Bitterness Extraction

[Wez]

Some of you may have seen in brewdays that bacause of a boiler element failure mid brew I have had to transfer the part-bittered wort to my thermobox for safe keeping and will do the boil tomorrow.

My question is, there was 30 mins of "sort of" rolling boil with FWH (Challenger) that i'd calculated to 30IBU for a full 90 min boil so how the heck do I work out what hops to add tomorrow to get somewhere near the 30IBU's I wanted? I'm not sure how extraction works..i.e. is there a high percentage of extraction done in those first 30 minutes?

 

[Wez]

Thank you to Aleman who while in chat has figured out given the hops, AA and wort gravity that I have about 11IBU at the moment, so tomorrow I'll bitter with the remaining 19IBU to get my 30.

:thumb:

 

[markp]

I'd be grateful if you would share the method and calculations please :grin:

 

[ano]

Some bits on utilisation as a factor of time can be found here, with a calculator here. Think it might be US gallons. The site's by a chap called Glenn Tinseth, whom I believe to held in some esteem for his mathematical hop utilisation models.

Obviously can't comment on Aleman's method but he usually knows his stuff

 

[Aleman]

I basically just used Tinseths calculator . . . with some tweaks . . . and then a bit of a fudge factor

The trick really is to account for the fact that it wasn't a proper rolling boil, plus the fact that the hops were 'First Wort'. . . .both of which lead to a reduction in the amount of isomerisation that occurs.