The role of diacetyl in beer

Written by Admin on June 12, 2016

By Moritz Kallmeyer
Chief Brewer of Drayman’s Craftbrewery, Silverton Pretoria, February 2003 (updated 22/10/2004)

Definition: Diacetyl is a powerful aromatic compound that imparts the flavour of butter or butterscotch to beer. It falls under class 6 of Morton Meilgaards beer flavour terminology together with soapy, fatty, rancid and oily. Subclass 0620 is diacetyl.


Diacetyl as by-product of fermentation is more characteristic of ales than lagers. Diacetyl is produced early in the fermentation, and then most of it is reabsorbed by the yeast and reduced to flavourless compounds later on. Yeast strains differ markedly in their diacetyl reduction ability. Some ales and a few lagers (such as the famous Pilsner Urquell) contain perceptible amounts of diacetyl, but as a rule modern brewers consider it as a fault. This is because certain bacterial infections and other errors in brewing technique will increase diacetyl levels resulting in unacceptable beer aroma and flavour profile. This parameter thus serves as a quality check. However, it is important to remember that diacetyl flavour is a natural by-product of yeast fermentation, and in some beer styles it is an optional or even required flavour component in low amounts.

Diacetyl (2,3 Butanedione) and 2,3 Pentandione

Diacetyl and the related compound pentanedione are produced from yeast metabolites that are secreted into beer – it is always produced during fermentation. Diacetyl and pentanedione together are called vicinal diketones (VDK) because they contain two ketone (oxo-) groups on adjacent (vicinal) carbon atoms. They occur as two significant by-products of fermentation because their oxo-hydroxy precursors can pass through the yeast cell membrane into the beer. The precursor of diacetyl is alpha-acetolactate and the precursor of 2,3pentanedione is alpha–ketobutyrate. These hydroxyl acid precursors are produced as intermediates in the biosynthesis of the amino acids valine (for diacetyl) and isoleucine (for pentanedione). Once in beer, the amino acids spontaneously undergo oxidative decarboxylation to yield the diketones which means that the decomposing of the precursors to diketones is a chemical reaction –no enzymes are involved.

The diketones then pass back into the yeast cell where they are reduced, (like other aldehydes and ketones) to the corresponding mono-acohols and di-alcohols. In the case of diacetyl these are acetoin and butanediol, which are much less intense in taste and aroma. Beer contains up to a thousand times more acetoin than diacetyl but it does not normally affect flavour. Some of the diacetyl precursor is chemically decarboxylated directly to acetoin without oxidation outside the yeast cell. Acetoin is however also produced this way enzymatically inside the yeast cell.

Pentanedione on its own has a sweet honey-perfume smell and diacetyl resembles butter or butterscotch. Of the two, diacetyl is more significant because it has a taste threshold 10 times lower than its partner does, and most yeast strains make more diacetyl than pentanedione. If the pentanedione fraction overpowers the diacetyl fraction I strongly suspect bacterial contamination. Pentanedione is very similar to diacetyl but only one-tenth as much is found in beer. For most people the flavour threshold for diacetyl is about 0.15mg/L and 0.90mg/L for pentandione.. Diacetyl is formed only when there is oxygen in the beer. It also means that it is, to some extent, inevitable, since the wort is usually strongly aerated at pitching.

Temperature, Time and Pressure:

One factor affecting VDK formation is temperature. The warmer the environment, the more VDK precursors will be expelled into the wort. In practise, VDK is formed during the initial aerobic stage of fermentation. During this stage, the yeast consumes all the oxygen in the wort, so there should be no further production unless air is re-introduced, like at transfers. Transfers thus create a new peak of diacetyl that has to be reduced by further maturation. However, all yeast’s can, to some extent, reduce VDK to flavourless diols. This is one of the key properties of yeast. Given time and the right conditions, most yeast strains can reduce VDK to below the flavour threshold level during the anaerobic (fermentative) phase of their activity. The reduction is very rapid but the breakdown is relatively slow, especially at low temperatures. For instance it can take well over 21 days at 1°C to break down the same amount of precursor decomposed in 16 days at 4°C, 8 days at 10°C and only 4 days at 18°C. There is thus a great advantage in decreasing the precursor level as much as possible before the temperature is lowered for conditioning. Another method for reducing the amount of precursor formed is by building up a controlled backpressure of about 30 kpa CO² in the fermenter. This reduces the amount of yeast growth so less pyruvic acid is converted to acetolactate in biosynthesis reactions.


The traditional German Kraüsening method involves adding 10% actively fermenting wort at the secondary fermentation stage. This addition of Kraüsen produces changes in beer composition, especially the removal of diacetyl and aldehydes.

Commercial enzyme:

Maturex L is used in the primary fermentation stage of beer. It prevents the formation of diacetyl by catalyzing the decarboxylation of alpha-acetolactate directly to acetoin. Thereby the diacetyl rest can be eliminated or at least greatly reduced. Maturex L is a purified alpha-acetolactate decarboxylase enzyme.

In the schematic below it is shown how diacetyl is formed from acetolactate in the fermenting beer outside the yeast cell. It is then removed from the beer by the yeast cell and enzymically reduced to acetoin and butanediol inside the yeast cell. Similar reactions involving alpha-ketobutyrate, the non-enzymatic formation of 2,3 pentanedione and its reduction by yeast to 2,3 pentanediol also occur.

The reason most commercial beers are essentially free from diacetyl is that fermentation is managed so as to discourage its formation and encourage its reduction. Higher than normal fermentation temperatures in the latter part of primary fermentation assist this reduction and However, not all lager breweries do this, they rely on lengthy conditioning or lagering. Pilsner Urquell, the granddaddy of all pale lager beers, has perceptible levels of diacetyl. Furthermore, many Belgian and British ales also have notes of diacetyl, where its sweetness and slickness on the tongue helps balance some of the bitter and harsh hop flavours.

Causes of diacetyl in beer:

  1. High pitching temperature >22º even if the chilling is set at normal fermenting temperature.
  2. High fermentation temperature; or runaway fermentations.
  3. Pitched too little yeast (at least 1% slurry).
  4. Pitched too much yeast (old, tired yeast cells, early flocculation).
  5. Oxygen exposure during primary fermentation, secondary fermentation, at transfers, or at packaging.
  6. Crash cooling of beer at the end of primary fermentation without including any diacetyl rest.
  7. Insufficient time allowed for warm conditioning for sufficient diacetyl reduction.
  8. Type of yeast strain used – some strains like Yorkshire Square fermentation strains (Old Speckled Hen) produce pronounced, but not unpleasant diacetyl levels in the beer.
  9. Too early yeast flocculation and settling in the primary ferment.
  10. Strains of Lactobacillus spoil beer by souring, producing turbidity and diacetyl. Diacetyl is generated by a different mechanism to that produced by brewer’s yeast. It most probably involves the condensation of “active acetaldehyde” and acetyl co-enzyme A. Pediococcus damnosus is especially common in breweries. It is particularly prevalent as a spoilage organism in beers fermented at low temperatures. Spoilage results mainly from the production of diacetyl. Other bacteria that produce diacetyl include Enterobacteriaceae and Obessumbacterium proteus.

Diacetyl Rest: (warm conditioning)

Maturation of beer flavour requires the presence of yeast as a catalyst. There are many methods of finishing that have the sole objective of prolonging the contact of beer with yeast after primary fermentation is completed. I want to emphasize that a diacetyl rest with most of the yeast lying at the bottom of the tank and not enough in suspension is of no use. Most lager breweries, especially those that use Weinhenstephan 308 or similar “diacetyl producing yeast’s” employ a long diacetyl rest, in order to minimize diacetyl in the finished beer.

Method 1
If a very cold primary fermentation was used it involves allowing the beer temperature to rise from the controlled primary fermentation temperature of about 10°C to 15-18°C when the primary fermentation is coming to an end. Normally, the time is determined by the attenuation of the beer. If, for example the wort starting gravity was 1050 and the expected terminal gravity is 1010, then the diacetyl rest would be commenced when the beer has attenuated to about SG 1023 when two-thirds of the total fermentable material in the wort has been consumed. The diacetyl rest normally lasts for 48-72 hours, until primary fermentation is over and secondary fermentation is under way. At this time the temperature is lowered when the more traditional method is followed, probably 1°C per day until the lagering temperature of 0-1°C is reached.

Method 2
If a warmer primary fermentation temperature was used for ale or lager the diacetyl rest involves either lowering the beer temperature 2 or 3°C at the end of primary fermentation or keeping it constant for up to 6 days. In lager yeast strains with low diacetyl production it is common practise nowadays to employ a short diacetyl rest followed by centrifuging to remove excess yeast and then crash cooling to 0°C. When brewing ales, that should have very low diacetyl levels especially German Ales like Alt and Kölsch, the implications are to not use highly flocculent yeast and to allow an extended primary fermentation, albeit at cooler temperatures until sufficiently low diacetyl levels are reached. Yeast that settles in the cone is still removed on a daily basis.


2,3 Pentanedione