IBU Impact from Parital Boil

BrewInVA · Aug 30, 2009

I've heard several conflicting opinions regarding this topic and I hope to get some clarity. When using a partial boil with the late extract addition method (say adding most of the extract at knockout), can you achieve the same IBUs as in a full boil recipe? I've heard that topping off with water in your primary fermenter will drastically reduce your IBUs vs an equivalent full boil.

Basic example for explanation:
1) Suppose I did a full boil with 6 gallons using 6lbs of DME and 1oz of Horizon (12%) hops. Accoriding to BeerSmith, I will get ~ 43 IBUs.

2) Suppose instead I did a partial boil with 3 gallons and 3lbs of DME using the 1oz Horizon hops. Because I have the same ratio of water to DME, I would expect similar bitterness (the only difference would be the evaporation for each boil would result in a slightly more concentrated wort in the partial boil). According to BeerSmith for this approach (which requires me to top off with water to achieve a 5 gallon batch), I get a similar looking 43 IBUs.

So in the end, do I have to same bitterness level or are my IBUs reduced when I top off?

Thanks for the help.

david_42 · Aug 31, 2009

can you achieve the same IBUs as in a full boil recipe?

Yes. What you can't do is achieve the very high IBU levels an IIPA might need. The maximum IBU level you can achieve in a boil is about 100 IBU, so if you do 3 gallons and top off to 5, your limit is 60 IBU. This is enough for 98% of styles.

If you need higher IBU levels, just boil part of your hops in the top-off water.

MMW · Aug 31, 2009

Now do a partial boil with 3 gal and 6 lbs DME topping off to 5 gal and look at what beersmith tells you.

There are gravity adjustments that lower your utilization as your gravity increases (here's a run down on the formulas) which is why your utilization is lower for a partial boil and also why adding extract later in the process can increase your utilization.

BrewInVA · Sep 4, 2009

To MMWs point, I understand that if I were to use all 6 lbs of DME in the 3 gallon partial boil that I would have lower IBUs due to the reduced hop utilization. But if I make the partial boil the same gravity as a full boil (by keep the water to DME ratio the same), why can't I achieve the same IBUs in the end? Are you basically saying I'm diluting it? Thanks for the replies.

MMW · Sep 4, 2009

That's exactly it.

david_42 · Sep 5, 2009

If you want 43 IBU AFTER dilution, you have to shoot for 72 IBU in 3 gallons.

BrewInVA · Sep 5, 2009

Great - thanks for the help.

BrewInVA · Sep 27, 2009

Another related question. My last two batches (3 gallon partial boils) have been a Nut Brown Ale and a Porter. Neither of them seem to have the level of "roastiness" to them that I would expect and they both taste similar. They're both a bit sweeter than I would want given I under-hopped the recipe (I didn't up the hops to account for the top off per the rest of this thread). I used established recipes so I don't believe there's a problem there. Is there any dilution that occurs to these flavors given the partial boil that requires a top off with water? I wouldn't think so but wanted to check. Thanks.