Beersmith 2.0 IBU calculation question

[beernutz]

I just noticed that if I adjust the boil time in Beersmith that also changes the IBU values assigned to each hop addition even though the addition has a specific time assigned to it.

For example if the boil time is set to 60 minutes and I have a Mosiac 12.8 AA addition for 60 minutes Beersmith shows 49 IBUs. However if I change to a 120 minute boil time, meaning I boil for 60 minutes then had hops for 60 minutes, that same 60 minute Mosaic addition shows 51.7 IBUs.

It seems to me that boiling time prior to the hop addition should not have any affect on the amount of IBUs added. What am I missing?

 

[Oginme]

I will assume that you are using the Tinseth model. The IBU calculation is responding to the specific gravity of the wort. When you move the boil time to 120 minutes, the program adds enough water to cover the added boil off time above your standard 60 minutes. This water addition coming from the mash means that the pre-boil gravity is lower for the 120 minute boil versus the 60 minutes boil.

 

[micraftbeer]

Interesting. It probably does have to do with how it's calculating volumes and boil-off rates. If I recall, it has both a % boil-off, or you can set it up as gallons/hour. If you have it set up for gallons/hour, and it's maintaining your final volume and OG constant, then I wouldn't think it would show a difference.

For example, if my boil-off rate is 1 gallon/hour, and my post-boil volume is 6 gallons, then at 60 minutes to go, I'd be at 7 gallons of wort. Whether my boil time was 60 minutes, 120 minutes, or 240 minutes. And the wort gravity at 60 minutes would be the same in all these scenarios.

 

[ScrewyBrewer]

I will assume that you are using the Tinseth model. The IBU calculation is responding to the specific gravity of the wort.

Agreed. The more sugars aka higher the specific gravity in the wort the less hop isomerization. I don’t think 2.7 IBUs difference is enough to worry about compared to losses related to hop age and storage conditions. Do a search on hop storage index (HSI) for the details.

 

[Silver_Is_Money]

The Tinseth (or any other) model is merely that, a math model intended to mimic and present an average reflection of measured reality. It is not in and of itself reality. And Tinseth himself stated that he never once tested pellet hops and he has absolutely no idea as to how they would actually perform. The error bars in hitting IBU's via the Tinsith (or any other) math model are huge. On rare occasions it is possible to be as much as 50-70% off. More typically you might be 25-35% off. Hitting IBU's on the nose via utilizing a math model is more luck than science. To improve accuracy somewhat you would need to use only newly calibrated as to AA's whole hops, and study the precise Tinseth procedure, and then follow it to a tee.

If you test the various implementations of the Tinseth model it is evident that many if not most of them yield slightly differing outcomes, and since there isn't a "correct" model here to begin with, it likely makes little difference.

 

[ScrewyBrewer]

The Bermuda Triangle of brewing beer. pH dissolved oxygen and IBU measurements. Plenty of theories about each. But consensus and hard evidence is scarce.

 

[Oginme]

The Tinseth (or any other) model is merely that, a math model intended to mimic and present an average reflection of measured reality. It is not in and of itself reality. And Tinseth himself stated that he never once tested pellet hops and he has absolutely no idea as to how they would actually perform. The error bars in hitting IBU's via the Tinsith (or any other) math model are huge. On rare occasions it is possible to be as much as 50-70% off. More typically you might be 25-35% off. Hitting IBU's on the nose via utilizing a math model is more luck than science. To improve accuracy somewhat you would need to use only newly calibrated as to AA's whole hops, and study the precise Tinseth procedure, and then follow it to a tee.

If you test the various implementations of the Tinseth model it is evident that many if not most of them yield slightly differing outcomes, and since there isn't a "correct" model here to begin with, it likely makes little difference.

^^^ This entirely! If you want to see how well the Tinseth model works, check out the Experimental Brewing podcasts and follow-up testing on "The IBU is a Lie". The interview they did with Glenn Tinseth is especially enlightening. He has always made the disclaimer that his IBU model is accurate for his process at the time he performed the testing and how he operated it.

The best thing to do is to self calibrate your system to the calculated IBU versus your perception of bitterness. Raise or lower your target based upon those results.

Edit: Links: https://www.experimentalbrew.com/podcast/episode-32-ibu-lie, https://www.experimentalbrew.com/podcast/episode-33-bitter-truth

 

[dmtaylor]

You guys are right. It's a volumetric effect. And, calculators are only good to about 1 significant digit, certainly not to the decimal places, so in reality you're not going to notice any difference in the end anyway.

 

[Silver_Is_Money]

To add to the fun, Tinseth is not the only 'math model' game in town for calculating IBU's. Others who have presented their own formulas include Mosher, Garetz, Noonan, Daniels, and Rager. All of them presume their math model to be the best. None of them calculate the same IBU's for any given mix of hops and boiling times. And vs. the real world none of them are going to be found to be correct sans by accident. All of them are capable of at least guessing what ballpark you are playing in. Perhaps the best thing you can do is pick one and stick with it. For your procedure, equipment, and hops, one may prove to be more accurate more often than the others. But to discover this you would have to get a bunch of your batches lab tested for IBU's. An expensive proposition.

 

[Silver_Is_Money]

Who knows who first fantasized that pellet hops have 10% more IBU's per ounce than whole hops, but Tinseth assuredly did not make any claims with regard to pellets. I seem to recall reading somewhere that Mosher thought pellets to yield perhaps 33% more IBU's than whole hops. And Daniels thought pellets to be perhaps 25% stronger than whole hops. and Noonan thought pellets could be anywhere from 0% to 50% stronger, with this being boil time dependent. No one really knows. If you get your beer tested (based upon the admittedly very scant data that I've seen) the odds appear to be perhaps somewhat better that your IBU's will be lower than calculated as opposed to higher than calculated, but both lower and higher can and do happen.

It appears that it has been at least 20 years or more since any of these formulas were proposed. Perhaps we are overdue for a new formula, particularly since in the past 20 years formerly shunned pellet hops are now pretty much the standard, and none of the existing formulas were built with pellets in mind, because they were seriously lousy back then. Having brewed with whole hops back in the 80's and 90's, I think pellet hops are lousy to this day (as to imparted flavor), but that's just me. Perhaps my taste buds are just getting old.