How is boil size defined for an extract brew?

[GreenGal]

I spent all day yesterday searching the boards for this and could not for the life of me figure it out:

Boil size seems to be an important factor when calculating hop utilization however I am not sure when the volume measurement is supposed to happen. Is it:

a) the total volume in the pot including any LME or DME at the Start of the boil

b) the total volume (as above) at the start and then maintained by topping up to account for boil off. In other words, the boil size should be roughly the same for the length of the boil. This would seem to be ill advised as it will lower the temp and potentially stop the boil.

c) the total volume at the End of the boil

If the answer is (A) then is there an assumption that we all have the same boil off rate, which seems a bit odd.

Incidentally my main IBU calculations are coming from Beer Calculus at the moment.

Thanks for the clarification.

 

[PintOfBitter]

This is not really much of an exact science. That being said, the answer you're looking for is A. Try calculating IBUs using your initial volume and then try with your final volume. You will likely see a fairly small difference.

 

[pcollins]

I'm not quite sure about the calculations for hop utilization but I do know a couple of other things.

Boil off rate is going to vary from brewer(y) to brewer(y) because of differences in equipment. The actual rate will be up to you to determine based on repeated brews. You won't be far off everybody else as there will be sort of a range of boil off rates. Mine is about a gallon over the course of an hour for a full boil. I go from about 6.5 gallons to 5.5 gallons during that time.

Your boil size is the amount that is in your kettle at the START of boil including all extract additions. Your boil should not be topped up for the exact reasons you stated. Your choice above labeled (c) is your end boil volume, even that will be less once you cool–not much, but measurable. Your pitching volume will likely be even less than that once you account for losses in trub and transferring.

Let me know if that's helpful.

 

[david_42]

If the answer is (A) then is there an assumption that we all have the same boil off rate, which seems a bit odd.

Not really. Once you are familiar with your gear, you'll find that about a gallon boil-off is nominal. A good rolling boil will result in a 15-20% reduction in volume for a one-hour boil. Violent boils are not necessary. Higher boil-off may be necessary with all-grain.

 

[GreenGal]

Thanks everyone! That is what I suspected. My boil off rate seems fairly in keeping with what you are saying as well - I think I had read something somewhere about different diameter pots having wildly different boil offs and it must have been stuck in my head. This info makes me feel more comfortable.

I know it is not a huge deal, but I also know that I find bitter beer undrinkable. So if there is anything I can do to save myself from making a whole batch that is too bitter for me, I should make the effort. I want to enjoy my creation! When I do the calculations for IBU using pre- and post-boil volumes I get about 7 IBU difference which depending on the style would seem to make a difference. Then again, maybe I'm just being a precision nerd .