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The Chemistry of Hops and Beer

Everything you need to know about hops. (Issue 35)
The Chemistry of Hops and Beer

With IPA as the leading craft beer style in America, even casual craft beer drinkers are aware that hops provide the bitter flavor in their favorite beers. But the way that hops are selected and their flavors and aromas are extracted is less well-known – more the territory of “mad chemist” brewers, who must draw on a knowledge of biology, chemistry and math to craft the next big thing. In fact, the ever-evolving chemistry between hops and beer is one of the most fascinating and intricate aspects of the craft beer universe. Here's an in-depth look at the science behind hops as well as how four popular hop varietals affect the flavor, aroma and other characteristics of beer.


Hops are the cone-shaped female flower of the vine-like plant humulus lupulus, and the important ingredients are concentrated in the cone’s lupulin glands. Hops are harvested, then dried and processed into pellets, plugs, extracts, or left in their cone form. 

During the brewing process, hops are added at the beginning of the boiling stage to give the brew its bitter flavor and more are added at the end of the boil for aroma and flavor. Hops also act as a preservative by inhibiting spoilage bacteria during a beer’s shelf life and they help to stabilize foam.

The four basic ingredients in brewing: malted barley, hops (in pellet form), a yeast slurry and water. Like scientists who experiment to learn how different substances interact, combine and change, modern brewers are known to get creative with any type of hop at any stage of the brewing process. Photo courtesy pFriem Family Brewers.

Europeans probably began cultivating hops for beer around the eighth or ninth century, and the hop heads of the 14th and 15th centuries are to thank for hops overtaking other herbs and spices when it comes to flavoring beer. Today there are many dozens of varietals, sometimes labeled “old world” and “new world.” Ideal growing conditions exist between the 35th and 55th parallels in both hemispheres, so a few regions produce the majority of the world’s hops. The United States (“new world”) and Germany (“old world”) are the leading producers.

Bruce Wolf, owner of Willamette Valley Hops, said that the growing region can have a significant impact on hop characteristics. “Not just the dirt, but the climate and the weather pattern throughout the spring and summer dictates a lot of what the hop profile is,” he said. So the challenge, he noted, is ensuring consistency for brewers who depend on certain hop varietals for their beers.


Those early hop heads knew hops made beer awesome, but thanks to science, we have a better understanding of why.

There are two general categories of hops: bittering hops, which have a higher alpha acid content and contribute more bitterness but tend to impart a less refined flavor and aroma; and aroma hops, which are lower in alpha acids, but contribute desirable flavor and aroma characteristics. Some hops are dual-purpose.

A sample is prepared for the analysis of bitter acids in hops at Haas, a hops supplier, hops breeder and research brewery. Photo courtesy Haas.

Alpha acids such as humulone, adhumulone and cohumulone are very important to understanding how hops add bitterness. They are expressed in a weight percent generally ranging from 2 to 19 percent. Many European hops have alpha acids in the 5 to 9 percent range, which means these hops are used for aroma and flavor, or they may be dual-purpose. Newer American hop varietals are typically higher in alpha acids, around 8 to 19 percent, and contribute more bitterness.

Boiling extracts the desired characteristics. In the case of bittering hops with higher alpha acids, longer boils (usually 60 minutes or more) will cause the alpha acids to isomerize. That’s the process by which a molecule is transformed into another molecule with the same, but rearranged, atoms.

With this chemical switcheroo complete, the alpha acids are now iso-alpha acids, also known as isohumulones. These bad boys are responsible for the bitter flavor of beer. So beers with high IBU counts (international bitterness units) are those that have a higher concentration of iso-alpha acids. The most common way to measure IBUs is called spectrophotometry, which can also be used to measure a beer's SRM (Standard Reference Method for specifying color) and usually involves fairly expensive equipment.

Humulone Isomerization