See list of hop varieties.


Hops are the flowering cones of a climbing vine Humulus Lupulus commonly called the hop plant. Hops provide a number of advantages when used as a beer ingredient, and using hops in beer has been standard practice for over 500 years. The most important advantage hops provide is protection from infection and spoilage by inhibiting the growth of bacteria. Hop compounds help to stabilize the bubbles in beer foam and lead to a long lasting head. They aid in the hot break by helping to precipitate proteins. Boiled hops produce bitter flavors that balance and compliment the sweetness of malt. Hops also provide numerous complex flavor and aroma compounds that can greatly enrich beers made to showcase hops.

History of Brewing with Hops

For thousands of years, fermented grain beverages were made without the use of hops. In Europe it was common practice for a time to season ales with a blend of herbs and spices. Some of these blends may have included hops, but using hops alone came to dominate brewing technique in Eastern Europe by about the year 1000. Hops probably gained popularity after brewers realized that beers made with hops had a longer shelf life – a very desirable trait before modern refrigeration. From there, the practice spread westward as far as Great Brittan by the 1500’s. In recent history virtually all beer is made with hops including some styles that retain the traditional use of other herbs and spices.


Humulus Lupulus is a flowering herbaceous perennial. During the growing season hop plants produce vines that can climb to approximately twenty feet in height. After each growing season the plant dies-back to the roots. This means they can exhibit an impressive rate of growth early in the season. Hop plants are dioecious meaning individual plants are exclusively male or female. Therefore unlike many other species of plant where all individuals produce both male and female reproductive structures, male hop plants produce only male flowers and female hop plants produce only female flowers.

Hops used in brewing are the female flower cones or strobiles. These resemble small green pinecones but are instead soft with high water content. Pinecones function similarly as the reproductive structures of coniferous trees. Male hop plants also produce flower cones, but these are not used in brewing. Hops cones have lupulin glands concealed underneath the green scale-like bracts. These glands produce a large number of different resins and oils including alpha-acids and beta-acids.

Hop Compounds in Brewing

Hops can be used to impart numerous flavors and aromas in finished beer. The exact effect produced depends on the chemical composition of the hops and the way they are used in the brewing process. Different varieties of hops have different characteristic compositions. These compositions vary substantially from harvest to harvest, are affected by growing conditions, and even vary between individual plants.

These three broad categories of chemicals are most important for hop character: the alpha-acids, the beta-acids, and the essential oils. These compounds are all components of the resins produced in the hop’s lupulin glands.

Alpha-acids have very low solubility in water and would have little influence on beer. However in the presence of heat they isomerize (each molecule reconfigures while keeping the same atoms) to form bitter and water soluble iso-alpha-acids. These iso-alpha-acids are the primary source of bitterness in beer. Hop utilization refers to the percentage of available alpha-acids that are converted to iso-alpha-acids. In the context of a brewery, as opposed to a laboratory, the maximum practical utilization is around 30 %. Typically hops are between 5 and 15 % alpha-acid by mass.

Beta-acids have more of an effect on bitterness over long periods of time as the beer ages. When oxidized beta-acids produce bitter compounds. Over time the iso-alpha-acids slowly deteriorate while simultaneously beta-acid bitterness increases. Beta-acids are thought to impart a somewhat harsher bitterness than iso-alpha-acids. Beta-acid bitterness can also be derived intentionally from aged hops. Photo degradation products of beta-acids are one source of skunky flavors in light-struck beer. Typically hops are between 3 and 10 % beta-acid by mass.

Essential oils are volatile plant oils that tend to vaporize around room temperature. They typically account for a small amount of hop mass 0.5 – 3 %, but they are extremely important for hop aroma and flavor. In their original volatile state essential oils produce particular hop aromas. When essential oils are heated they oxidize and degrade into new more water-soluble compounds that result in different hop aromas as well as hop flavors.

Hop Uses

Bittering Hop Addition (60 – 45 min boil or longer)

The most basic use of hops is to provide a simple bitter flavor to balance the sweet flavors from malt. Unchecked the malt sweetness could be unpleasantly overpowering sometimes described as cloying. With an appropriate amount of refreshing hop bitterness an enjoyable mix of flavors can be achieved. Hops boiled for 60 to 45 minutes or longer will produce a substantial bitter flavor. This amount of boil time allows for high utilization of alpha-acids while largely boiling off the essential oils that contribute to hop flavor and aroma.

Flavor Hop Addition (40 – 20 min boil)

When hops are boiled for an intermediate length of time the high temperatures favor production of hop flavor and aroma compounds by oxidizing and degrading the essential oils. The flavor compounds will have less time to boil off than in the case of a bittering addition. There will also be some utilization of available alpha-acids in this time.

Aroma Hop Addition (15 min boil or less)

Hops boiled for very short times or even added around the time the heat is turned off (flame out) are thought to favor hop aroma compound production and more importantly retention of these compounds. Aroma hop additions are similar to flavor hop additions except with less time to boil off the more volatile aroma compounds. There will also be some tiny amount alpha-acid utilization for these additions.

Dry Hopping

This refers to the addition of hops directly to the fermentation vessel without prior boiling. It is a traditional method for imparting the aroma of hops. Since dry hopping takes place at fermentation temperatures it can have a substantially different effect from boiled aroma hop additions. Dry hopping mostly contributes essential oils in their un-oxidized form. Typically dry hops are added after vigorous fermentation has completed as the bubbling carbon dioxide from fermentation can scrub away hop aromas.

Hopback and Whirlpool

These are both techniques for adding hops as the wort is cooled to fermentation temperature. Both impart very little bitterness and have a similar effect to flavor and aroma additions. The moderate temperatures are ideally cool enough to retain essential oils and warm enough to promote oxidation of those essential oils. This potentially makes these techniques a more effective way to produce the hop flavors and aromas that are associated with oxidized essential oils.

First Wort Hopping

First wort hopping is the practice of adding hops to the wort for a period of time prior to boiling. Where a separate mash vessel is used this is usually achieved by adding hops to the boil kettle during the transfer of mash liquor from the mash tun to the boil kettle. This will have an effect on bitterness that is similar to boiling the same quantity of hops for the entire duration of the boil. However the perception of this bitterness is thought to be smoother and mellower. The chemical explanation for this difference in perception is not well understood. First wort hopping is thought have a bigger impact on flavor than a bittering addition. This takes place by providing time for volatile essential oils to dissolve and oxidize at lower temperature. The oxidized products are more soluble and survive as flavor compounds in the finished beer.

Wet Hopping

Wet hopping is the use of fresh off the vine hops that have not been dried in the usual fashion. Fresh hops are approximately 80 % water as opposed to about 10 % for dried hops. With a much larger water weight fresh hops will have a much lower alpha-acid percentage by mass compared with equivalent dried hops. Since wet hops have not been exposed to heat in the drying process they have slightly different character.

Hop Forms

Pellet hops – dried hop cones that have been chopped up and then extruded into pellets. The extensive processing may subtly harm some of the hop character. The compressed form leads to superior storage properties and ease of measurement.

Whole hops – minimally processed whole hop cones that have been dried to remove moisture. Some people consider them to have better aroma for dry hopping.

Hop Plugs – similar in most respects to whole hops but loosely compressed into plugs of predetermined weight.

Hop Extract – concentrated solution of hop compounds. Generally only used by very large-scale breweries for precise control of hop bitterness.

Measuring Hop Bitterness with IBU

International Bitterness Units (IBUs) are defined in terms of the concentration of isomerized alpha-acids specifically milligrams of iso-alpha-acid per Liter. This is equivalent to parts per million (ppm). Most beers have between 10 and 60 IBUs and the perceptible difference is around 5 IBUs. The minimum detectable level of bitterness is around 5 IBUs. Laboratories can measure IBUs using a spectrophotometer, but generally home brewers do not have access to this technique. Often IBUs must be estimated using formula rather than measured empirically.

Factors Affecting IBU

Hop bitterness is determined primarily by the amount of alpha-acids added to a batch and the degree to which the alpha-acids are isomerized and remain in the beer as iso-alpha-acids. The percentage of alpha-acids turned into iso-alpha-acids is known as hop utilization. There are a number of factors that influence utilization and a few different formulas that take some of these factors into account. Unfortunately some of the factors are difficult to quantify and the formula results can only be as good as the measurement accuracy of values plugged into them. Consequently it is very difficult to predict IBUs with a high degree of accuracy. However an approximate back of the envelope calculation is useful when making a recipe for the first time.

The most important factor influencing utilization is boil time. Other important factors are total alpha-acid quantity, wort gravity, and vigor of the boil. A large total amount of alpha-acid can lead to high concentration of iso-alpha-acids that interfere with the solubility of iso-alpha-acid and reduces utilization. Higher wort gravity is correlated with reduced utilization. This is possibly due to higher gravity wort generally producing a larger mass of trub; this trub in turn has more surface area for hop oils to coat and get removed from the beer. The geometry of the kettle and the boil volume will also determine how much kettle surface area is exposed to the boiling wort and will remove some amount of hop oils. A more vigorous boil will result in greater utilization by mixing of wort thoroughly and by providing more energy for isomerization.


  1. Daniels, Ray. 2000. Designing Great Beers: The Ultimate Guide to Brewing Classic Beer Styles. Boulder: Brewer's Publications.
  2. Tinseth, Glenn. 1995-1999. “Glenn’s Hop Utilization Numbers.” Glenn Tinseth’s Hop Page. Retrieved 2012-11-29. http://realbeer.com/hops/research.html
  3. Palmer, John. "Chapter 5 - Hops." How to Brew. Retrieved 2012-12-19. http://www.howtobrew.com/section1/chapter5.html