Homebrew Science: The Biology and Chemistry of Hops

Beer, in America, has long been a favorite alcoholic beverage. Inexpensive and readily available, beer is associated with "the common man." Times have changed for the humble American beer. No longer is it limited to the Pilsener variety-rather, everything from a mellow rich porter to the light "hoppy" India Pale Ale can be found in supermarkets and liquor stores, depending upon where you live. As an outgrowth of this, brewing one's own beer as an amateur brewmeister, is now very popular.

Know Your Hops

Good homebrew beer consists of only a few, well-chosen ingredients, including, but not necessarily limited to malted grain, water, yeast, and hops. Each of these ingredients is important in making good beer. Although there is much to learn, generally the enthusiast is already familiar to a reasonable degree with the first three of these ingredients. Oddly however, most beer drinkers have probably never seen hops growing; yet they are one of the most important factors in making beer. What is the botany-what the chemistry-of hops?

The Biology of Hops

Although already utilized in beer making before that, hops were first introduced to the state of Virginia in 1648. Hops is a perennial plant-actually a vine-that can be trained to grow along a path, such as a string, wire, or pole-vertically. Growing conditions must include deep, rich soil, plenty of sun, with only moderate wind. The vines twist counterclockwise, growing to a height of twenty to twenty-five feet. The female hops flowers, sometimes called "cones" are of particular importance to brewers. These cones have scales. On the surface of the scales is a coating called "lupulin"-a golden oily-resinous substance-the desired ingredient! The hops flowers are gathered and dried. If too warm a temperature is used, some of the precious aromatics are lost, so a very low temperature is preferred to achieve the task.

The Chemistry of Hops

Lupulin contains a host of chemicals. Among these are three "alpha-acids"-humulone, adhumulone, and cohumulone. These occur in varying levels and ratios, depending upon the particular variety of hops, and are largely responsible for the various bitter flavors imparted to beer. The oil fraction is a mixture of so-called "essential oils," terpenes, which impart the tangy, spicy, and herbal essence to the beer. They include myrcene, carophyllene, farnesene, and humulene.¹ The brewer changes the flavor of beer not only by choosing the particular cultivar(s) of hops used, but also by the timing of when they are added. As he processes his beer, some hops may be added at the beginning of cooking the wort,² some during the heating, and some added toward the finale. The alpha-acids may change chemically and some of the more volatile components may escape.

¹ Essential oils such as these are generally constructed of isoprene units joined together. For a discussion of the isoprene rule, see the article, "The Chemistry of Nature: The Isoprene Rule."
² Wort is the combination of malt and water that is heated to destroy harmful microorganisms, which would ruin the finished product.

References and Resources:

USDA / ARS - Hops Genetics

Argonne National Laboratory, Newton Ask a Scientist - "Hops"

Hop Growers of America - USA Hops