Comparing Alpha Acid Utilization Formulas

by Bob Jones

Over the years there have been several articles and formulas to help the homebrewer calculate or predict the final IBUs in their finished beer. One of the first was by Jackie Rager that appeared in a special edition of Zymurgy, 1990. A lot of us have used his formula with mixed success. Most of us felt the utilizations were on the high side resulting in too low of an IBU calculation.

Then along came Mark Garetz with his book on "Using Hops" that included yet another formula to calculated IBUs. His formulas and techniques stirred things up a bit and most brewers felt his IBU numbers were too high or the utilizations were too low.

The one thing both of these authors did not have was hard data to back up their claims. I think each had done a few tests, but not really extensive enough, in my opinion, to really convince me that the calculations were right.

Well along comes Glenn Tinseth and he has yet another formula plus a utilization table. I know both Mark Garetz and Glenn Tinseth pretty well, but personally I think that Glenn has the resources, friends and equipment to better explore and measure hop utilization. He has friends and tools at the USDA hop labs at Oregon State University.

I knew Glenn was working on this project as we spoke about it over 3 years ago. Well I was excited when I heard about Glenn's new utilization numbers. The first thing I questioned was how do these new utilization numbers compare with Rager's and Garetz's utilizations? The differences can be best shown in a graphical form. The following plot shows that Tinseth's utilization numbers generally fall between the old Rager and Garetz utilizations. This is initially reassuring.

It's interesting to note that the old Rager numbers peaked out at 30% utilization for a 45 minute boil. Garetz's utilization numbers didn't show any utilization for boils of less than 10 minutes and peaked out at 90 minutes. Both Rager’s and Garetz’s plots follow a familiar "S" shaped curve. Tinseth observes that it doesn't seem to accurately reflect what's happening in the brew kettle. Note the new Tinseth curve indicates low utilizations for short boil times and continuing utilization for boil times to 2 hours. Granted that these short and long boils produce diminishing returns, but I have always felt there would be some utilization for short boil times and increasing utilization for long boils as well. Incidentally, the choppiness of the Rager and Garetz data are due to the coarseness of the original data.

I like the look of Glenn's curve. It fits and looks more like a classical chemical reaction as well as a lot of other things in nature. Let’s take a look at the new IBU calculation as see how it works.

First we must calculate the concentration of alpha acid (AA) each hop adds in the wort:

mg/l of added AA = decimal AA of each hop * ozs of hop * 7490 / gallons of wort

Then we calculate the IBU contribution for each hop addition:

IBUs = decimal AA utilization (from table) * mg/l of added AA (calculated above)

The sum of the IBUs for each hop addition then equals the total IBUs in the wort.

I ran the Tinseth calculations for a recent IPA I made. The Rager numbers indicated an IBU of 78. The new Tinseth calculation came up with 60 IBUs. I think the beer is much closer to 60 than 78.

For brewers that are also Web surfers, the hard numbers and an IBU calculator can be found at Glenn's World Wide Web page, along with a lot of other interesting information on hops. The address is:

http://www.teleport.com/~gtinseth/

At Glenn Tinseth’s Hop Home Page you will also find the utilization table I have included here. The table lists the utilization numbers for wort gravities that range from 1.030 to 1.130. He also has formulas for the tables for those that are so inclined. I believe Glenn has plans to formally publish this data in an upcoming issue of Brewing Techniques. Look for this article, I think it will be one of the most definitive works on hop utilization yet.