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IBU Impact from Parital Boil

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BrewInVA

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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.
 
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.
 
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.
 
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.
 
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.
 
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