Quick Promash question

[Twofox]

I noticed that in promash, when you add hops at 15 mins left in the boil, it adds them in to the IBU calc. I understood it that hops added at that point don't add any bitterness? I saw someone show a recipe that appeared to just put the 15 min hops in as 0 mins due to the difference in IBU.

So the questinon is: Do I basically change the 15 min addition to 0 so it shows the correct IBU value? (Or am I smokin crack?)

 

[Sreidy12]

Hey twofox,
What happens when you add hops to the boil, alpha acids are released which are the bittering agents. The longer the hops are boiled, the more AA's are converted. Bittering hops are usually hops with high AA ratings, so when they're boiled for, say, 60 minutes more alpha acids are released which adds the most bitterness. Adding hops at 15 minutes will still convert some AA's, just not as many. The formula for determining IBU's is as follows:
1)Calculate Alpha Acid Units (AAU): This is done by multiplying the weight of hops (oz) by the Alpha Acid (AA) content.
2)Determine the AA's converted: 60 minutes=approx. 30%, 30 minutes=aprox 21%, 5 minutes=approx 10%. You can estimate in between those figures.
3)Multiply AAU's by converted AA's
4)Take that number and divide by the batch size (gallons)
5)Divide that number by .01335

TADA!!! IBU's have been calculated. You probably didn't care to know that, but its food for the brewers mind!
Cheers

 

[Twofox]

Heh, actually that explains what Promash is doing so that helps!

I am also looking at previous batches and comparing the numbers to this https://cdn.homebrewtalk.com/gallery/data/1/ibuguchart.jpg Where one batch should be slightly hoppy, it was more malty. The rest of the batches don't match the chart either... The one where I took the 15min add to 0 seemed to put it right on.