Beer Fact Friday: Alpha & Beta Acids

Hop with lupulin

Hey, guess what? It’s Beer Fact Friday. Today, at the request of one of our readers, we’re discussing alpha and beta acids.

In the picture above, you’ll notice a yellow substance close to the middle of the hop cone. That stuff is called lupulin, a sticky resin principally made up of alpha and beta acids—complex molecules that give beer its bitterness, under the right conditions.

You see, neither alpha acids nor beta acids will contribute any bitterness to a beer without some help. In order to extract alpha acid bitterness, brewers must boil their hops, which causes them to undergo a process called isomerization that literally changes their molecular structure from a compound that won’t dissolve in beer at all to one that dissolves easily and remains in beer until it’s time to drink. (You may have seen the letters IBU in a beer description before; this stands for International Bitterness Units and is literally the parts per million of isomerized alpha acids found in a beer.)

Beta acids, on the other hand, won’t contribute bitterness to a beer even if they’re boiled. In order for beta acids to isomerize, they have to be exposed to oxygen and stored for a long period of time. Alpha acid levels tend to decrease over time at roughly the same rate as beta acids isomerize, so the amount of bitterness a hop can contribute to a beer remains pretty static, but most brewers consider the bitterness contributed by beta acids to be harsh and unpleasant, so they try to avoid letting their hops get too old.

Hop growers measure the levels of alpha and beta acids in a hop as a percentage of the hop’s weight, and that number varies from as low as 2.5% to 20% or higher. This number is a useful piece of information for brewers, because even in the same hop variety—Cascade, say—acid levels can change from one year to the next. Knowing the alpha acid percentage in a given crop allows a brewer to adjust hopping rates to achieve the same level of bitterness batch after batch, year after year.

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