Question re Hop-stand /Whirlpool

[G-Town Brewer]

When formulating a recipe using brewing software, I can calculate the IBUs provided by the hops added at the hop-stand stage. As an example, a 20 min hop-stand at ~79 degrees using hops of 11.6% will add 12.5 IBUs to my brew. My questions is this; do I also need to consider the extra IBUs from hops which have been added earlier in the boil when doing a hop-stand. So if I added 20g of the same 11.6% at 5 mins giving me 9.9 IBUs, if these are added loose to the boil and cannot be taken out will they continue to add IBUs during the hop-stand and if so how do I calculate the net IBUs for hops added during the boil and that also present in the hop-stand. Maybe I'm over complicating things here, but hopefully someone has the answer.

 

[Agentgonzo]

Yes, you do need to consider these. Your 5-minute additions will yield the 9.9IBUs during the 5-minute boil, but then they will continue contributing to the IBUs throughout the hopstand (though because some of the IBUs have come out of the hops during the 5-minute boil, they won't contribute as much IBUs as the fresh hopstand hops will add).
There are calculators on the internet and built into tools like brewfather and beersmith that should do all of this for you and just give you the final IBU. But I wouldn't worry too much about being accurate, as a lot of factors contribute to IBUs, pH, age of hops, how they were stored, your hops will likely deviate from the IBU on the packet anyway as the stated IBU is taken from a sample from the grower and the your specific hop cones will have come from a different plant and may vary. The IBU calculations are an estimate (and lab analysis shows it is pretty inaccurate), so use IBU calculations as a rough guidestick, rather than expecting an accurate result from it.

 

[Hazelwood Brewery]

If you were talking about obscene quantities of hops you might want to think about it but you’re adding small quantities that will make no discernible difference.

Those hops boiled for 5 minutes will already have lost some of their alpha acids. What’s left (90%?) will be available to extract but at 80C the IBUs will be a quarter of the IBUs at 100C. A quick back-of-an-envelope calculation suggests you might add 4IBUs from those late addition hops but I believe 5IBUs is the threshold for most people to tell any difference.

 

[The Baron]

Yes as said above all hops that are left in will add extra IBU's and need calculating in to the equation especially if larger amounts. I always put my additions in bags etc if you are not sure so that you can take them out

 

[G-Town Brewer]

Thanks all for the replies. That all makes perfect sense. I've just proved this to myself (I don't know why I didn't do this before!!) but in Brewfather I removed my whirlpool hop addition and it recalculated my 5 min addition from 9.9 IBU to 6.5 IBU, so it does takes this account and recalculate accordingly. Thanks for clarifying though!