Thu May 28, 2009 4:19 am
IBUs are determined by extracting the bittering principals into gasoline (spectrographic grade - it's even more than $3 a gallon) and then measuring the absorbtion of UV light at 275 nm in the extract. Clearly this requires equipment not found at your local HBS but some homebrewers have jobs or relationships with larger breweries that give them access to this equipment. If you are fortunate enough to be one of these you will quickly find that arguments about whose formula to use for calculating IBUs are a waste of time. The main issue is that despite what it says on the package you have little idea about what the actual alpha acid content of your hops are becuase it may have been a year (or more) since the hops were picked, processed and stored. Also home brewers, at the scale we brew at, have difficulty with repeatability in process. Obviously things like kettle pH and length of boil effect hops utilization but consider that things like a healthier crop of yeast drag more isohumulone to the bottom than would be the case in a beer with a lower cell count are also part of the story.
If you can get your hands on a set of beers for which the bitterness is known you can almost calibrate yourself. Time was when I think I could estimate to 5 IBU or better just by tasting.
