Originally Posted by bobbytuck
Any idea on how to calculate the IBUs from the HopBack? Is it essentially the same as flame-out hops -- but decreased slightly because I'm using leaf instead of pellet hops?
I think the calculation is essentially 0 IBUs from the hopback. You are not boiling the extracted oils from the hops, so you are not isomerizing the alpha acids the way you would in the kettle. You are definitely getting some hop aroma and flavor, but none of that counts towards IBUs.
I personally find this to be a confusing argument as hop flavor is bitter. I don't understand the difference between bitterness in an IBU and bitterness in the hop flavor. Nevertheless, the IBU equations generally work out that if boil time = 0, then IBU = 0. It generally isn't as clear cut as multiplying by the boil time, but the effect is the same.