Style and flavor
The many styles of beer have evolved for many reasons; however, they each have a flavor profile where the ingredients complement each other to deliver a pleasurable brew. On the other hand, many hops can also clash if given the chance. My brother Joel, a professional chef, once quipped about creating "chocolate-covered fish sticks" which, understandably, never came to be. Some things just don't belong together, on your plate or in your glass. Understanding how hops deliver their goods to your glass can help you be creative yet complementary of the rest of the flavors in your beer.
Most of the bitterness contributed by hops occurs during the boil. This is a simple yet important point to remember. Using a variety of hops with a high alpha acid content does not guarantee lots of bitterness. During the boiling process alpha acids in hops are isomerized (the configuration of the molecules are rearranged without changing their content). The resulting isomers, or iso-alpha acids, are more water soluble, stable and bitter than the original alpha acid found in the hop before the boil. Hops boiled for a long period of time (i.e. an hour) will have their alpha acid isomerized into bitter iso-alpha acid to a greater extent than hops boiled for a short period of time (i.e. five or ten minutes). Hops that are not boiled at all will contribute little bitterness to the beer, but can provide many other flavors and aromas (more about that later).
There are two major alpha acids that occur in hops; humulone and cohumulone (adhumulone, prehumulone and posthumulone make up a small percentage of the total alpha acids and have a correspondingly lesser impact on bitterness and flavor). Generally speaking, humulone contributes a "soft" bitterness, while cohumulone lends a somewhat "sharper" bitterness in beer.
Most European "noble" hops such as Hallertauer and Tettnanger have a relatively low level of cohumulone, typically less than 25% of the total alpha acid content. Whereas Cascade hops (an English/Russian cross) may have in the neighborhood of 40% of total alpha acid represented by cohumulone. This subtle difference between the type of bitterness produced by humulone and cohumulone in "kettle hops" (hops used in the boil) may be a consideration when choosing hop varieties for a particular recipe.
Lest you think that cohumulone is not a good thing to have in beer, cohumulone has been linked to the favorable retention of beer foam. Please see the references at the end of this article for sources of detailed hop chemistry that include information on alpha acids.
In addition to alpha acids, beta acids contained in hops also contribute bitterness to beer. However, rather than isomerization, beta acids undergo oxidation during aging prior to use or in the boil to produce their bitterness. Ounce for ounce (or gram for gram) oxidized beta acids are not as bitter as isomerized alpha acid and so provide a lesser, yet significant, contribution to overall bitterness.
Though beta acids take a back seat to alpha acids in their relative impact on beer bitterness, it is important to note that alpha acids are not the only contributor to bitterness when evaluating hops used in a brew.
Flavor and aroma
It is difficult to separate the elements of flavor and aroma as they are inextricably linked with how our brain processes the senses of taste and smell. However, as brewers we can control the impact that hops will have on our tongue and our nose by when and how we incorporate a particular hop into a recipe.
The closer hops are included prior to serving a beer, the greater their contribution to aroma in the beer. Dogfish Head Craft Brewery of Milton, Delaware has gone as far as inventing a device known as a Randall, which is a cylinder packed with whole hops that the beer passes through between the keg and glass to maximize hop aroma in the beer.
A similar device to the Randall is a hopback, which allows beer to pass through a charge of hops between the boil kettle and a wort chiller prior to the beer being fermented. A hopback will result in a beer with more hop flavor and less aroma than beer that is dry-hopped (a practice of adding hops to the secondary fermenter) or passed through a Randall. Many flavors and aromas from hops come from the essential oils contained in the lupulin glands (the yellow powdery stuff) of hops. There are over thirty essential oils in hops that have been identified as contributing to beer flavor and aroma. Those of primary interest include; humulene, myrcene, caryophyllene and farnesene. It is these same terpene-type of compounds that produce many flavors and aromas enjoyed in wine. These essential oils are relatively volatile and can therefore be lost from boiling wort. If the goal is to retain aroma from hops, they should be added near the end of the boil, to a hopback, or added as dry hops in the secondary fermenter.
Humulene is the essential oil most prominent in the European "noble" hops, which gives them their signature aroma. After more than several minutes in the boil, humulene lends a spicy and herbal flavor (think Saaz) to the finished beer.
The myrcene content of American hop varieties is essentially the same as European "noble" hops, but because the Europeans historically processed and stored hops differently than the Americans, noble hops were often considered to contain less myrcene. Myrcene lends a strong citrusy aroma (think Cascade) when added late in the boil, in a hopback, or most prominently as a dry hop in the secondary fermenter. When high myrcene-containing hops are boiled in wort they provide pine and citrus flavors. Incidentally, myrcene is highly prized by the perfume industry. No wonder you can smell that pale ale as it comes out of the tap from across the bar!
Caryophyllene is an essential oil in a variety of plants including hemp, caraway, cloves, basil, oregano, pepper, rosemary and cinnamon. While the precise impact on beer aroma from caryophyllene is not well defined, the list of plants just mentioned may provide some clues to a fertile imagination. When used for a significant period in the boil, caryophyllene produces flavors similar to humulene.
Farnesene usually comprises a small amount of the total essential oil in hops but is more prevalent in European "noble" hop varieties than American hop varieties. The effect of farnesene on beer flavor is not well understood, but two naturally occurring isomers are known to produce the odors associated with green apple and sweet gardenia.
Hop selection and heritage
Armed with this basic understanding of where and how the bitterness, flavor and aroma of hops comes from, one can delve into the information of hop alpha acid, beta acid, essential oil, etc. (provided in the references at the end of this article) to have an idea what to expect when using different varieties of hops. As a general guideline, staying within the bounds of hop pedigree (and therefore similar alpha acid and essential oil characteristics) may be a good place to start if you plan to experiment with combining different varieties of hops.
Most of the hops used in brewing can be traced back to Germany, England and other parts of Europe, and America. These groupings by geographical origin usually correlate to similarities of alpha acid and essential oil content and chemistry, and not surprisingly, beer styles. Therefore, many English types of ale rely on Admiral, Bramling Cross, Fuggle, Kent Golding, Northdown and other hops that originated in the United Kingdom. Likewise, many German beers were built around Hallertauer, Tettnanger and Spalter hops, and Czech beers were designed with Bor, Saaz and Styrian hops.
American brewers have borrowed these techniques from across the Atlantic, depending on the heritage of the brewer, but we have since developed styles of our own with hop varieties such as Amarillo, Cascade, Cluster and others.
Choose your own
Now that you know some of the components that make up a hop's characteristics, it's time to start putting it all together. Keep in mind that staying within a similar hop pedigree or geographical place of origin can help prevent making a potentially incompatible combination of hops with an undesirable beer flavor profile. However, by using information on hop chemistry and pedigree, the artistic brewer can employ his/her imagination to create a truly unique and satisfying beer outside of normal beer style guidelines.
By combining an understanding of how hops contribute bitterness, flavor and aroma with knowledge of the various hop pedigrees and their signature chemistry, you can experiment with lots of different hop combinations to create a signature beer of your own.
Detailed information on the alpha acid content, flavor and aroma producing oils, pedigree of individual hop varieties can be found online. Visit Brew Your Own's new online hop chart (see right), refer to the Best of BYO Hop Lover's Guide or check out some of these hop outlets' Web sites:
Freshops: http://www.freshops.com/usda_hop_desc2.html and http://www.freshops.com/hvariety.html
Jon Stika is an avid homebrewer from North Dakota. He frequently writes "Techniques" for Brew Your Own.