The answers have appeared in posts before yours, read backwards and forwards.
Ok, the two replies have so far contradicted each other. So much for clarity. I'll try again - if I FWH and boil for 90min, is it calculated as a 90min bittering addition + 10% (but seeming lower due to 'integration') YES, or as a 20min bittering addition? NO
Mikedub admitted he wasn't really sure.
Florian is on the money.
Lets think about it logically for a moment.
(1) At a certain lower temp range of wort the bittering qualities of hops are released into solution (isomerised), it may be around 85oC. Below this temp and bitterness is not rereleased. Normally you would put hops in wort some time after hot break has occurred; this is when you start the clock for the count down. Hot break occurs some time after the wort has started boiling. There is likely to be a length of time between the wort reaching 85oC and when it gets up to boiling and then hot break occurring. For arguments sake let us say that this is 20 minutes (it will vary from brewery to brewer and this is just a pretend time for now).
(2) We know that hops in the wort for 15 mins give little IBU's, the same amount in for 30 mins contribute more IBU's and that the same amount of hops contribute even more IBU's if left in for 60 minutes. It takes time to extract IBU's, the longer the time the more IBU's you can extract. The longer the hops are in the boiling wort, the more IBU's they contribute.
(3) There are only so many IBU's that an amount of hops can physically contribute, then they have no more to give. Hops seem to stop contributing extra IBU's somewhere above 60 minutes of boil time. There is little difference in amount of IBU's in wort that is boiled for 60 minutes or 90 minutes. The different times that peolpe boil their wort for (60 vs 90 mins) is for other reasons.
To put it together (and add some): If you put hops in the kettle before the wort (FWH) and then drain your running's on top of them they are likely to get sparge temps of around 75oC or so for maybe up to an hour whilst you continue to sparge. This is not hot enough to extract IBU's but perhaps other compounds, flavours, aromas and change in chemical structure of the hops is occurring during that period they are subjected to these temps. These things all end up in the wort.
Then you turn the heat on to the kettle and it gets up to 85oC, the hops start contributing IBU's to the wort but it a takes a further 20 minutes for your wort to get to the boil and hot break to form. At this point you would normally add 60 min hop additions but your FWH hops were already in there and you could say that the IBU's have had a chance at 80 minutes (60 mins + 20 mins) worth of escaping into the wort. The FWH hops have been in for a longer period of time and have thus had a chance to contribute more IBU's than if they were in for 60 mins. The extra amount of IBU's is probably not a whole heap more because 60 minutes is close to where they stop contributing IBU's to the wort.
See this chart below. After 60 minutes the amount of IBU's contributed to wort really slows down. Yes they will contribute more IBU's over time but it is relatively less per unit of time.
Notes on the chart. I used Beersmith 2 to calculate the IBU's over time of Centennial hops (10% AA) in a Pilsner wort of 1.056 SG. I then charted it in Excel (it has serious limits to it's charting ability). Yes there are other factors that affect isomerisation, let's ignore them just for now.
As an example; the difference in IBU's between 20 mins and 60 mins is large but the difference between 60 mins and 100 mins is quite small.
So Beersmith says that if you FWH, the IBU's contributed by those hops is going to occur in the period of 85oC-100oC which normally you would not time during a boil and this amount is approximately +10% more bitterness (IBU's). I don't know the maths behind it but it seems logical to me that it would be an AMOUNT extra, I trust them to tell me HOW MUCH extra. Their maths may not be perfect but let's say that it is close enough.
