Never mind IBUs. How many AUs were in that last IPA you brewed?
Almost 30 years ago, Gail Nickerson at Oregon State University and Earl Van Engel of Blitz-Weinhard Brewing proposed establishing an Aroma Unit comparable to the International Bitterness Unit. They identified 22 hop compounds that would be used to calculate the Aroma Unit and divided them into three broad categories: oxidation products, floral-estery compounds, and citrus-piney compounds.
The concept was seen as controversial and never found broad acceptance. Nonetheless, it gave brewers a new way to think about hop aroma at the outset of the 21st century, just as interest was expanding in the broader range of flavors and aromas that hops can produce. As the authors wrote, “Hop-oil analyses that emphasize varietal identification do not provide results that the brewer can use to control hop aroma because the major hop-oil constituents are not present in beer.”