Almost every time I add hops to a recipe in beersmith I cringe when I see the hop stability index. Most of the hops I use have a 25-40% HSI. (meaning thats 25-40% alpha lost in six months). I cringe because the hops I buy are typically a season old. So how would it be remotely possible that my beers' IBUs are anything close to what I calculate in beersmith?
Note that HSI is an indication of AA degradation at 68F in a totally unsealed environment (e.g. sitting in an unsealed paper bag on your kitchen counter).
Every decrease in temperature of 27F halves the speed at which hops age. Vacuum sealing also almost halves that speed.
A hop with a 40% HSI in a vaccuum-sealed package in the freezer would lose:
4.4% of its AA after 6 months (e.g. a 10% AA would be down to 9.56%)
16.5% of its AA after 2 years (e.g. a 10% AA would be down to 8.35%)
If you know the harvest time and storage conditions, you can account for that in your brewing (
http://brewerslair.com/index.php?p=brewhouse&d=calculators&id=cal16&u=eng has an aging calculator).
This is usually far from the biggest source of inaccuracy in calculating IBUs, though; if you know it, it's worth controlling, but even guys like Palmer say they use hops they've had a ziploc in the freezer for over a year without correcting and they make darned good beer.
Basically, use your estimates as a guideline: use the same formula every time, and use it the same way every time. Even if it doesn't give you a really exact IBU number, it will give you a standard baseline that you're familiar with to work from.
Sidebar on where IBU estimates can go awry for those interested:
For one thing, the algorithms themselves are pretty rough estimates; you can count on them to probably get you within 30% for a pretty standard full-boil brew. If the software says a recipe is 50 IBU, that means it's probably in the 35-65 range; certainly don't expect it to be spot-on-the-nose.
For another, AA% in a bale of hops is usually measured by pulling a single plug. Hops obviously aren't uniform, so if that hits a particularly high or low AA% part of the bale the measurement's going to be off. That's counterbalanced by the fact that growers know the season's typical yield and the variety's range, so it won't be outlandishly different, but the original values aren't always correct to the tenth of a percent (as they're labelled).
For another, isomerized alpha acids are not the only bittering compound that contributes to IBUs (if your brewing book defines an IBU as " one part per million of isomerized alpha acids" or similar, it's wrong). As one example, oxidized beta acids play a significant role in bittering. Cascades have about a 1:1 alpha:beta ratio. Columbus is in the 3.5:1 range. The formulas used by brewing software _completely ignore_ beta acids--you don't even enter that ratio. So how much they're off by is going to vary based on what kinds of hops you use and any aspect of their chemistry other than AA% that affects IBUs.
For another, the amount and concentration of break material you have plays a factor in overall IBU utilization. That means whether you brew extract vs. all grain, use wheat or rye vs. barley vs simple sugars, etc all play a role in your utilization. So does the geometry of your kettle, your fermenter, and even what yeast you use. All of those are ignored by the software.
Many of those are pretty small effects, but they add to the overall uncertainty of the final calculation.
