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Full boil vs partial boil and hop utilization

I have read many posts discussing using a full boil or partial boil with extract batches. The full boils obviously improve the beer in many ways over a partial. My questions concern hop utilization. Does this mean “you will get better flavor and bitterness from the hops”? or does it mean “Watch out you will get more bitterness than is intended in the recipe”?

It is stated that you will get better hop utilization when using a full boil, but usually doesn’t make mention of cutting down the bittering addition, or how much less to use. Wouldn’t it be true that the bittering addition would need to be smaller if using a full boil? Aren’t most of the kit recipes from NB intended to use the bittering amount at 60 minutes with only a 2.5 gallon boil? How much more bitterness would be expected if a brewer used the amount called for in the 2.5 gallon recipe but increased the boil to full? Im sure the answer would be dependent on style and other factors, but is there a “rule of thumb” answer to this?

Yes, yes, and roughly 20-40%, depending on gravities and on whose IBU correlation you prefer.

[quote=“a10t2”]
Yes, yes, and roughly 20-40%, depending on gravities and on whose IBU correlation you prefer.[/quote]

+1

Changing the size of the boil makes a huge difference in the IBU calculations using the free brewing software at hopville.com. I’m not sure how accurate that program is, but it indicates a huge effect. If you enter the standard NB kit partial-boil instructions, then change it to a 6.0 gal. boil, the final IBU figure is often (at least) doubled. A similar effect is caused by using the “late addition” feature for extract batches. I have a partial mash recipe for this weekend, and adding the 5 lbs of DME to “late addition” from the normal (beginning of boil) addition moved the IBU’s from the upper 30’s to the upper 50’s.

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