Troubleshooting - Diacetyl

Diacetyl is a chemical that is found in many different foods and drinks. It is part of a class of chemical compounds called vicinal diketones (VDKs) and has a buttery or butterscotch flavor. It is produced as a byproduct of fermentation under certain conditions. To learn more about diacetyl, check out the links at the bottom of this article. 

How is diacetyl formed? 

Diacetyl is mainly produced through the cell trying to make the amino acids Valine and Leucine. Anything that increases the need for the cells to produce those amino acids will drive Diacetyl production. The main yeast factors that correlate with higher diacetyl production are: 
1. Lower FAN/amino acid content in the wort resulting in more amino acid synthesis by the yeast.
2. More divisions/growth required by the yeast cells as a result of under-pitching. 

The yeast cell's capacity for diacetyl degradation is mainly determined by wort aeration. Diacetyl reabsorption is ATP-driven and diacetyl degradation is mainly NADH driven. Both ATP and NADH are increased with wort aeration.

Forced Diacetyl Test 

The forced diacetyl test is a great way to confirm that your beer does not have the potential to form diacetyl before you cold-crash, carbonate, and package it. 

Root causes of Diacetyl issues:
1. Fermentation cooled too soon, resulting in incomplete diacetyl reuptake.
2. Unhealthy yeast unable to take up diacetyl from the beer.
3. Incomplete yeast nutrition resulting in heavier valine synthesis and therefore more diacetyl production.
4. Beer containing flavourless diacetyl precursor alpha acetolactate held warm resulting in chemical conversion to diacetyl.
5. Contamination by lactic acid bacteria such as Pediococcus damnosus.
6. Refermentation resulting from dry hop amylase enzymes (hop creep) and incomplete diacetyl uptake by yeast. 

Questions we will ask to help troubleshoot a Diacetyl issue: 

1. What is the yeast generation? If you have it, what was the viability? 
2. What was the yeast pitch rate? 
3. What was the oxygenation rate/method? 
4. What was the temperature and timing of diacetyl rest? 
5. Is the beer dry hopped? How long is it dry hopped? Do you see additional fermentation during dry hop (hop creep)?
6. Was the forced diacetyl test performed?

Helpful Resources:

How To Do A Forced Diacetyl Test
Diacetyl Deep-Dive on YouTube:

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