Ask Mr. Wizard

Explaining Hop Utilization


Greg Wilhelmi — Boone, Iowa asks,

I was recently reading an article from the BYO archives about recipe formulation, and the part about hop prediction lead me to a question: What is meant by “boil efficiency”?


The term “boil efficiency” typically refers to “hop utilization” and relates hop alpha acids added to wort to iso-alpha acids in beer. I will use the standard term “hop utilization” in this answer to prevent any confusion with the term “boil efficiency” as that makes me think of other things like evaporation rate and heat transfer rate.

For starters, let’s digest the basic definition of hop utilization and review why hop utilization has a practical maximum of about 40% for large commercial breweries and about 30% for homebrewers and most craft brewers. Hops contain alpha acids, among other hoppy components, useful to brewers. When hops are boiled, alpha acids contained in hop lupulin glands soften and dissolve into wort. It turns out that alpha acids have very limited solubility in aqueous solutions because they are hydrophobic (water fearing). Hydrophobic compounds tend to literally stick together in aqueous solutions and when they are less dense than the solution they float to the surface. Think of vegetable oil added to a pot of boiling water; the oil is visible as large, amorphous, balloon-looking blobs. When foam develops in a pot of boiling pasta, chicken stock, or wort, oils are carried with the foam and some of these oils are deposited on the surface of the pot above the liquid level. We see this when cleaning pots after boiling wort, pasta, and chicken stock. This relates to hop utilization because oil, aka alpha acid, loss is one reason why hop utilization is less than 100%.

Aside from getting hop acids to stay in wort and away from the surface of the kettle, another key of hop utilization is the conversion of hop alpha acids into iso-alpha acids. When alpha acids are heated above about 176 °F (80 °C), they begin to morph or isomerize into a different group of compounds named iso-alpha acids. Turns out there are several types of alpha acids and each type morphs into a specific iso-alpha acid. Chemistry aside, this transformation depends on temperature, time, pH, and wort density. As wort density increases, isomerization rate decreases (although iso-alpha acid solubility increases). And as temperature, time, and pH increase, so does the rate of isomerization. It’s worth noting that while increasing wort pH increases isomerization and hop utilization, it also decreases the quality of bitterness, increases wort color, changes the way malt proteins precipitate, and has a negative effect on beer stability. Suffice to say, increasing wort pH is not something brewers do to increase hop utilization.

Because hop utilization compares hop alpha acids added to wort to iso-alpha acids in beer, wort boiling is just one part of the process where losses occur. Iso-alpha acids are more soluble and less hydrophobic than alpha acids, but they are still sticky and have limited solubility in wort (that’s why the limit of beer bitterness is about 120 IBUs, regardless of marketing claims made on beer labels). Alpha and iso-alpha acids are also lost to trub, fermenter surfaces contacted by beer foam during fermentation, some beer finings, filtration surfaces, and anytime in the process when beer foams. One practical way to increase hop utilization is to use anti-foams during wort boiling and fermentation. But your question is not about how to increase hop utilization, just what it is.

The next question, how does a brewer know what value to use when calculating hop additions, is easy to answer but far from easy to assess. The easy answer is to compare iso-alpha acids in beer to hop alpha acids added to wort. Duh! However, most small-scale brewers, both home and craft breweries with limited production, don’t know the iso-alpha acid content of their beers because the testing method requires special equipment, is time-consuming to perform, and uses a solvent that must be properly captured and disposed of after use. The other challenge is knowing how much hop alpha acids were added to the kettle.

Let’s tackle this last point first. Because the hop alpha acid content is printed on all packages of hop pellets and hop cones, knowing how much was added seems simple. Right? The problem is that alpha acids oxidize during hop storage and the value printed on the bag changes over time. When hops are packaged and stored properly, the rate of alpha acid change is slow. Therefore, using the value on the package is not a bad place to start if we recognize what we are dealing with. And as luck would have it, alpha acids oxidize into a group of bitter compounds called humulinones. While humulinones are less bitter than iso-alpha acids, they do preserve some of the bitterness and do not represent a complete loss when hops oxidize. In practice, however, this poses a challenge because it means that hop storage plays a role in hop calculations. Let’s just leave that idea and keep moving!

The real challenge in this whole thing is knowing the iso-alpha acid content of beer to develop prediction models. The homebrewing hop models developed by Rager, Tinseth, and Garetz in the early 1990s are all problematic because none of the methods make any mention of how utilization was measured. Please correct me if I am wrong, but I cannot find anything written by these authors documenting how they went about determining utilization. Not to cast shade because their efforts helped many brewers with hopping. The good news is that larger breweries do routinely measure iso-alpha acids in beer, and we know enough to be able to come up with some pretty safe assumptions.

Table 1 is a synthesis of utilization rates from several sources plus some massaging. It’s important to know that any utilization rate from a table is some form of a guesstimate; without knowing what goes into the kettle and what ends up in the beer, we simply do not know what happened. This is why many brewers don’t get too worked up over calculated IBUs. When calculations are performed consistently and adjustments are made based on perceived bitterness, the numbers are simply a means to an end. I hope this sheds a useful light on your question!

Response by Ashton Lewis.