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My understanding is that wort gravity does not affect hop utilization. Instead, a larger boil volume will increase hop utilization, while a higher amount of break material will lower hop utilization. This would mean that for all grain batches, high wort gravity is correlated to low hop utilization, but for extract batches it would be irrelevant.

Of course, this contradicts a number of brewing resources, such as Palmer's How to Brew and multiple articles in BYO. I've read that Palmer has since changed his mind (but I have no good reference for this). I've also read that BYO and Basic Brewing Radio did an experiment with extract that determined boil size, not wort gravity, was the contributing factor to hop utilization (again, can't find the reference).

So my question is this: Does wort gravity affect hop utilization? In particular, I'm looking for a solid reference (maybe even with a solid experiment) rather than general brewing knowledge or forum posts.

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The podcast "High gravity brewing" with John Palmer discusses this at the 18 minute mark.

To paraphrase, John says that hop utilization does decrease with higher wort gravity, but not because of solubility as originally thought (the difference in solubility between 1.040 and 1.080 is negligible), but mostly because there are more solids in the wort that the iso-alpha acids stick to, keeping them out of the wort and so reducing bitterness.

The original tables of hop utilization were made on an empirical basis, and they are still valid. What has changed is the cause of the decrease in bitterness is now better understood.

There's also a podcast by John P. and Jamil Z. and discussion of the utilization issue here.

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Does the quantity of solids increase with gravity only for all-grain worts? If so, then the answer to the original question must be that wort gravity does not affect hop utilizarion for extract beers. –  Tobias Patton Jul 23 '12 at 0:19
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All worts, whether extract, all grain, cereals etc.. have non-dissolved solids, such as proteins and polyphenols - it's part of their composition. So hop utilization does decrease, even with extract worts. –  mdma Jul 23 '12 at 9:19
    
Thanks for this! Straight from Palmer was what I was hoping for. –  KISS Brew Jul 23 '12 at 12:46
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You're welcome. One other factor influencing utilization is how the hops are placed in the kettle, i.e. free boiling vs using hop bags. I recently built a large 10" stainless hop strainer to avoid using hop bags. I used to think 40 IBUs was assertive bitterness, but not unpleasant. Now with my latest brews I find it much stronger. The fibers in the muslin of hop bags has a fairly large surface area, which I imagine absorbs a significant amount of the alpha acids, reducing utilization. –  mdma Jul 23 '12 at 16:33
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