Better to decrease boil time or amount to achieve hop ibu?

I was brewing a recipe this weekend that called for "X" amount of hops for the full 60 minute boil. However, the hops used in the original recipe were only 6.5% alpha acid while the hops I bought at the LHBS were 9.4% alpha acid. Therefore, in order to hit the IBU target for the batch I had to either decrease the amount of hops used for the 60 minute boil or decrease the boil time for the hops.

If I am still able to hit the target IBU for the batch by going either way is there going to be a difference? Which is the better option?

 

Decrease the amount. If you play with the time in the boil, it can change the flavor if the beer.

 

In a technical sense, I agree with AgentHubcap. If you cut the boil time, you get the same bitterness but more aeromatics.

However, if you cut the amount of hops, you end up with some fraction of an ounce of hops leftover. Do you have a plan for the leftover fraction? If not, I'd try to find another place in the beer to put them (FWH, dry hop, flameout, etc.). It also changes the flavor, but I'd prefer to do that than waste hops.

 

Don't forget about boil-off. If you decrease boil time, you end up with a greater volume, lower gravity wort, unless you take this into account when you're planning. I would either move the addition later, which can achieve the same bitterness but have more aroma, or decrease the amount added for the bittering addition.

What type of recipe is it? That might have an effect on the answers given. If it's an IPA, use the same amount, just move it later in the boil. If it's something with little to no hop aroma, use less hops.

 

I assumed he meant keeping the same boil time and reducing the hop time, (ie 60 minute boil, 50 minutes hops)

 

Yes, you are correct!

I brewed three 5 gallon batches this weekend, two of which I encountered this scenario. The first was a Jeremiah Red. OG was 1.071 and on this one I kept the same hop boil times, but reduced the amount of hops to hit the target IBU for the recipe.

The other batch was Slim's Lemon Lime Hefe. On this one I had to substitute Citra hops for the Motueka. The Citra hops have a really high alpha acid, so an adjustment had to be made. On this batch I decided to reduce the boil time for the hops. I still boiled the wort 60 minutes, I just didn't make my first hop addition until 30 minutes into the boil.

I figured going this route would be the better way to go since I would probably end up with a stronger aroma and citrus flavor by keeping the hop amount relatively the same by adding towards the end of the boil as opposed to reducing the amount of hops and keeping the full boil time. I still hit the target IBU for the batch though, so I would assume I would still have the right level of bitterness, but perhaps a little more citrus aroma?

 

I think that's exactly what you'll end up with. I think you'll be pleased with the results.

In my pumpkin saison, I got 8 of the 29 IBUs from an ounce of Citra added in the last 5 minutes. It's very nice.

 

Yeah, I read that wrong.

 

For me, it really depends. My scale only reads to 1/8 of an ounce, so I try to keep to 1/4 ounce increments whenever possible. If it's a little over or a little under, I'll tweak the addition time, but no more than 10 minutes for bittering and 5 minutes for flavor. Aroma hops don't add much in terms of IBU, so I kind of play fast and loose there.