First off, make sure you take photos so that we can help troubleshoot! There are some troubleshooting steps we can take to diagnose what is happening. It is possible that this is a real result. 2,3-pentanedione and/or acetoin, which are vicinal diketones (VDK's) similar to 2,3-butanedione (diacetyl) sometimes show up as a dark circle, especially with certain types of yeast. You won't necessarily detect any diacetyl via sensory because 2,3-pentanedione and acetoin are not as potent as diacetyl. However, before we can say that, I want to make sure everything else about the test is running smoothly.
Download the white paper below to see how you can use the VDK+ Test Strips to improve your beer quality program!
Follow this video to learn how to perform a VDK+ Test Strip reaction.
2,3-butanedione (diacetyl) and 2,3-pentanedione, colloquially known as vicinal diketones (VDKs), are metabolic by-products of yeast fermentation that impart buttery flavor characteristics to fermented beverages. Although desired in certain cases, VDKs are frequently denoted as an off-flavor in finished beer, cider, kombucha, and wine when present at concentrations between 100-4000 parts per billion (ppb).
Organoleptic perception of VDKs varies greatly between people. For example, trained sensory experts can detect VDKs down to 40-100 ppb in beer depending on the style, while a substantive fraction of the population cannot discern VDKs present even at levels exceeding 4000 ppb. This range in sensory perception has led to the development of several chemical assays specific for VDKs in beer.
Current methods for monitoring VDKs generally utilize either gas chromatography paired with a specialized detector (e.g. GC-MS or GC-ECD) or distillation of a beer sample followed by colorimetric reaction and detection via spectrophotometry. Methods utilizing gas chromatography require expensive instrumentation, while methods utilizing spectrophotometry are technical and require large amounts of time for a single measurement. Both general workflows require sufficient chemical training. This is why we developed our proprietary VDK+ Test Strips.