Hopefully you are familiar with IanH's spreadsheet. In the Hops section, adding in hops choices puts a value in a section labelled %AA - with low values. e.g. Kent G gives 4.9 or Amarillo gives 8.9.
It doesn't seem this is the ABV reading so is it the AA change in points caused by adding the Hops?
I don't use Ian's spreadsheet, but have looked at it.
Calculating the bitterness of a beer is problematic, often far from an exact science.
Bitterness is usually defined in IBU's (International Bittering Units) or mg/L (milligrams/L) of isomerised Alpha Acids in the finished beer. That's a fairly modern definition but if we work with it we will all be on the same page.
Where it starts to get complicated is most people are calculating the end of boil bitterness. Forgetting that during the ferment about 1/3 of the dissolved Iso-Alpha gets lost, it sticks to the fermenter and the yeast, gets blown off in foam... and that 1/3rd can vary by +/-15% (roughly) depending on pitch rate, temperature, dry hops...
We might have calculated that we will have 45IBU (mg/L) but really we are calculating that we will have 45IBU +/-15% or 38.25 to 51.75IBU (probably).
To calculate the amount of Iso-Alpha we need to know how much Alpha Acid (pretty much insoluble in wort) we are adding. Hops all have different amounts of AA, this can vary from ~2% to nearly 20% by weight. Clearly the same amount of a 5% AA hop will give you less AA than the same weight of 15% AA.
Traditional hops like Goldings tend to have lower bitterness (AA) than some of the more modern cultivars, but they tend to have more other flavours (taste and aroma components).
The mg of Alpha Acid available can be calculated by mass*AA, if we used 25g of EKG with 4.9%AA. we would be adding
25*(4.9/100) = 1.225g or 1,225mg of AA.
The same addition of say Amarillo at 8.9% gives 2,225mg
Naturally this will vary from year to year, as it will with the age of the hops, as they get older they loose AA, depending on how they are stored up to ~50%/year in Cascade stored at 20oC (much less if air and light are excluded and its stored cold, lower loss rate in pellets than flowers).
Only 20-40% of the AA added to the boil will get isomerised and survive to get sent to the fermenter, this amount is referred to as Utilisation.
Utilisation is the product of a bunch of factors mainly, Length of the boil, Vigor of the boil, pH, Wort gravity, Mass of AA added, amount of protein in the wort...
Put it all together and we get a simple equation Predicted IBU's = (mg of AA added * Utilisation)/L
Remembering of course, that its not really an equation, more an educated guess. Big breweries use experience to tell what sort of utilisation they will get with well known ingredients on the same equipment with lots of practice. They still measure IBU's and blend beers or adjust with prepared pre-isomerised AA, not something we can do at home.
Worth noting that there is a limit on the solubility of Iso-AA its about 103 IBU at 20oC (less colder), not more than 90 IBU in beer that has-been chilled, no-matter how much hops you add, how long you boil...
When you buy hops it should come with the AA% value, you should enter this value rather than rely on one that is only a typical value. Never forget that you aren't really getting an exact answer, more of a guidance value.
I call it a Goldilocks answer, aim for just right.
Sorry to rabbit on, sometimes a simple answer to a question can be very misleading, especially when its a simple answer to a very complex question.
Worth reading: -
The HBD Palexperiment Results Lab Analysis, Part I
Mark
