Iso alpha acids (iso) are misunderstood little molecules. Some people think that all bitterness is iso but if you ever get to taste a solution of iso in water you’ll be very glad it’s not. Iso even at low concentrations exhibit a harsh bitterness. They remind me of the vomit I produced when I was 16 and drank a 3 litre bottle of Ruddles County on an empty stomach. The bitterness associated with high rates of copper/kettle or dry hops is much more gentle and fruity.
EBU and IBUs are the concentration of iso in milligrams per litre. Unless you are bittering your beer with pure iso, BUs are only an indication of the level of sensory bitterness. Boasting that your imperial IPA has 100 BUs is all well and good but that doesn’t mean it will taste ‘hoppy’. Making a beer just to have a very high level of anything isn’t big, clever or worthwhile. It’s like chefs trying to make a soup with the greatest concentration of salt possible. Beer is about flavour not impressive numbers and labels.
Iso are formed during boiling from alpha acids which come from hop resins. It is easy to distill and purify Iso so some brewers (the retarded ones) buy pure iso and add it to the beer as it is packaged. This saves a lot of money by preventing losses of iso during fermentation and filtration. There aren’t any beers that are worth worrying your liver with that are made in this way.
When I was at university I was immencely proud of my ability to memorise the structure of iso and their formation during the boil from alpha acids. I have forgotten all but one of these but I am a lot better at making beer now. I can live that trade off!