Diacetyl in Home Brewed Beer – The Butterscotch Flavor

by Brad Smith on April 21, 2012 · 11 comments

Diacetyl is the butterscotch or buttery flavor that can ruin your home brewed beer. This week, as part of my ongoing series on beer flavors and off flavors, we’ll discuss diacetyl in your beer and how you can control it. Earlier articles in the series include DMS in home brewed beer, Esters in Home Brewed Beer and Judging Beer.

Butter and Honey

Diacetyl tastes like butter, Pentainedione tastes like honey

If you want to learn more about diacetyl I also recommend listening to my podcast/video interview with Charlie Bamforth – he does a great job of explaining it.

What is Diacetyl?

Diacetyl is a natural by-product of fermentation. It is one of two major Vicinal Diketones (VDKs) produced during fermentation, the other being Pentainedione. Diacetyl tastes like butter or butterscotch and in fact is used in the production of artificial butter flavors. Pentainedione provides a honey like flavor. Both can be detremental to the finished beer, and are considered off flavors if too much is present.

Diacetyl is most noticible in very light flavored beers, such as light lager, due to its relatively low flavor threshold. It is far less of an issue in full bodied english ales or dark beers because ales are fermented at higher temperature which helps break down VDKs after fermentation. Also other flavors in ales often mask the flavor of remaining diacetyl.

Diacetyl can also be produced by bacterial infection, and in fact bacteria produces more diacetyl than pentainedione. It is most often produced by contaminated keg lines (particularly in bars), but can occur even in home brewed beers that are infected.

 Controlling Diacetyl

Diacetyl is a natural byproduct of fermentation, so it cannot be completely eliminated. However healthy yeast can also “mop up” or break down both diacetyl and pentainedione into other substances that have a much higher flavor threshold. To do this, the yeast needs to remain in contact with the beer, and also it needs to remain healthy.

This brings us to the first important method for controlling diacetyl, which is simply to pitch a proper quantity of healthy yeast with a good yeast starter at the start of the fermentation. Properly pitching the right amount of yeast will result in a strong healthy yeast concentration after fermentation and allow the yeast to reabsorb the VDKs (diacetyl and pentainedione). Underpitching can lead to diacetyl.

A second method, called “krausening”, involves pitching fresh active yeast after the beer has completed fermentation. This active yeast will break down the diacetyl and pentainedione and significantly reduce the levels of both in the finished beer.

A third method, often used with lagers, is to add a diacetyl rest. A diacetyl rest is allowing the fermentation temperature to rise slightly to 57F or 14 C for lagers (roughly a few degrees higher than your fermentation temperature) near the end of the fermentation to help activate the yeast so it can absorb some of the VDKs. Diacetyl rests are rarely needed with ales, as ales are already fermented at a high temperature that promotes VDK reduction.

Diacetyl can also come from bacterial contamination, so it is important to sanitize everything that might touch your beer. Dirty keg lines also contain bacteria that produces diacetyl, so keeping your keg lines clean is very important.

Finally, some commercial brewers use a product called Maturex that is a chemical that helps reduce VDKs. In some cases, microbrewers and home brewers can obtain this as well, but it must be used with care in the appropriate quantity.


  • Diacetyl (butterscotch flavor) and pentainedione (honey flavor) are a natural biproduct of fermentation, but both can be broken down by healthy yeast when fermentation completes.
  • The primary method for controlling diacetyl (and pentainedione) is to pitch the appropriate quantity of healthy yeast from a yeast starter and ensure that the yeast remains healthy.
  • A diacetyl rest for lagers (raising it a few degrees during fermentation) can also help reduce diacetyl in the finished beer by helping the yeast break down VDKs.
  • For some really light lagers where eliminating all of the diacetyl is important, a method called krausening can also be used. In krausening, fresh active yeast is added after fermentation has completed to break down remaining VDKs. This method is relatively quick – taking less than a day in most cases.
  • If you are kegging, be sure to keep your keg lines and taps clean as bacteria often creates diacetyl within the dirty lines and taps.
  • Poor sanitation can lead to bacterial infection which will also produce large quantities of diacetyl. Keep your equipment clean.
  • Finally, some commercial brewers use chemical additives (Maturex) to chemically break down diacetyl and pentainedione. This may be available for microbreweries and home brewers if you can find a good source of it.

Thank you for joining me this week on the BeersSmith blog – please subscribe to the newsletter or listen to my video podcast for more great material on homebrewing.

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{ 7 comments… read them below or add one }

Marty Nachel April 22, 2012 at 10:44 am

Overall a good article, but I have an issue with your description of “krauesening” (note spelling). In the traditional/historical sense of this process, a small portion of young, actively fermenting beer is added to aged beer to induce natural carbonation. The process didn’t involve re-pitching of yeast.

Am I quibbling over semantics?

Marty Nachel– “Beer for Dummies”, “Homebrewing for Dummies”

Brad Smith April 25, 2012 at 9:06 am

Marty – I’m going to Dr Charles Bamforth here (listen to BeerSmith podcast 31). He claims this technique is a proper word of the word krauesening, even though the term is commonly used for the carbonation method you refer to.

Chris McNally May 14, 2012 at 12:53 pm

very timely article. I brewed a pils and after force carbonating it I found it full of Diacetyl. I had done a short 2 week 50 degree ferment with only a 2 day diacetyl rest at a warmer temperature. So I made a two liter starter with a German Ale Yeast and put it in the keg and gave it another week or so, cleaned it up substantially. Everyone says they cannot taste it, but I feel like it diminished but was still there. This is what happens when you try to rush the yeast.

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Adam February 11, 2014 at 9:15 pm

Hi Brad… In regards to Maturex you mention “it must be used with care in the appropriate quantity.”

Do you know what adverse effects can be expected from using more than the recommended dosage?

Jorge Sarabia September 30, 2014 at 9:58 pm

What if I want the diacetyl flavor in my beer? Is there a method to produce in such way that it will stay in the finished beer for a long time? I think I’ve read that diacetyl is usually broken down by the yeast after a while.

Pharmerdavid June 4, 2019 at 10:43 pm

I read in “Knowing Beer”, by Jeff Alworth (should have been “Aleworth”) that as Pilsner Urquell gets older the yeast used produced more diacetyl as it ages, which I had previously noticed without knowing that factoid. I always check the date, and older Pilsner Urquel usually has a more pronounced butterscotch flavor, sometimes a bit of spunkiness, but that usually dissipates in the glass (pour it vigorously). The brown bottles they use now, even better the cans, and the fact they keep it refigerated during shipping, gives it a long shelf life. The Pilsner Urquell I just bought says it’s good until January 2020, which is over six months away, and I’ve seen them since last month, so the shelf-life is over six months (if refrigerated). Pilsner Urquel is my favorite beer, but Prima from Victory is a close second. It’s perhaps more like honey, rather than a butter flavored aftertaste.

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