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Master Brewers Association of the Americas > BREWING RESOURCES > Ask the Brewmaster > Posts > Using VDK Levels to Determine when Fermentation is Complete
February 15
Using VDK Levels to Determine when Fermentation is Complete

​Question

I have a question about using VDK levels to determine when fermentation is complete and the beer is ready to be filtered. We talked about this briefly in the Brewing and Malting course and I was wondering how this is commonly done. I've found a method for testing VDK using a spectrophotometer in the ASBC methods cd. I was wondering if the common method is to test VDK levels and release the beer for filtration after the levels are below flavor threshold? Or is it common to also test for VDK precursors like alpha-Acetolactate to determine that VDK levels will not increase? Thank you for your time and any help on clearing this up.

Answer

The ASBC spec method is not all that reliable although may be good enough to give you some baseline measurements, you could also correlate this to a GC by sending out duplicate samples to an outside lab. It is difficult to test for acetolactate as it easily oxidizes into diacetyl. If diactyl is above 0.1 ppm most people will taste it. In most warm, ale making breweries the yeast will mop up the diacetyl by reducing it to 2,3 butanediol which has no noticeable flavor. In my old brewery we usually yielded about 0.04 ppm diacetyl or less and not even the most sensitive diacetyl taster (me) could pick it out. 
 
A quick easy method for checking a sample by taste is to warm your sample in a 150F water bath for 30 minutes. Then cool down and taste. This quickly converts precursor and brings out the diacetyl character that has potential to develop. I hope this helps answer your question but let me know if you have any others.

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