Brewing Today: IBU calculations and hop utilization

[FifteenTen]

Heretofore I have used beer kits to brew with. I’m trying my first extract recipe gleaned from the internet. The recipe calls for the following:

Perle: 1oz at 9.2 AAU for 90 mins
Cascade 0.8 oz at 3.3 AAU for 15
Cascade 1.2 oz at 3.3 AAU for 5
Cascade 1 oz at 3.3 AAU dry hop

So what did I do, I went out and bought 1 oz of Perle and 3 oz of Cascade only to come home and realize that the Perle I have is 8 AAU and the Cascade is 5 AAU. Thus, my Perle is weaker than recipe requires and my Cascade is stronger than the recipe requires.

My hypothesis is I can use less of my Cascade for flavor and aroma and use some of the left over Cascade for bittering along with the Perle.

Using the Weight x AAU = IBU formula my math for the recipe is as follows:

Perle 1 x 9.2 = 9.2
Cascade 0.8 x 3.3 = 2.64
Cascade 1.2 x 3.3 = 3.96
Cascade 1 x 3.3 = 3.3

The hops I have are as follows:

Perle 1 x 8 = 8 (deficiency of 1.2)
Cascade 0.8 x 5 = 4 (surplus of 1.36)
Cascade 1.2 x 5 = 6 (surplus of 2.04)
Cascade 1 x 5 = 5 (surplus of 1.7)

To use the Cascade I have I get the following numbers:

0.528 x 5 = 2.64 as called for in the recipe
0.792 x 5 = 3.96 as called for in the recipe
0.66 x 5 = 3.33 as called for in the recipe

I have 3 oz of 5 AAU Cascade, per recipe I only need 1.98 oz leaving me 1.02 oz in reserve.

My questions are as follows:

1. Since I don’t have enough Perle can I use 0.24 oz of the 5% Cascade to make up the difference for bittering?
2. Will that affect flavor?
3. Should I even worry about it?
4. Can I use all remaining Cascade for dry hopping in excess of the 1 oz per recipe?

Thanking you in advance for you consideration, FifteenTen.

 

[DarkBrood]

First and foremost, if this is a custom recipe (as opposed to a clone recipe), how much do you REALLY care about following it carefully? Are there tasting notes or reviews of this recipe? If not, is the source at least credible? There are a lot of creative, well-meaning people out there inventing brews with software and sharing them before testing them out....

Assuming that you trust the recipe as credible, are you sure that you don't want to tweak it in some way? Using the weights called for would yield the same beer with less bitterness and a little more hop flavor/aroma.

If you really want to match it exactly, then your calculations are good (although to be technical, you are calculating in HBUs, not IBUs). HBUs are standardized for 5-gallon batches of homebrew and are calculated as: weight (oz) X alpha acid (%) = HBUs.

Rather than trying to sum it all up like you have, I find it easier to look at it on a per-addition basis with basic algebra. It's really nice to have it broken down when you look at it months later and try to remember why you added what when.
My notes would look something like this:

Recipe Perle 90-min: 1oz X 9.2% = 9.2 HBUs [8.0% ON HAND]
Actual Perle 90-min: 1oz X 8.0% = 8.0 HBUs [-1.2 HBUs]
Extra Cascade 90-min: ##oz X 5.0% = 1.2 HBUs [1.2 / 5.0 = 0.24oz]
90-MIN ADDITION: 1oz Perle @ 8.0%, 0.24oz Cascade @ 5.0%

Recipe Cascade 15-min: 0.8oz X 3.3% = 2.64 HBUs [5.0% ON HAND]
Actual Cascade 15-min: ##oz X 5.0% = 2.64 HBUs [2.64 / 5.0 = 0.528oz]
15-MIN ADDITION: 0.53oz Cascade @ 5.0%

Recipe Cascade 5-min: 1.2oz X 3.3% = 3.96 HBUs [5.0% ON HAND]
Actual Cascade 5-min: ##oz X 5.0% = 3.96 HBUs [3.96 / 5.0 = 0.792oz]
5-MIN ADDITION: 0.79oz Cascade @ 5.0%

Recipe Cascade Dry-Hop: 1.0oz X 3.3% = 3.3 HBUs [5.0% ON HAND]
Actual Cascade Dry-Hop: ##oz X 5.0% = 3.3 HBUs [3.3 / 5.0 = 0.66oz]
DRY-HOP ADDITION: 0.66oz Cascade @ 5.0%

This notation also easily accommodates using multiple strengths of the same hop (i.e some Cascade @ 5% and some at 3.5%). You can, of course, always use BeerSmith ProMash, or BeerTools to do the work for you.....

 

[FifteenTen]

Thats awsome...thanks for taking the time to give a detailed reply.

 

[DarkBrood]

Oh, no worries....most people here are happy to help. When you're starting out, try to find a couple more things to document on each batch. Don't overwhelm yourself and try to record everything at once....but as you phase into it and start planning future batches, you'll be glad for any additional reference that you can look back on. Most brewers start using software or setting up a spreadsheet within the first year or two..

 

[ajf]

Your formula (weight * AAU = IBU) is not quite correct. Replace IBU (International Bitterness Units) with HBU (Homebrew Bittering Units) and it would be correct. Your math would be correct for the bittering hop addition (90 minutes), but not for the late additions. The 15 late hop additions contribute very little to bitterness, but do contribute to flavor and aroma, and the dry hops add no bitterness.
I ran the original hop schedule through Promash, adding sufficient fermentables to get an OG of 1.050. It predicted 44.1 IBUs (using Tinseth).
I then changed the AAU of the hops to what you have, and without changing anything else, the IBUs came out at 44.1.
You will lose a few IBUs because your perle has a lower AAU than the recipe, but you will make up for that with the 15 and 5 minutes additions because your cascade has higher AAU than the recipe. Keeping the cascade proportions the same as the original recipe will preserve the intended flavor and aroma contributions, so using the original weights and ignoring the AAU differences should give you exactly the same results.

-a.

 

[Yooper]

Your formula (weight * AAU = IBU) is not quite correct. Replace IBU (International Bitterness Units) with HBU (Homebrew Bittering Units) and it would be correct. Your math would be correct for the bittering hop addition (90 minutes), but not for the late additions. The 15 late hop additions contribute very little to bitterness, but do contribute to flavor and aroma, and the dry hops add no bitterness.
I ran the original hop schedule through Promash, adding sufficient fermentables to get an OG of 1.050. It predicted 44.1 IBUs (using Tinseth).
I then changed the AAU of the hops to what you have, and without changing anything else, the IBUs came out at 44.1.
You will lose a few IBUs because your perle has a lower AAU than the recipe, but you will make up for that with the 15 and 5 minutes additions because your cascade has higher AAU than the recipe. Keeping the cascade proportions the same as the original recipe will preserve the intended flavor and aroma contributions, so using the original weights and ignoring the AAU differences should give you exactly the same results.

-a.

I always keep the "late hops", any added after 20 minutes or less to the end of the boil, the same. The IBU contribution is negligible, and the flavor/aroma is very important in those late hops. So, I ignore any differences in AAUs at 15 minutes or less. You want to keep an ounce of flavor hops, for example, since the bitterness is negligible.

Since you're doing a 90 minute boil (which I almost never do, so I have to wonder why.......especially in an extract batch which I would recommend against, but anyway....)

The bittering hops ARE different. But with a 90 minute boil, even that is negligible. I would just proceed with the recipe and not worry about it. Even if there IS a difference, 5 IBUs is not perceptible anway. I'd brew as is.

But I'd definitely question any extract recipe that had a 90 minute boil!

 

[FifteenTen]

I love this forum....thanks my peps...

The 90 min boil comes from an all grain coverted to extract. So I take it that since I was doing extract I would have been fine with a 60 min boil.

 

[DarkBrood]

It really depends on your beer style. For an extract recipe, the extra boil time really will only serve to further isomerize the hops' acids to bitterness (iso-alpha acids).

The calculations discussed above determine the HBUs that are theoretically contributed to the recipe - in reality, the actual IBUs also take into account the length of the boil, the vigor of the boil, the density/gravity of the wort, and how much water is lost to boiloff (boil efficiency). This can quickly become very complicated (a part of the reason for the early hop/late hop regimen was more reliable calculations) to calculate accurately. General good rules of thumb are:
>90min. very raw and harsh bitterness, absolutely no flavor or aroma contributed
75-90min. raw bitterness, some semi-tannic harshness
60-75min. pure bitterness
45-60min. mostly bitterness, very light flavor
30-45min. some light bitterness, moderate flavor, very light aroma
15-30min. hints of bitterness, good flavor, moderate aroma
5-15min. no bitterness, strong flavor, good aroma
2-5min. strong flavor, strong aroma
0-2min. intense flavor with vegetal hints, intense aroma with grassy hints

Bearing in mind that these are empirical descriptions, actually calculating your usage is simplified for homebrewers by simply comparing wort gravity and planned boil time with a table such as John Palmer's at http://www.howtobrew.com/section1/chapter5-5.html. This gives a multiplier for your HBUs that will let you determine your IBU contributions from each hop addition (it can be interesting to adjust additions by 10 or 15 minutes each way to see the effects).

Bottom line, the extra boil time will suck all of the flavor and aroma components out of your first hop addition, in exchange for a bit of extra bitterness. Does your beer style/recipe support that result?