How is the IBU calculated? Part 2

Continuing our exciting study of IBU calculations, today we will explore the most modern models to calculate it. Before reading, click in part 1 to read the others IBU model we studied in the last article, and subscribe to this newsletter to receive all new articles!

Mark Garetz

In the excellent ‘Using Hops, The Complete Guide to Hops for the Craft Brewers’ from 1994, Mark presented his model to obtain IBU. His equation is similar to Rager’s approach; however, instead of having a ‘gravity adjustment (GA), Garetz uses a more generic ‘Adjustment factor’ (CA).

To get the adjustment factor, it’s necessary to calculate the height factor (TF), which as the name suggests, depends on the height where the brewery is settled down, the hopping rate factor (HF), that results of the desired IBU, and the gravity factor. In the gravity values, differently from the Ranger model, we will have only 1 or 1.05.

The table of %utilization is also different from the previous and you can check it below:

The table was extracted from the Mark’s book. In the second column is presented the Rager %utilization value, for comparison, and in the third column the Mark value. Note that until 10 minutes, the % is 0, meaning that this model presents no results for the first minutes.

This model is still being used today, and you can see it as an option if you use Beersmith (it's possible change in the options).

Glenn Tinseth

This is the gold standard of IBU calculation. Glenn Tinseth published his model on his personal website (https://realbeer.com/hops/) for the first time in 1997. This model is the most used today because it has the most consistent results in different conditions (time boilers and wort densities).

As you can see in the equation, it is very similar to the other models, in fact, it’s indeed simpler because it has no gravity or adjustment factor multiplying the volume.

However, we have a second equation to obtain the %utilization, which differs a lot from the other models that uses tables (we may found a table of %utilization extract of Tinseth equation). In this way, it’s possible to obtain more precise and adjusted values for our process. The Tinseth model is more sensitive to the time and wort density, making it a better model for general purposes!

Comparing Results

Just for fun, I plotted a graph comparing the results of the calculation using the models of Rager, Ray Daniels, Mark Garetz, and Tinseth, for 30 L of wort with a density of 1.06, with 50g of hops with 11.9% of Alpha Acids. I range the time jumping from 5 minutes until reach 90 minutes and you can see the results below.

Here are some observations:

• All models, except Tinseth, form some ‘steps’, that because the %utilization data comes from a table (problably an approximation)
• The Ray Daniels table has the data more gapped, so the ‘steps’ became greater.
• The Mark model starts null until 10 minutes.
• The Rager model finished at 60 minutes because the data also ends there.
• All curves started sloped and decreased as time passed.
• After 60 minutes, the %utilization (and the increase in IBU) almost did not have any gain.

Limitation of IBU Calculations

• Only predict iso-alpha acids; nonetheless other compounds, such as beta-acids, oxidized compounds, and polyphenols, among others, also add bitterness to beer. This is a limitation of the IBU itself.
• The models only calculate the isomerization during boiling; however, during the whirlpool step and cooling phase, even at a slow pace, the conversion continues. There are some hybrid models, for example, SMPH, that use Tinseth as a base and calculate it.
• Techniques of adding hops in flame-out (for aroma) or late-hopping (at lower temperatures, e.g., 80°C), in a classical approach would present 0 IBU; nevertheless, they also add some iso-alpha-acids to beer.

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