Diacetyl is a flavour-active molecule found in beer, which for most tasters appears as a buttery, popcorn or butterscotch-like flavour and is undesirable in most beer styles. Yeast produce the potential for diacetyl during fermentation as the cells grow, in the form of α-acetolactate (alpha-acetolactate) which is initially colourless and tasteless. When α-acetolactate is exposed to the beer environment over time, it will be broken down into diacetyl which can be reabsorbed by the yeast cells and broken down into 2,3 butanediol, a much less flavor-active molecule (Figure 1). The process of reabsorption will continue after the beer has reached its terminal gravity and in some cases this period may extend beyond the point of terminal gravity. This is typically known as a diacetyl rest.

Figure 1. Diacetyl metabolism in yeast. adapted from http://publications.lib.chalmers.se/records/fulltext/163053.pdf



Common problems regarding diacetyl



1. A beer has reached terminal gravity and there does not appear to be any diacetyl present according to sensory evaluation. The beer is cooled, filtered, carbonated, packaged, shipped and placed on a shelf for purchase. When the customer takes the beer home, the beer has diacetyl off-flavor. This is an example of a beer that has not finished fermenting or had other residual diacetyl issues as not all of the α-aceto has been turned into diacetyl and taken up by the yeast prior to cooling.

2. A brewer who is unknowingly insensitive to diacetyl smells and tastes a beer at F.G. and it does not appear to them to have diacetyl. The beer is cooled and further processed due to the brewers judgement, but customers complain about diacetyl upon purchase. This is not a fault of the brewer - everyone's palates are different, and diacetyl sensitivity has been linked to genetics. However, it is important to have a range of tasters during the critical stages of beer production, and there are ways to help insensitive diacetyl tasters to identify potential faults. The forced diacetyl test can help prevent the above problems from occurring.



How do I perform the test? How does it work?

In its simplest form all the test requires is that a sample of beer is gently heated to 60-70ºC inside a sealed container for 10+ minutes ( Figure 2).

Figure 2. steps in a forced diacetyl test



This can be achieved several ways:

1. A sealed flask on a heated stir plate or water bath

2. A mason jar in a bucket of hot water (this can be done at any brewery!)



Once heated for the appropriate time all you must do is open the container and smell. Some tasters prefer to cool the sample back down to room or beer temp as other flavors are also pronounced at hot temperatures.



If you smell butter: diacetyl uptake is not complete



If you do not smell butter: diacetyl uptake is complete



This tests works due to the heat being added which accelerates the breakdown of α-acetolactate into diacetyl. Do not boil, as this will boil off the diacetyl and lead to a false negative result!



How do I use the forced diacetyl test within my brewery?

The forced diacetyl test is commonly used as a `”go/no-go test” to determine if a beer has completed diacetyl reuptake and can thus have the cooling turned on and proceed toward crashing, transfer, filtration and packaging. If the cooling is turned on before diacetyl uptake is complete, the diacetyl will remain within the beer as the yeast cells will have flocculated and become metabolically dormant, less able to take up the remaining diacetyl.



This test is superior to simply smelling a beer for diacetyl for 2 reasons:



1. Heated diacetyl is much more volatile than when cooled, making those with low sensitivity to the molecule able to identify its presence more easily.

2. Flavourless α-acetolactate with the potential to turn into diacetyl during storage is also accounted for, ensuring the product will not develop yeast-derived diacetyl during storage.



Common Questions



Who should be performing this test?

Anyone can perform the test! However, it is best analyzed by someone who is more sensitive to diacetyl than others. We recommend multiple people assessing the heated sample to ensure that all diacetyl is removed and that no personal limitations or biases are skewing the data.

Can I microwave the sample to heat it up?

Microwaving the sample is not recommended as it tends to flash off the the diacetyl very quickly.

If I have diacetyl present, how long do I need to let it rest for?

This is heavily strain-dependent. As a general rule, the more diacetyl present, the longer it will take to be reabsorbed. Allowing the beer to “free rise” without any cooling will accelerate the process however this is best performed at the very end of fermentation to avoid flavour changes.. If diacetyl is present, we recommend raising the temperature slightly (1-2ºC for ales, possibly greater for lagers) and holding the beer until diacetyl is no longer detected.

Can I do anything else to reduce my diacetyl content of the beer?

Diacetyl is mainly produced during the beginning of fermentation while the cells are within their exponential growth phase. If this phase occurs at a lower temperature, then less will be produced. Healthier cells will produce less diacetyl along with properly pitched cells, proper aeration levels and generally anything that can be done to improve cell health. High finishing pH, high residual FAN (free amino nitrogen), and dry hopping have all been linked to elevated diacetyl levels in beer.

My beer reached terminal gravity several days ago and the diacetyl is still there!

This is usually an indication of poor yeast cell health. In order for the diacetyl to be absorbed there must be healthy cells in your beer. If the cells are unhealthy or have flocculated out of solution then the diacetyl will not be absorbed. Try rousing the yeast within your fermentor with CO2 and removing the cooling to try and reawaken the yeast.