What is Diacetyl?

Tasting Diacetyl

  1. Go to the grocery store and purchase a bottle of imitation butter flavor, usually in the spice section, and twocans of light beer (Coors Light works very well). You want to use beer packaged in cans to ensure that it has notbeen exposed to light, which causes beer to skunk.
  2. Chill the beers to about 45 °F. Open both cans, pour them into separate glasses, and add about six drops of theimitation butter flavor to one of the beers.
  3. Now smell both beers. If you cannot easily detect the difference in the aromas, add butter flavoring three dropsat a time until you clearly smell the difference.
  4. It is important to use the flavoring chemical in beer rather than water so that you can learn how to distinguish itfrom the other beer characteristics. Up front, it is good to use a light beer for this lesson because the loadedflavors that occur in bold beers will confuse your senses.
  5. It is easy for the novice beer evaluator to confuse the buttery flavor of diacetyl with the caramel flavor of certain maltsand the toffee−like character of beers that have gone stale. Keep trying this test with increasingly bolder styles of beer.Until you are comfortable picking this flavor out.
  6. A few beer styles may benefit from the additional complexity afforded by a small amount of diacetyl, but manyprofessional brewers dislike the chemical. One reason so many brewers scorn diacetyl is that it is impossible to controlthe amount that will end up in the consumer’s glass of beer; it tends to increase with age. Brewers also dislike thiscompound because it can be indicative of poor brewing habits. To fully appreciate the brewer’s point of view, one mustunderstand how diacetyl is formed in beer.

There are three ways that 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 todiacetyl is accelerated by high temperature. At cool temperatures it will still occur, but more slowly.

Today’s brewing practice dictates that beer be aged on live yeast until the vast majority of AAL is converted intodiacetyl. Brewer’s yeast, while unable to metabolize AAL, will readily absorb and break down diacetyl intorelatively flavorless compounds. By giving the beer enough contact time with the active yeast, the brewer caneliminate the diacetyl. It generally takes only about two weeks of aging an ale to assure that it will have nobuttery flavors.

Lager beers can take a bit longer to “diacetyl proof,” because they are usually fermented at lower temperaturesthan ales. The cool environment slows the conversion of AAL to diacetyl. Some brewers will warm their lagers to55?60 °F to help speed the oxidation of AAL to diacetyl and its subsequent metabolism by the yeast. Still, it cantake 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 is calledrespiratory 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.
  • Bacteria called pediococcus can also form high levels of diacetyl in beer. While this bug cannot hurt humans, itcan make beer sour as well as buttery. Tartness is desirable in Lambic beers, but it is most unwelcome in mostother beer styles. The vast majority of brewers do their best to avoid pediococcus!
  • If a bottle of beer leaves the brewery with exactly the butteriness the brewer intends, that does not mean theconsumer will taste the beer in this condition. It is more than likely that the beer will continue to becomebuttery over time, until it becomes the dominant flavor. This can very well throw the beer’s flavor profile out ofbalance 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 ofthe beers. Decide if you think the flavor is in tune with the rest of the beer’s character, adding complexity, or if itis dominant, making the beer seem one dimensional.

    Diacetyl Test

    How do you know if your beer contains an excessive amount of alpha acetolactate (AAL), thus necessitating a diacetylrest to yield a shelf- stable beer? The test is not only easy and accurate, but it requires only some glassware, hot water,and your nose.

    This test is based on the fact that heat will rapidly oxidize the relatively flavorless AAL into butter−like diacetyl. You willneed two glasses with covers; aluminum foil works well in this role. You will also need a hot water bath big enough tohold one of the glasses. The water should be heated to 140 - 160 °F.

    1. Place a sample of your young beer in each glass.
    2. Cover and put one in the hot water bath while keeping the other at room temperature.
    3. Keep the beer in the water bath for 10 to 20 minutes.
    4. Cool the hot beer to about the same temperature as the cool sample; a cold water bath can be used to goodeffect for this.
    5. 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 andyour 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 floatingaround your beer, and you should age it at 60 °F, or so, for a few days to allow diacetyl to form and then bemetabolized 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 an indication of a pediococcus infection, inwhich case you should dump the batch and start over, or it can mean that your yeast is incapable ofmetabolizing diacetyl.
    4. It could also mean that your beer is still kind of young and you should try the test again after a few more days ofwarm aging. Hopefully the diacetyl will fade. Kräusening with fresh yeast may also help, unless the problem isbacterial.