The international bitterness unit (IBU) test was developed to measure the iso-α-acid concentration in finished beer and give the brewer a general sense of his beer’s sensory bitterness. This test method was not designed for nor did it take into account beers that are heavily dry hopped. Craft brewers performing the IBU test on dry-hopped beers are finding that the IBU test results do not correlate with the bitterness they taste in their beers. Many brewers are unaware that dry hopping can significantly alter the hop acid composition within their beer. High-performance liquid chromatography was used to accurately measure the hop acid composition of dry-hopped beers, and purified hop compounds were added to non-dry-hopped beers to simulate the hop acid composition of dry-hopped beers. The IBUs of the spiked beers were measured to see what effect non-iso-α-acid hop acids have on the IBU test. The results of this work will explain why and how dry hopping affects the IBU test and beer bitterness.
Keywords: Dry hopping, International bitterness unit (IBU), Bitterness