Diacetyl has long been seen as a fault in beers and is still considered as such for Lager type beers.  On the other hand, in certain grape varieties such as Chardonnay, diacetyl is an intrinsic characteristic of this type of wine.  What is diacetyl really? Where does it come from?  We will answer theses questions in this short article and give you a little bit of insight on its metabolism.

Diacetyl is a by-product of yeast fermentation

Diacetyl, as its name implies, consists of a molecule composed of 2 acetyl groups with the chemical formula: CH3-CO-CO-CH3. It can also be referred to as 2,3-butanedione. Diacetyl is produced during the course of yeast fermentation or its presence can also be the result of a Lactobacillus or Pediococcus infection. The molecule α-acetolactate, which comes from the catabolism of glucose into pyruvate, is the direct precursor of diacetyl. Α-acetolactate is a small compound that is involved in the production of the amino acid valine. However, excess α-acetolactate can be exported out of the yeast cell where it is oxidized into the compound diacetyl which is responsible for the buttery and butterscotch aroma in beer. Being a small molecule, diacetyl can re-enter yeast cells and undergo a transformation into acetoin and 2,3-butanedione, 2 compounds with minimal aroma.

Diacetyl metabolism

Diacetyl rest. Helpful to decrease the levels of diacetyl

From the diagram above, you see that re-absorption of diacetyl by yeast cells is an efficient way to decrease the amount of diacetyl in beer. This process occurs as long as yeasts are viable and in good health. This is the reason why many brewers will include a “diacetyl rest” during Lager production. In practice, brewers will increase temperature slightly towards the end of fermentation in order to drive α-acetolactate oxidation into diacetyl which will then be reabsorbed by yeasts and degraded into acetoin and 2,3-butanedione. Buttery aroma may develop after canning or bottling especially if fermentation is not totally complete. Some individuals are very sensitive to diacetyl perception while others have a much higher threshold for detection. Ideally, diacetyl levels in Lagers should be lower than 0.1 mg/L.

To conclude…

OENOSCIENCE can now assist you with sensory and analytical diacetyl testing. By using gas chromatography coupled with a mass spectrometry detector, diacetyl levels as low as 0.025 mg/L can be accurately measured. As your partner in risk management, the laboratory can assess the risk of diacetyl development before and after packaging.  We can also evaluate α-acetolactate and diacetyl together as total diacetyl.  Contact us at 514-564-2050 for more info.

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