I recently did a few days' worth of reading on this, trying to come up with a logical way to at least estimate whirlpool IBUs using the data from that paper as well as others. I ran into some walls and ended up giving up. They were:
1) Lack of real brewery data. The Malowicki paper used a "model wort" (which was just an acetate buffer at pH 5.2) and alpha acid extracts. They did a nice job of looking at alpha acid isomerization kinetics
independent of other wort compounds and hop compounds. That is, they were investigating the nature of isomerization kinetics, not trying to come up with a way to calculate IBUs in a real brewhouse. The thing is, other compounds present in wort and hops have been shown in other studies to impact isomerization considerably. All we can really say from the Malowicki paper is yes, isomerization happens at sub-boiling temperatures and the rate drops off steeply as temperature decreases. Without measurements from real worts, I wouldn't trust any IBU estimates derived from these equations alone.
2) Brewhouse variation. Like
drummstikk said, you would have to factor in the rate of temperature drop during the whirlpool, which will vary from person to person and brew day to brew day. I actually don't think this is as much of a problem as (1) because once you get below say 180°F the isomerization rate is so slow that for our purposes you can call it zero. But still, everything from the volume of the wort to the weather outside is going to change the rate of temperature change significantly. I think this is where the whirlpool of a pro brewer is going to be very different from a homebrewer. They are likely going to get a lot more whirlpool IBUs (e.g. Jamil's Evil Twin) than a homebrewer because their wort stays hot for a much longer period of time. I bet a pro brewer could whirlpool for two hours and the temp would still be above 200°F.
So... I thought about it a lot, and eventually said "f*ck it, if my beer is too bitter I'll just add less hops next time." Not very scientific. But if anyone else is more dedicated than I am and wants to give it a shot, please do
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