacteria produce diacetyl when they reproduce, and when that happens in beer, the beer can acquire a flavor that tastes like spoiled butter.

Because lactic acid bacteria can live in alcohol and without oxygen, they can continue reproducing in beer that’s already been bottled. The bacterial contamination causes pressure to build up inside the bottles, and it can turn beer sour within just a couple of months of bottling.

How to Control Diacetyl in Homebrewing

If you’re a homebrewer, controlling the diacetyl content in your beer is a challenge you’re going to have to tackle. Here are a few tips that can help you prevent diacetyl from ruining a batch of beer.

  • Sanitize your equipment and bottles carefully. As mentioned above, bacterial contamination can ruin a beer after it’s bottled; proper sanitation is the only way to prevent that from happening.
  • Keep your bottled beer cold. Alpha acetolactate is a precursor to diacetyl, and it’ll convert to diacetyl in a bottled beer that’s stored in warm conditions.
  • Introduce a diacetyl rest into your fermentation process. You can do that by finishing your fermentation at a slightly elevated temperature before lowering the temperature again as the beer approaches its final gravity. Alternatively, you can simply wait. Instead of racking the beer when it reaches its final gravity, leave the yeast in the mixture for a week. During the diacetyl rest, the yeast consumes any diacetyl present in the beer. In consuming the diacetyl, the yeast produces acetoin and 2,3-butanediol – neither of which greatly affect the flavor of the final product.

Remember that once you’ve removed the yeast from your beer, there’s no longer any way to remove diacetyl from the product – so the diacetyl rest is a crucial stage of the brewing process if you want to be certain that your beer contains no diacetyl.

During the diacetyl rest, you can test your beer for diacetyl simply by sampling it. Draw two samples and put one sample in the refrigerator. Heat the second sample to about 140 degrees Fahrenheit for 20 minutes. Put the second sample in the refrigerator. When the samples are chilled, drink them both. If you can taste butter in the sample that you heated, the yeast needs more time to consume the diacetyl in your beer.