Ive been looking at bitterness for some time; frankly its the most contentious subject in brewing.
All the research $s come from big brewers and they arent all that interested in what happens after 30-35 IBUs, further they are making the same beer over and over with what amounts to the same ingredients, in ever increasing numbers they are using hop extracts rather than raw hops. In any case they have labs that measure the results, then they can either blend beers or add extracts to nail down the exact number they are looking for. In consequence there is a lot less known about what happens at higher IBUs.
Its really us home and craft brewers that really are interested in the subject, first as with any problem solving exercise we have to pin down what we know, before tackling the great un-knowns.
Here is an outline of what I am using as working data; I know there will be some who disagree of put different emphasis on other factors.
Lupin Glands, are special glands (containing nearly all the hop constituents we are interested in), the majority of these are located at the base of the Bracts (the small leaf like petals that make up the hop cone).
They are reasonably easily detached, if you are ever handling hop flowers this is the sticky greenish yellow powder that gets all over your hands.
This is also why we get better utilisation from pellets than from flowers, the milling of the hop cones to powder exposed all the lupin glands, in the case of flowers some of the lupin glands are covered in a mat of wet bracts and simply dont get the chance to be involved in the reactions that lead to bitterness.
Hop Resins, well there are a bunch of those, the two we look at most are alpha and beta acids, alpha acid (AA) is the prime bittering substance in beer.
The roll of the other constituents shouldnt be ignored but if we dont we could be here for a fair fraction of forever.
There are three chemicals called AA they are all relatively insoluble; during the boil they get rearranged into a soluble form, this rearranging process is called Isomerisation.
Utilisation, is simply how much of the insoluble AA is converted into soluble Iso-Alpha and is found in the finished beer expressed as a percentage
IBUs are by definition the milligrams of Iso-Alpha / Litre of finished beer, (its worth noting that 1mg/L is the same as 1 ppm (part per million)).
Just to complicate matters Iso-Alpha sticks to everything, yeast cells, trub, the fermenter walls and particularly some filters, between wort and finished beer something like up to 30% of the Iso-Alpha can be stripped out.
Yes that counts as part of utilisation, yet all the equations only relate to wort bitterness with no mention being made of fermentation losses, except in a vague application of Utilisation as some sort of fuzzy fudge factor, without ever pinning down what it is or its components (yes this is the bit that has me banging my head on the wall).
Degradation, Ok we turn insoluble Alpha Acid into soluble Iso-Alpha, just for fun Iso-Alpha degrades in the kettle into what are called Trans Iso-Alpha breakdown products.
So to a kettle we add a known amount of AA (usually expressed in ppm) during the boil it converts to Iso-Alpha, from there it breaks down. This process is pretty thoroughly understood. The rate at which it undergoes both transitions means there will be a time when there is the highest amount of AA in the wort; the main governing factor is wort temperature, in general the peak AA concentration will be 120-140 minutes into the boil at normal wort boiling temperatures
Other factors
Temperature is the main governing factor (should be of particular interest to those talking about cube hoping) Tinsith refers to boil vigour; I suspect that this can be exchanged for wort temperature.
Availability, anything that prevents the AA from getting involved in the reactions that we want will reduce the resulting IBUs, this includes using flowers, hop socks or other devices that confine the hops.
Solubility Limit, there is a limit to how much Iso-Alpha will stay in solution, (in the same way as adding sugar to a cup of coffee, 1-6 teaspoons full make it sweeter and sweeter, from then on the sugar lies on the bottom and the coffee stays at the same sweetness). That limit is about 90 ppm (90 IBUs). That may well vary in hot wort, but when the wort cools (same for the cuppa above) ISO-Alpha will come out of solution.
Note
1/ that 90 ppm is as measured by HPLC, older methods (UV/VIS) report up to 110 IBUs this is because other hop constituents are being recorder.
2/ anyone that tell you they are making beer over 100 IBUs is having either you or themselves on. But remember all those other hop constituents? They do play a role in what the beer tastes like; I certainly cant define what will happen if you add massive amounts of hops early in the boil, except to say that the bitterness wont get over 90 IBUs so it might be worth looking at when you use your hops to get the most effect, ideas like early bittering additions with late Hop Burst, might be a better more economic of getting IBUs and other hop characters.
3/ Wort pH, that solubility limit is at a wort pH of 5.2, the solubility limit goes down either side of that point.
I have been looking at the work by Malowicki and Shellhammer this is a really fascinating piece of research
View attachment Isomerisation.pdf
I have also put the two equations (11 and 12)
View attachment IBU__s.xls into an excel spreadsheet if you want to have a ply, Tab 2 (equation 12) should put to rest the idea that a quick boil will get rid of the unwanted bitterness in a kit, you can plug in the Iso-Alpha concentration for a kit at the dilution you are going to boil it in and see what really should happen to the IBSs.
For those interested in the question of what happen in a no-chill cube when you add hops to the cube, by combining the changing the wort temperature as a function of time, someone with better maths skills than me should be able to write a simultaneous equation that relates all the variables. We know from TBs results that bitterness does go up, the equation above should allow for reasonably good prediction of how much.
But back to the original question, I dont know how much AA carries over from the kettle to the cube and whilst in the cube continues to be converted into Iso-Alpha, if you have whirl pooled well and arent carrying too much trub over with the wort, my guess considering the relative insolubility of AA would be Not Much; but thats a guess.
Dam but I do love brewing
MHB
