A spreadsheet (Apple Pages) used to calculate alpha acid loss over time.
In the previous post, I related a quick and dirty way to estimate the loss in alpha acids of hops over time, assuming they were stored in a freezer. That method was based on two measured variables (initial alpha acid percentage and amount left after 6 months at 68 °F/20 °C), plus a couple “guesstimations” — how the rate of loss changed at colder temperatures and a linear extrapolation from the initial condition through the one “data” point.
We would expect the loss of alpha acids to be an exponential function. So, it’s almost certain the simple model underestimates hop losses prior to six months and overestimates them after 6 months — although the deviations should be small. Using the simple method is better than not accounting for the losses at all, but there is a more accurate way of estimating the alpha acids if you’re willing to put in a little more work.
The Garetz Equation
Back in 1994, Mark Gartez published an article in Brewing Techniques that gave an equation for alpha acid loss in hops. His equation was this:
where AA% is the percentage of alpha acids in hops (both current and at harvest), k is a rate constant based on the hop variety, TF is a temperature factor based on storage temperature, SF is a storage factor based on oxygen exposure, and t is time (in days)
The Variables
The alpha acid percentage at harvest should be given on your bag of hops. You need to look up k, TF, and SF from a series of charts to plug these into the equation. And you need to estimate the time from harvest in days. (Recall that most hops are harvested in early September in the northern hemisphere. Small errors in guessing the harvest date do not add up to much in this equation, so don’t worry about that.)
You can do the calculation on any scientific calculator that calculates exponents. There are also online calculators based on this equation, and some homebrew recipe formulation software packages allow you to calculate this. You can also do this on a spreadsheet. The EXP() function will give you e raised to whatever power is within the parentheses. (See the first picture in this post for an example.)
Using the Garetz Equation is a little more work than the previously presented easy method, but it is more accurate. It still uses a few assumptions, but likely gives an estimate more precise than is required in any homebrew setting. Advanced homebrewers who wish to be more consistent in hitting their IBU targets should incorporate this calculation into their recipe formulation routine.
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