Super64

Recipes 

I just read on line that if you do a partial boil, say 2.5 gallons for a 5 gallon batch, that you need to build your recipe with twice the IBU values to compensate for the fact that you will be adding 2.5 gallons of top off water to the primary.

Can anyone confirm this?

I've been brewing small batches starting with 2.5 gallon boils that can end up with 1.75 gallons of wort. That would mean I'd need to adjust my hop value up by about 42%.

Yooper

Recipes 

No, that's not so. You might need 15-25% more hops to get the same bittering, but even that is debatable.

One thing you can do easily is add the bulk of the extract at the end of the boil. That way your wort doesn't darken and get carmelized-tasting and the hops utilization will improve as well.

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hardmike

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have you ever done that? i did it with my last to beers. it really sucks. the extract do not want to dissolve plus it get hit with so much steam on its way into the pot it basically falls in chunks!
besides that it is impossible to claculate the change in utilization for the last 10 minutes of the boil at full gravity. So it really just gets more confusing.
Do you have a smart phone? find an app that claculates hop bitterness and SG and everything. find your boil gravity and calculate what your utilization is gonna be.
The problem is that with a partial boil, the gravity is much higher and you wont get as much bitterness from your hops.
If you are building a recipe...try using the beer recipator. I know it asks for the boil volume so i think it might calculate all that stuff for you.

Redpiper

Recipes 

An easy way I've found to add the late extract is to use a second pot. I heat up a gallon or so of water in one of my wife's cooking pots when I start my boil. Once the main boil is going, I mix the extra extract into the hot water. 30 minutes later it is well dissolved.

Then I bring this to just under a boil and ladle it into the main boil in the last 10 minutes. You do have to leave a little space in the main pot (evaporation makes more) but this hasn't been a problem.

Super64

Recipes 

I typically do late adds and use DME. I put a couple of pounds in at the start of the boil along with the bittering hops, then add the balance of the extract at the 15 minute mark.

I have also found a hot water slurry makes the addition a little easier.

ayoungrad

Recipes 

When I use DME I add all of it at flameout. I use a large wisk and with the wisk I have no issues with dissolving it - even up to 5 pounds at flameout (IIPA).

In re: utilization and partial boils. I think the OP is partially correct about the dilution factor IF the OP is talking about 100+IBU. I have never found a definitive answer to this but here is the best I can come up with...

It seems to be agreed upon that there is a maximum taste threshold for bitterness - around 100 IBU. However, some seem to also think there is a maximum IBU threshold that can dissolve/form in the wort. So, if, for example, you get 100IBU in your wort in the boil, when you dilute it in half, you will get 1/2 the IBU - so the maximum is 50 with a 50:50 dilution.

I'm not saying this is true with definity but it seems at least close to the mark based on my experiences with dilution and IBU. I now do full boils (I even have to do two boils for each batch to achieve this with my set-up) and I really, really love the results.

Just another thought. There are also those who debate the truth of wort concentration affecting IBU utilization. Some feel utilization is related to other factors including, from what I understand, break formation. I do not know exactly all of the factors that they say contribute, but I use Tinseth which uses concentration as a major factor and I find it to be fairly accurate.

jkarp

Recipes 

Nope. Gravity has no direct impact on hop utilization. This myth was debunked years ago.

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ayoungrad

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One other thought - I just read my post. If the idea of dilution is real, it is probably true for small beers too. The only issue is that you really can't do a big IBU beer at all. Again, not sure how accurate this is, but full boils made a big difference in the bitterness of my beers.

jkarp

Recipes 

BBR/BYO did an experiment on extract partial/full boils and sent the results to be lab tested for IBUs. No difference was found. Recipe was an IPA too.

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ayoungrad

Recipes 

Interesting. I guess its one of those things where I must have also changed something else along the line that made the true difference.