How To: Diacetyl Test

What is a Diacetyl Test

Compounds known as vicinal diketones, or VDKs, are aromatic compounds that are byproducts of fermentation. The most well-known VDK is diacetyl, which presents a characteristic butter, buttered popcorn, or butterscotch flavor and aroma.

Yeast will usually “clean up” these byproducts by further metabolizing them into other compounds when given enough contact time with the beer. One way to speed this cleanup along is to raise the temperature at the end of fermentation and perform what is called a “diacetyl rest”.

But how does the brewer know when the yeast is finished cleaning up diacetyl and other VDK byproducts?

One way is to perform a VDK or diacetyl test.

How to Perform a diacetyl test

To perform a forced fermentation test, here are some of the equipment you’ll need:

  • Two jars with sealing lids
  • Hot water bath (65 °C/150 °F) (or microwave)

Diacetyl Test Procedure

  1. Take a beer sample, approximately 100 mL/3 oz, and put it in one of the jars.
  2. Seal this jar
    • This will be your “Heated Sample”
  3. Take a beer sample, approximately 100 mL/3 oz, and put it in the other jar
  4. Seal this jar
    • This will be your “Unheated Sample”
  5. Place the “Heated Sample” jar in a hot water bath for 15 minutes.
  6. Cool the “Heated Sample” to room temperature
  7. Perform a sensory analysis on each sample, starting with the “Heated Sample”
    • If the “Heated Sample” has butter and/or butterscotch aromas, and the “Unheated Sample” does not, continue with a diacetyl rest. Retest the beer in a few days
    • If both the “Heated Sample” and “Unheated Sample” do not have butter and/or butterscotch aromas, the diacetyl rest can be considered complete and you can proceed to bottling or kegging.

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