Determining the bitterness units in beer is often done by calculations based on raw materials and boil time. After all not everyone has easy access to a well equipped brewing lab.
But how accurate are the calculations, and which method of calculation should you use? Several different methods exist, each giving different results.
As it's something I've pondered myself I was interest to see that someone has carried out research into this question:
Multiple small batches (20–80 L) of beer were brewed, varying different factors each time, including boil time, starting gravity, hop variety, hop addition (time and amount), hop type (pellet versus whole leaf), and yeast variety. The α-acid concentrations of hops used in the study were measured using HPLC, and the IBUs were calculated using the three formulas based on the measured concentrations. The BUs of the finished beers were determined by isooctane extraction followed by spectrophotometric absorbance at 275 nm. The iso-α-acid content of the finished beers was also measured using HPLC.
Three methods of calculating Bitterness Units were used and:
Interestingly, the measured BUs for most of the beers analyzed matched the Garetz calculation of IBUs most closely.
I still prefer to have hard data rather than estimates but sometimes you have to make do. The rest of the abstract can be read here.