IBU Reduction during fermentation

[spears104]

I recently read Mitch Steele's IPA book. On pg 203 he mentions that all of the utilization calculations performed are for wort and that there can be as much as a 33% reduction in IBU's during fermentation. I had never seen this before in any other book that I read about doing the utilization calculations. Do you guys take this into account when reporting your estimated IBU's? Or take it into consideration when you are formulating a recipe? Or is it even true?

[mr x]

First I have heard of it.....

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[berley]

Mitch seems to have done a lot of research, interviews, etc. for this book, so I'm sure he has at least an idea of what he's talking about...

But I had never heard that before, either, and I don't take it into account for my recipes, and I seriously doubt the high majority of brewers (professional and otherwise) do either. IBU calculations are pretty vague in the long run anyway, so go with what works for you!

[BBrianBoogie]

First I have heard of it.....

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Hmm, me too. You'll definitely lose some aroma from the hops, but the actual bittering compounds I'm not so sure on.

[pet lion]

From reading that book I get the impression that the interaction between yeast and hop compounds is rather complex. Also, because of this it seems a lot is still to be learned about these types of interactions as well as others that affect the end hop experience.

[wortly]

This is actually a little known and emerging phenomenon, but when the yeast (especially lager yeast) run out of the more "consumable" sugars, they can actually cleave some of the glycosidic bonds associated with different hop compounds. This is one of the reasons why it is virtually impossible to nail down an accurate IBU number for a lager. Thom Shellhammer did a bunch of work on this a few years ago.

[Graham.C]

I thought the only way to get a real estimate was to sample and test. I was under the impression the calculations don't actually correlate with isomerized alpha acids. Then again from what I know, perceived bitterness is probably more then iso-alpha acid. I'm no chemist though, so take my ramblings as such.

[chalmers]

Wortly, I'm looking forward to hearing about more NMR and/or FTIR results for acids/IBUs.

[NASH]

Yeast and trub both have considerably higher concentrations of iso-alpha acids than what is found in the wort or beer from which it was harvested. Acids are adsorbed by yeast. Products like Fermcap S help with higher BU efficiency rates during the boil and fermentation mostly by way of diminishing adsorption, and it really does work. The numbers we calculate as brewers (Pros at least) are meant to represent the finished product, not what we are dumping in the kettle.

Professional brewers that don't have the means for IBU testing generally estimate by taste, adulterating software and calculations to more accurately represent what they taste in the finished product. Brewer X and Brewer Y may use the same software, the same recipe et al but the resulting beers could be dramatically different in actual BUs. Different systems, brewing style, heat sources etc can have a HUGE effect on final BU count. I recommend everyone taste your own beers against known samples that have been tested, Central City tests all their beers along with most all the larger craft breweries. Grab a red Racer IPA (assuming they list the BUs somewhere), take a gravity sample, this will give you the OG as per listed ABV. Brew an IPA to match the numbers then do a side by side taste comparison and you'll get a pretty good idea of how close (or not) your BU calculations are. Of course it's important that the OG/FG are bang-on so sugars don't interfere with your perception. Water salts can have an effect as well but that's a whole other ballgame. None of the local breweries here actually test theirs to the best of my knowledge.