How does the grain bill effect IBU?

[Polyphaeon]

Just noticed that changing the grain bill changes the IBU of a beer even with the same hops. Also, it seems to have the opposite relationship I would have expected - making a beer bigger brings the IBUs down. Can anyone explain why this is the case? Thanks!

 

[dzlater]

Hop utilization decreases with increasing wort gravity
http://www.howtobrew.com/section1/chapter5-5.html

 

[GrogNerd]

Hop utilization decreases with increasing wort gravity
http://www.howtobrew.com/section1/chapter5-5.html

some would argue with this, including Palmer. he disagrees with himself in the latest print edition

but, you can't find a hop utilization/IBU calculator that doesn't take gravity into account

 

[Epimetheus]

The link above Chapter 5 of How to Brew states how to calculate IBU vs. gravity. It does not explain why.

Elsewhere (I forget the link, I think it was a post by Yooper) I have read that 1. hops utilization requires some, not a lot of sugar for isomerization, and 2. the higher gravity means it is increasingly more difficult to dissolve anything in the water including hop oils.

If that is true, then theoretically the hops could be used with greater efficiency if they are boiled separately in a very low gravity wort for the sugars, which is then added to the high gravity wort after boiling.

Looking at Table 7 on that page, the utilization for 60 minutes, compare SG of 1.030 to 1.100, the higher gravity gets 0.53 or roughly half of the utilization of the lower gravity. Theoretically again, a hop tea of 1.030 could use half the hops required by the high gravity, which could be added at the end, adjusting for combined gravity.

Very importantly, this does not account for the perceived difference in bitterness due to the higher malt flavor and higher alcohol.

--- edit OT ---
Excellent comment in GrogNerd's signature
"critique your recipe? OK; brew it, good luck with it, please report back with the results, I'm stealing the recipe, thank you!" - Rob Grog

 

[Polyphaeon]

dzlater thank you for the reference. Looks like I will have a little reading for the weekend.
Epimetheus thank you for the explanation. I don't expect I will be doing separate boils anytime to save hops but is nice to have a reason to believe the calculator.

 

[biertourist]

If that is true, then theoretically the hops could be used with greater efficiency if they are boiled separately in a very low gravity wort for the sugars, which is then added to the high gravity wort after boiling.

Yep, that's exactly why a number of traditions do that. Utilization is also impacted by pH. You'll see some old British recipes separately boiling the hops in pure water; but oddly enough the German tradition gets more IBUs by first wort hopping while still "breaking all the rules" as first wort hopping exposes the hops to even higher gravity wort and lower pH than normal prior to the kettle reaching full volume, but this is offset by longer exposure at high temps.


I'd love to see the quote from Palmer because my guess is that it's being taken out of context; there is far too much data that shows that higher gravity worts == less utilization so I really doubt that's what Palmer is actually saying. (The utilization formulas come from curves that are derived from experimentation values so they're actually driven from real data which shows less dissolved iso alpha acids per ml in the final beer as gravity increases.)

-I tend to agree that the ability for the oils to dissolve into solution is one factor for lower bitterness rates as the gravity goes up; even oxygen dissolves less readily as gravity goes up so this makes sense.

Another factor that's been discussed is that isomerized alpha acids get pulled out of solution via hot and cold break and you get a larger quantity of hot and cold break when you have strong gravity beers. (You have larger quantities of insoluable proteins in higher gravity beers because you have a higher % of malt vs. water.) --I would bet that if you performed a hop-free intense boil and quickly chilled the beer and removed it from the hot and cold break and then brought that wort back up to boiling and added your hops for a 60 minute boil that you'd see dramatically increased hop utilization; it wouldn't be "worth it" as you'd spend way more on electricity than you would on just buying more hops but it could help illustrate how much break proteins are responsible for the decline in utilization in high gravity beers.


We know from the overwhelming amount of data and experimentation that's been done that higher gravity worts result in less IBUs in solution; what I'm most curious about is whether higher gravity worts actually end up with less isomerization; that is to ask whether or not you end up with less alpha acids converted to iso alpha acids when you have a higher gravity wort that is boiled for the same amount of time... -My understanding is that isomerization is only dependent upon heat energy and you'd have the same rate of isomerization in either wort but it's other factors like soluability and %break material that are driving the resulting decline in IBUs in your glass. (But I haven't seen data to support either possibility.)


I'm also curious whether any of the experiments tested wort directly out of the kettle but prior to fermentation or if the major experiments that the majority of our utilization formulas are based upon looked at simply the quantity of dissolved iso alpha acids in the final BEER. (From memory I think at least most tested BEER and not wort so there are a great many yeasty factors involved in that testing methodology, too. (Tinseth specifically comes to mind but I'm doing this from memory so please fact check me.))
Adam