Beer Flavor Primer #1: Diacetyl
by George de Piro
Author’s note: This is the first in a series of articles about different flavors that occur in beer. Each piece will discuss how to recognize the flavor and how it is formed during the brewing process. A sensitive and well-trained palate is critical to being a good brewer.
A bartender pulls a pint in London
Diacetyl. Most people don’t even know how to pronounce this word, and yet all know the flavor of the chemical it represents. (By the way, I used to be a chemist. Two common pronunciations are die-ASS-a-teel and DIE-a-see-till.) This chemical gives butter its characteristic flavor, and it also can be found in beer.
Many consumers are familiar with beers that have a buttery flavor, as it is common in many imported ales and some American craft beers, but don’t worry if you are uncertain of diacetyl’s character. It is easy to learn how to discern its aroma and flavor in beer.
Step one is to go to the grocery store and purchase a bottle of imitation butter flavor (found in the spice section) and two cans of light-tasting beer (Coors Light works quite particularly well). The beer used for this lesson should be packaged in cans to ensure that it has not been exposed to light, which causes beer to skunk (click here to read how light damages beer).
Step two is to chill the beers to about 45 °F. Open each, pour them into separate glasses, and add about six drops of the imitation butter flavor to one of the beers. Now smell both beers. If you cannot easily detect the difference in the aromas, add butter flavoring three drops at a time until you clearly discern the diacetyl character.
It is important to spike the flavoring chemical into beer rather than water so that you can learn how to distinguish it from the other beer characteristics. At first, it is good to use a light beer for this lesson because the rich flavors that occur in bold beers will confound your senses.
It is easy for the novice beer evaluator to confuse the buttery flavor of diacetyl with the caramel flavor of certain malts and the toffee-like character of beers that have gone stale. Practice with spiking different types of beer and drinking with experienced tasters will quickly reveal the differences.
A few beer styles may benefit from the additional complexity afforded by a small amount of diacetyl, but many professional brewers shun the chemical, most especially German brewers. One reason so many brewers scorn diacetyl is that it is impossible to control the amount that will end up in the consumer’s glass of beer; it tends to increase with age. Brewers also dislike this compound because it can be indicative of poor brewing procedures. To fully appreciate the brewer’s point of view, one must understand how diacetyl is formed in beer.
Three pathways lead to the creation of diacetyl. The first is through normal yeast metabolism. Brewer’s yeast form a precursor called alpha acetolactate (AAL), which is tasteless. This compound is converted to diacetyl as the beer ages. The reaction that changes AAL to diacetyl is accelerated by high temperature. At cool temperatures it will still occur, but more slowly.
Modern brewing practice dictates that beer be aged on live yeast until the vast majority of AAL is converted into diacetyl. Brewer’s yeast, while unable to metabolize AAL, will readily absorb and break down diacetyl into relatively flavorless compounds. By giving the beer enough contact time with the active yeast, the brewer can eliminate the diacetyl. It generally takes only about two weeks of aging an ale to assure that it will have no buttery flavors.
Lager beers can take a bit longer to “diacetyl proof,” because they are usually fermented and matured at lower temperatures than ales. The cool environment slows the conversion of AAL to diacetyl. Some brewers will warm their lagers to 55-60 °F to help speed the oxidation of AAL to diacetyl and its subsequent metabolism by the yeast. Still, it can take a minimum of four weeks to produce a stable lager.
Diacetyl is also formed by mutant yeast. Brewer’s yeast that has lost its ability to properly utilize oxygen are called respiratory mutants, or petite mutants (because they form abnormally small colonies on laboratory plates). These yeast are also unable to properly metabolize diacetyl, thus leaving it in the beer.
A bacteria called pediococcus can also form high levels of diacetyl in beer. While this bug cannot hurt humans, it can make beer sour as well as buttery. Tartness is desirable in Lambic beers, but it is most unwelcome in most other beer styles. The vast majority of brewers do their best to avoid pediococcus!
Even when diacetyl is formed by normal brewer’s yeast, it can be a scary thing. Certainly, a small amount of butteriness can add depth and complexity to some beer styles, but the level of diacetyl in packaged beer cannot be easily controlled by the brewer. That is why so many of us scorn this otherwise innocuous compound.
If a bottle of beer leaves the brewery with exactly the butteriness the brewer intends, that does not mean the consumer will taste the beer in this condition. It is more than likely that the beer will continue to become buttery over time, until it becomes the dominant flavor. This can very well throw the beer’s flavor profile out of balance and embarrass the brewer.
Now that you know how to recognize diacetyl in beer, go out to your favorite pub and see if you find it in any of the beers. Decide if you think the flavor is in tune with the rest of the beer’s character, adding complexity, or if it is dominant, making the beer seem one-dimensional.