Diacetyl In Beer-Butterscotch and Buttery Flavors We All Try to Avoid

Diacetyl in beer, or those butterscotch and buttery off flavors we have all heard about, are normal byproducts of the fermentation process.

Too much can create off flavors which are easily detectable and hardly ever welcome.

There are varying methods of reducing or eliminating this unwanted flavor in the final product, but it is always produced.

Ales produce more than lagers and some ale styles, such as the English Milds and American Common Beers, may even find slight amounts acceptable.

It is almost never acceptable in lagers, Bohemian Pilsener is one exception. So why are these butterscotch flavors shunned by brewers?

It’s because its formation can also be caused by some bacterial infections. If you have brewed your beer and used brewing practices which minimize or exclude diacetyl in beer, but notice it anyway, it can be an indicator of possible sanitation problems or other brewing errors.

With the term “bacterial infection” floating around, it’s no wonder that brewers shun diacetyl in beer.

Learn How to Detect Diacetyl in Beer

A picture of butterscotch candy.

If you are a little unsure of how to detect this unwanted byproduct of fermentation, don’t worry. It is fairly simple to learn to discern its buttery flavor and just takes a little practice. Here is how to do it.

  • Purchase a bottle of imitation butter flavor, usually found around the spices.
  • Get two cans of flavorless American lager, Coors light works well, buy them in cans so you can rule out skunked beer from light exposure. Chill the beers to about 45°F (7°C).
  • Open each can and pour them into separate glasses.
  • Add about 6 drops of imitation butter flavor into one of the beers.
  • Smell both beers.
  • If you can’t smell the difference, add 3 drops at a time until you can.

Using a light American lager is the easiest way to begin to learn its flavors and aromas because darker and bolder beers can mask the flavors somewhat.

It is easy confuse the flavors and aromas with some of the caramel and toffee-like flavors and aromas of some other beers. Practice the exercise with different types of beer and drink with experienced tasters or judges to quickly hone your skills.

How Diacetyl is Produced

Most diacetyl in beer is produced early in the fermentation process and if left alone, will be reabsorbed by the yeast and converted to flavorless compounds.

Chemically, the compound is called a vicinal diketone (VDK), because it contains two ketone (oxo-) groups on adjacent (vicinal) carbon atoms.

It occurs as a common byproduct of fermentation because its oxo-hydroxy precursors will pass through the yeast membrane and into the beer. Its precursor is called alpha-acetolactate (AAL) which is a hydroxyl acid produced midway through the synthesis of the amino acid valine.

Once it gets into your beer, the amino acid reacts with oxygen and in a chemical reaction (no enzymes are involved) will yield the diketone diacetyl. Diacetyl in beer then passes back into the yeast cell where is is finally reduced to the mono-alcohol called acetoin and di-alcohol butanediol.

Both of these have a much lower flavor threshold and a much less intense flavor and aroma than diacetyl. The flavor threshold for diacetyl is about 0.15 mg/L for most people.

Diacetyl Formation Can Only Occur if Oxygen is Present

Formation can occur only when there is oxygen present for the amino acid valine to react with. Since beer is normally heavily oxygenated prior to pitching yeast, it seems that the buttery notes are inevitable.

It is formed early on, when the yeast is absorbing oxygen during reproduction. Once all the oxygen is gone, there will be no more production unless more air is introduced, such as when the beer is transferred form primary to keg, or primary to secondary.

How Temperature Affects Its Formation

More diacetyl in beer is produced with higher fermentation temperatures (thus why ales have more than lagers). As mentioned earlier, yeast have the ability to reduce the levels to below the flavor and aroma thresholds during the anaerobic phases of fermentation, given the right conditions and enough time.

Once in your beer, it takes some time for it to be broken down to flavorless diols. It occurs much slower during the lagering phase due to the low temperatures.

Thus, it is important to reduce the compound as much as possible before you lager your beer, to reduce the amount of time it takes to fully condition them.

Using a Diacetyl Rest

One way to accomplish the reduction is to use a diacetyl rest. This rest is employed at the end of primary fermentation by slowly ramping the temperature up 5-10°F (1-2°C) above final fermentation temperature, to invigorate the yeast and reduce the level to below it’s flavor threshold.

It generally takes about 2-3 days at the warmer temperatures to reduce the levels. The reduction is fairly rapid, but the breakdown is usually pretty slow, especially at lagering temperatures.

For this reason, it usually takes at least 4 weeks for a lager to stabilize. Since ales are usually conditioned warm, it takes only about two weeks of aging and conditioning to assure there are no buttery or butterscotch flavors left.

Petite Mutants

Sometimes brewers’ yeast loses its ability to utilize oxygen.  In this state they are called respiratory mutants, or petite mutants (because the yeast colonies are very small when cultured in the laboratory).

Petite mutant yeast cells produce the compound but are unable to metabolize it in the later phases of fermentation, thus leaving it in your beer. Temperature shock of greater than 10°F will cause formation of these petite mutants in your fermenting beer.

The remedy is to allow the yeast to acclimate to the same temperature as the wort before pitching. You can use thermometers to measure the temperatures, but this is another source of possible contamination.

Pediococcus Bacteria

Pediococcus bacteria will produce copious amounts of diacetyl. Using pedio in your beers, such as lambics to produce the characteristic sour or tart flavors, can be a delicate process since it is sometimes added after the majority of fermentables have been consumed, limiting or controlling the sourness in the beer.

Diacetyl Increases with Aging

Another problem is that the aromas and flavors increase with aging (if left in the beer after fermentation). If you want to add a little buttery note to your beer, it may be difficult to control when a little becomes too much (as evidenced by the score-sheets in homebrew competitions). Timing is everything in beers which display some diacetyl.

Krausening to Reduce Diacetyl 

Another method of reducing the buttery notes is
krausening.  Krausening is the addition of actively fermenting wort from the krausen stage of primary fermentation, usually at 5-10% by volume of the green beer. This is the traditional method of carbonation by German brewers. There are many advantages to carbonating by krausening, and one of those is a reduction in diacetyl. 

If you would like to learn how to do a simple test for the diacetyl precursor alpha-acetolactate (AAL) in your green (or young) homebrewed beer, click here.

If you do a search for Butterscotch in Beer, you may be surprised to find this Butterscotch Beer, made by Flying Cauldron since 1374.  This is a non-alcoholic drink made for children or the “Young at Heart” in Hogsbreath England.  

References:  This article was about diacetyl in beer was adapted in part from Beer Flavors #1: Diacetyl by George de Piro, Brewmaster C.H. Evans Brewing Company at the Albany Pump Station, and THE ROLE OF DIACETYL IN BEER By Moritz Kallmeyer, Chief Brewer of Drayman’s Microbrewery, Silverton Pretoria, February 2003 (updated 22/10/2004).

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