IBU Reduction - Yeast Strain Differences
Wait, yeast drops IBU?!
This might be a bit of a surprise, but yeast is capable of reducing IBU in beer wort. This means the final IBU of your beer is often lower than the original IBU of the wort. It is theorized this is due to the yeast cells interacting with the bitter acids during fermentation and potentially pulling them out of solution when the yeast flocculates. This may also explain why yeast slurry can taste so bitter (for those who dare to taste it).
While we were not the first, we did observe this phenomenon in our 2019 yeast comparison study. In a wort with starting IBU of 35, most yeasts reduced measurable IBUs.
In this study most yeasts dropped measurable IBU by about 20-25%. We observed that IBU reduction is higher in British and American strains and lower in some Lager and Saison strains. This may explain why some British or American strains produce a "softer" or more refined hop profile.
We have included graphs below with the strains we tested. Note that strains released since 2019 will not be present in this data set.
Tracking IBU Through the Brewing Process by Justus (requires MBAA membership)