Diacetyl. Most people dont 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 dont worry if you are uncertain of diacetyls 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 consumers 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 brewers 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. Brewers 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. Brewers 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. Brewers 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 brewers 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 beers 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 beers character, adding complexity, or if it is dominant, making the beer seem one-dimensional.
For Brewers Only: The Diacetyl Test
How do you know if your beer contains an excessive amount of alpha acetolactate (AAL), thus necessitating a diacetyl rest to yield shelf-stable beer? The test is not only easy and accurate, but it requires only some glassware, hot water, and the very sophisticated aroma analyzer found in the middle of your face!
This test is based on the fact that heat will rapidly oxidize the relatively flavorless AAL into butter-like diacetyl. You will need two glasses with covers; aluminum foil works well in this role. You will also need a hot water bath big enough to hold one of the glasses. The water should be heated to 140-160 °F.
Place a sample of your young beer in each glass. Cover and put one in the hot water bath while keeping the other at room temperature. Keep the beer in the water bath for 10-20 minutes.
Cool the hot beer to about the same temperature as the cool sample; a cold water bath can be used to good effect for this. Remove the covers and smell each sample. One of the following conditions will exist:
1. Neither beer smells buttery. This is good! It means that all of the AAL has already been converted to diacetyl and your beer is ready for packaging.
2. The heated sample smells buttery, but the cold one does not. This means that there is excessive AAL still floating around your beer, and you should age it at 60 °F or so for a few days to allow diacetyl to form and then be metabolized by the yeast. Repeat the test to determine the proper time for packaging.
3. Both samples smell like butter. This can be a bad thing. It can be indicative of a pediococcus infection, in which case you should dump the batch and start over, or it can mean that your yeast is incapable of metabolizing diacetyl (see respiratory mutants, above).
It could also mean that your beer is still kind of young and you should try the test again after a few more days of warm aging. Hopefully the diacetyl will fade. Kräusening with fresh yeast may also help, unless the problem is bacterial.