Dry hopping does weirder things to beer than we thought. While conventional logic—and all existing software models for calculating theoretical IBUs in beer—say that IBUs can only be generated on the hot side of brewing (since alpha acids can only be isomerized by heat), brewers have long suspected that dry hopping can, indeed, make an impact on the perception of bitterness.
Last year, Stan Hieronymus wrote in the August-September 2017 issue of Craft Beer & Brewing Magazine® about evolving research into IBUs and the strange way that extreme dry-hopping regimens can actually reduce iso-alpha acids in beers, ostensibly by causing more of those acids to precipitate out with the dry hops material.
In 2017, Jason Perkins of Allagash Brewing presented at the Craft Brewers Conference on a study they conducted with Oregon State University to test the impact of dry hopping on beer attenuation and found that by adding dry hops to a fully attenuated beer (their sample was Coors Banquet), they could cause significant additional attenuation in the beer. Over 40 days, those dry hops were able to drop the finished Coors Banquet from about 1.014 SG to 1.007, taking the beer from 4.9 percent ABV to 6.2 percent by creating enzymatic activity that broke down nonfermentable dextrins in the beer.