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(Generally I’ll try to use both metric and Imperial units in this post… according to WordPress, about half of the people reading this are from the U.S. Where U( BG, t 1) is the hop utilization (which is a function of both boil gravity and time), D( AA, W, V) is the density of alpha acids in the wort (in mg/l, a function of the alpha-acid rating, weight of hops, and post-boil volume of wort), b( BG) is a “bigness factor” that is a function of the boil gravity ( BG), f( t 1) is a “time factor” that predicts the isomerization of alpha acids as a function of time (in minutes) ( t 1), AA is the alpha-acid rating (in percent) of the hops added, W is the weight of the hops (in grams), and V is the volume of the wort (in liters). Tinseth’s formula is as follows: IBU = U( BG, t 1) × D( AA, W, V) I’ll use Glenn Tinseth’s formula for predicting IBUs in this post, because this formula is “considered very accurate” ( BeerSmith: Calculating Hop Bitterness: How much Hops to Use?), but also simply because I haven’t yet worked as much with the formulas from Rager or Garetz. It seems possible to take Tinseth’s formula for IBUs, combine it with a measure of how utilization is affected by temperature, and calculate the additional IBUs that occur after flameout… and that’s exactly what this section does. (If you start your wort chiller immediately at flameout and have a small volume, then you’ll probably get pretty close to the Tinseth estimation of 0 IBUs with a hops addition at flameout, but most people leave the hops in the naturally-cooling wort for at least some time before forced cooling.) Therefore, there is (usually) additional utilization happening after flameout, and the degree of this additional utilization depends on when the hops were added and how quickly the wort cools. In a separate blog post, I look at relative utilization as a function of temperature.) It’s also very clear that when the gas (or electricity) of the boil is turned off, the temperature of the wort doesn’t instantaneously drop to room temperature. (I’ve seen claims that utilization drops to zero at around 180☏ (82☌), and claims that the decrease in utilization follows an Arrhenius equation. Hop utilization decreases as a function of temperature, with more utilization at boiling and less utilization at lower temperatures.
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The question then becomes how to model post-flameout utilization in the prediction of IBUs. There is apparently some utilization happening after flameout it’s just not quite clear how much. In short, we have reported post-flameout utilization (and consequent bitterness) ranging from 10% to 30%, and a lot of numbers in between. In another discussion at probrewer, utilization rates between 13% and ~30% are reported for whirlpool additions. In a discussion at probrewer, one user says “Matt Brynildson, a former hop chemist, claims to get 22% whirlpool utilization at Firestone Walker (50-bbl system) and to have gotten 15% whirlpool utilization on a 10-bbl brewpub system”. In a BYO article from Mar/Apr 2013, it is reported that large systems (15 bbl = 465 G = 1760 liters) get 16% utilization from hops added at flameout, while smaller systems (11 G or 42 liters) get around 10% utilization. However, a quick glance at various internet sources indicates that this is one area where formulas and observations do not always line up.
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So it’s worth taking his work and formula very seriously. Tinseth had “access to some handy tools and knowledgeable friends at the USDA hop labs and the Flavor Perception labs at Oregon State University,” and he has “had quite a few worts and beers analyzed.” His formula is very widely used, presumably because it does as good or better a job at predicting bitterness levels compared with other available formulas. If one takes a standard formula for predicting IBUs, such as Tinseth’s, hops additions at flameout contribute nothing to the final IBU measurement. The SMPH model incorporates the post-flameout technique described here. While the technique described here is still correct, I’ve since developed another IBU calculator (called SMPH) that accounts separately for the contribution of isomerized alpha acids and auxiliary bittering compounds to the IBU. John Koopmans has made an implementation of the mIBU method available in an Excel document. I’ve created an on-line calculator for the mIBU method here: with a description of this calculator in the blog post An On-Line Calculator for the mIBU Technique. This new metric is referred to as “maximum IBU” (or “mIBU”), for reasons explained below. This blog post proposes a modification to the Tinseth IBU formula to account for hops added late in the boil and/or at flameout. This is in conflict with widespread experience, which shows that adding hops at flameout does add significant bitterness. The predicted IBU contribution when adding hops at flameout is usually zero.