by Peter Garofalo, updated in 2017 by Scott Bickham


Hops are the spicy and bitter counterpart to the malt backbone of beer; they are essential to beer as we know it. Prior to the widespread acceptance of hops, various bitter herbs, seasonings, and spices were used to balance the malt sweetness. Hops also contribute many secondary attributes to beer: they provide a measure of bacteriological stability, aid in kettle coagulation, and contribute to a stable head.

Brewers’ hops are the cone-like flower of the Humulus lupulus vine, a relative to the cannabis plant. The essential ingredients are concentrated in the lupulin glands, located at the base of the bracteoles, or leaves of the cone. The bracteoles are attached to the central stem of the hop cone (strig). The lupulin resin contains alpha acids and essential oils that contribute the characteristic bitterness, flavor, and aroma that are associated with hops in beer. The amount of alpha acid is usually expressed as a weight percent, and is determined by extractive and chromatographic methods.


Many varieties of hops are known, though they are generally divided into two subsets: aroma and bittering hops, although some are considered to be “dual-purpose.”  The finest of the aroma hops are referred to as “noble,” due to their prized aromatic and subtle bittering properties; the noble varieties include Saaz, Spalt, Tettnanger, and Hallertauer Mittelfrüh, although some sources list other varieties. Aroma hops are generally lower in alpha acid content, but contribute desirable flavor and aroma characteristics. Bittering varieties are higher in alpha acid content, but their flavor and aroma characteristics are generally considered to be less refined. There are no hard and fast rules about aroma, bittering, and dual-purpose hops; the categorization is subjective. Generally, aroma hops consist of such varieties as Saaz, Tettnanger, Hallertauer, Spalt, East Kent and Styrian Goldings, Fuggles, Cascade, Willamette, Liberty, Crystal, Ultra, and Mount Hood. Bittering varieties include Brewer’s Gold, Nugget, Chinook, Eroica, Galena, and Bullion. Dual-purpose varieties include Northern Brewer, Columbus, Cluster, Perle, and Centennial, among others.

Hops were introduced in beer making prior to 1000 A.D., and came into widespread use in the 16th century when they were legislated as a required ingredient in the famous Reinheitsgebot, or German Beer Purity Law of 1516. Hops are still grown in many of the traditional regions, such as the Zatec region of the Czech republic, home of Zatec Red, or Saaz variety. Hop varieties have been enriched through intensive cross-breeding, which has given us many of the newer, disease-resistant varieties.

Bitterness arises from the alpha acids, which consist of humulone, cohumulone, and adhumulone; the proportions of each will vary according to hop variety. They are isomerized into iso-alpha acids in a vigorous boil, rendering them much more soluble in the wort, in addition to increasing their bitterness. The essential oils, which contribute to flavor and aroma of the finished beer, consist of dozens of compounds. Many of these are volatile, and hence do not survive extended boil times. For this reason, flavor and aroma hops are generally added during the last 30 minutes of the boil.

Brewing hops are available in many forms: whole hops, plugs, pellets, and extracts. Whole hops are simply dried hop cones, and are the most traditional form of hops. Plugs (also known as type-100 pellets), are whole hops compressed into 1/2-ounce (15 gram) disks. Pellets are ground into powder, and then extruded through a die. Hop extracts include isomerized extracts, which may be used to add bitterness; hop aroma essences are also available.

Bitterness from Hops

The bitterness imparted by hops is quantified in various ways, with varying degrees of precision. The simplest method is the Alpha Acid Unit (AAU), also known as the Homebrew Bittering Unit (HBU). This basic measure is simply the weight of hops in ounces (one ounce = 28.35 grams) times the alpha-acid content, expressed as a percent. In order to be meaningful, the brew length must be specified when using AAUs or HBUs. The main downfall of the AAU/HBU quantification method is that it describes the potential bitterness without accounting for many critical factors which determine the actual bitterness.

The more precise method of quantifying hop bitterness is the International Bittering Unit, or IBU. The IBU is a measure of the concentration of isomerized alpha acids present in the finished beer, and is expressed in milligrams per liter, or parts per million (ppm). The relationship between the quantity of hops used and the IBU level depends on many factors: length of the boil, wort gravity, vigor of the boil, wort pH, age/condition of hops, hop form (whole, plugs, or pellets), hopping rate, plus several other less important elements. The relative IBU level does not always translate directly to the perceived bitterness of the finished beer. The ionic makeup of the brewing water, particularly carbonate and sulfate levels, directly affect the perception of bitterness. The degree of attenuation also plays a role in the amount of bitterness that is needed to reach a balance for a given style.

The IBU content of a beer may be expressed as: IBU = 7489 x (W x A x U)/V, where 7489 is a conversion from milligrams/liter to ounces/gallon, W is the weight of hops in ounces, A is the alpha acid content as a decimal, U is a percent utilization factor, and V is the final volume of beer, in gallons. [In metric, this formula is IBU = 1000 x (W x A x U)/V where 1000 is a conversion from milliliters to liters, W is the weight of hops in grams, A is the alpha acid content as a decimal, U is a percent utilization factor, and V is the final volume of beer in liters.] The most important variable in the equation is the utilization factor, which depends on the aforementioned parameters. Utilization normally tops out at about 30 % in the home brewery; often, it is significantly lower. Some additional factors which affect the value of U are boiling temperature, whether or not hop bags are used, and filtration losses. U is the product of all correction factors and may be estimated by any of several methods for each set of conditions. In any case, a different utilization is typically assumed for each hop addition (when multiple additions are used); in this manner, the IBU contribution for each hop addition may be estimated, and then totaled. It should be noted that the only way to determine the IBU level in the finished beer is through a direct measurement in the laboratory.

The relationship between the various correction factors and hop utilization is often not simple, but certain tendencies are well known. Utilization is reduced by: reducing the contact time of hops with boiling wort; reducing the boiling temperature of the wort; increasing the wort gravity; using whole hops instead of pellets; increasing the hopping rate; using hop bags to contain the hops during the boil; using older hops; decreasing wort pH; using more flocculent yeast; and filtering the beer. Some bitterness is also lost to oxidation or staling of the finished beer.

The desired level of bitterness, as measured by IBUs, varies widely for different styles. For example, an Festbier is expected to have about 18 to 25 IBU, while a Czech Premium Pale Lager might have 30 to 45 IBU. Each style has different bitterness, flavor, and aroma expectations; only the a-acid level may be accurately quantified. Another way to characterize the bitterness of a given style is the BU:GU ratio introduced by Ray Daniels. This is simply the IBU content divided by the last two digits of the original specific gravity.

Hops are often added at different points in the brewing process, with the goal of contributing bitterness, flavor, or aroma to the finished beer. Bittering hops are usually most efficient at yielding their iso-alpha acids with 60 to 90 minutes of vigorous wort boiling. Hops boiled for 10 to 40 minutes are often referred to as “flavor hops,” since they contribute less bitterness, but retain some essential oils which contribute characteristic flavors. Hops added at or near the end of the boil contribute little or no bitterness, some flavor, and aromatic quality to the finished beer. Hops added during or after fermentation (“dry” hops) contribute a fresh hop aroma.  Hop-derived compounds can also be altered in the finished beer. Oxidation (staling) reduces bitterness, and may also add a harsh edge to flavor, as well as diminishing aroma.

First Wort Hopping

The technique of first wort hopping is also gaining favor among homebrewers. It essentially consists of adding a portion of the hop charge (some insist that most or even all of the hops should be added at this point) to the first sweet wort runnings from lautering, during which time the higher pH is thought to extract some of the finer qualities of the hop flavor. The hops are kept with the wort throughout the boil, and contribute a more refined bitterness, though the exact amount is a matter of debate. What is beyond debate is the fresh hop flavor imparted by first wort hopping; some have speculated on possible formation of stable complexes, or perhaps esters, at the temperature range encountered in the mash runoff. Another possibility is the removal of undesirable, somewhat volatile constituents during the extended heating and boiling time; this coincides with the observation that even with increased IBU levels provided by first wort hopping, the resulting bitterness is usually described as smoother and more pleasant. Surprisingly, the technique also contributes aroma; in fact, first wort hopping has been suggested as a replacement for late hop additions. Less clear is how the aroma boost compares to dry-hopped aroma. The technique is an old German method that was originally used for hop-centered styles, such as Pilsener; recently, it has gained favor for a wide range of homebrewed styles. It was originally intended as a means for extracting more bitterness, and it has been found (analytically) to provide a favorable bittering and flavor compound profile.


Hop varieties are often associated with particular beer styles; in fact, some styles are virtually defined by their hop character. British ales are normally associated with native hop varieties (East Kent Goldings, Northern Brewer, Challenger and Fuggles, for example), and most are expected to embody the characteristic flavor and aroma attributes associated with these hop varieties.  Golding hops provide an earthy, peppery and lemon-like character; Fuggles hops are also earthy, but with cedar, tobacco and floral notes; while Challenger hops have a tea-like earthiness with light citrus notes.

Continental styles, particularly the more hop-oriented ones, are also often associated with more local Continental hop varieties. Czech Premium Pale Lagers, for example, are partially defined by the characteristic spicy Saaz aroma and flavor. On the other hand, German pale lagers are usually associated with German hop varieties, such as Tettnanger, Hallertauer Mittelfrüh, and Spalt. All of these noble hop varieties provide a slightly peppery or woody spiciness, often accompanied by a soft floral or light citrus character.  Altbiers, which often have subdued hop aromas and flavors, have a clean bitterness that is associated with the use of low-alpha Spalt hops. Even less hop-accented lager styles such as Dunkles Bock and Munich Helles benefit from the soft hop bitterness produced by low-alpha noble hop varieties.

American styles, especially such hoppy examples as American pale ale and American brown ale, benefit greatly from the floral, citrusy character of the dominant American varieties such as Cascades, Centennial, Columbus or Chinook. In fact, it is often the hop character that sets these styles apart from their European prototypes.  Cascade and Centennial hops both provide grapefruit and floral notes, with the latter usually being a little more floral.  Columbus and Chinook are both high-alpha hops that provide assertive pine-like or pine resin character, along with herbal and light citrus notes. 

Many hoppy beer styles such as American IPA, Double IPA and American Strong Ale are made with generous additions of the so-called “New World” hops.  Amarillo is the eldest member of this family and provides orange-blossom notes when used in modest amounts, but become catty when larger amounts are used as finishing hops or for dry-hopping.  As the name implies, Citra hops impart an orangey or orange-rind character that is often accompanied by tropical fruit notes such as mango, passion fruit and pineapple.   Simcoe is another popular hop that has notes of grapefruit, pine and tropical fruit when used in small amounts, but at higher levels, has an aroma that recalls raw onions.  Another New World hop variety that judges should recognize is Mosaic, which has a distinctive blueberry-like aroma, along with notes of tangerine, pineapple and ripe peaches.

It is important to note that the region of cultivation is as important as the hop variety in determining the character of the crop. Classic European hop varieties exhibit different characteristics when they are grown in the United States than when the same varieties are grown in European soil. Therefore, the place of origin is every bit as important as the genealogy when selecting the appropriate hop variety for a particular application.

Further Reading

  • Haunold and G. Nickerson, “Factors Affecting Hop Production, Hop Quality, and Brewer Preference,” Brewing Techniques, vol. 1, no. 1, 18-24 (1993).
  • Mark Garetz, “Hop Storage: How to Get–and Keep–Your Hops’ Optimum Value,” Brewing Techniques, vol. 2, no. 1, 26-32 (1994).
  • Glenn Tinseth, “The Essential Oil of Hops: Aroma and Flavor in Hops and Beer,” Brewing Techniques, vol. 2, no. 1, 33-37 (1994).
  • VanValkenburg, “A Question of Pedigree–The Role of Genealogy in Hop Substitutions,” Brewing Techniques, vol. 3, no. 5, 54-59 (1995).
  • Don Put, “Home Brewery Basics: The Pursuit of Hoppiness–Part I: From Farm to Market to Brewery, Hops Lead a Fascinating, Delicate Life,” Brewing Techniques, vol. 4. no. 2, 12-19 (1996).
  • Don Put, “Home Brewery Basics: The Pursuit of Hoppiness–Part II: The Care and Feeding of Hops in the Brewhouse,” Brewing Techniques, vol. 4. no. 3, 18-23 (1996).
  • W. Lemmens, “Hops in America: a 20-Year Overview,” Brewing Techniques, vol. 4, no. 6, 56-65 (1996).
  • Jim Busch, “How to Master Hop Character–Exploring Hop Flavors and Aromas for More Targeted Recipe Formulation,” Brewing Techniques, vol. 5, no. 1, 30-33 (1997).
  • Mark Garetz, “Boost Hop Bouquet by Dry-Hopping,” Zymurgy, vol. 16, no. 2, 42-52 (1992).
  • The Classic Guide to Hops, Zymurgy, vol. 20, no. 4 (1997).
  • George Fix, Principles of Brewing Science (Brewers Publications, Boulder, CO, 1989).
  • Gregory J. Noonan, Brewing Lager Beer (Brewers Publications, Boulder, CO, 1986).
  • Gregory J. Noonan, New Brewing Lager Beer (Brewers Publications, Boulder, CO 1996).
  • Dave Miller, The Complete Handbook of Homebrewing (Garden Way Publishing, Pownal, VT, 1991).
  • Mark Garetz, Using Hops: The Complete Guide to Hops for the Craft Brewer (Hop Tech, Danville, California, 1994).
  • Ray Daniels, Designing Great Beers-The Ultimate Guide to Brewing Classic Beer Styles, (Brewers Publications, Boulder, CO, 1996).
  • Randy Mosher, The Brewer’s Companion, Alephenalia Press, 1994.