Hop cones

Hops

The bittering powerhouse of beer

Boiled Down Version

  • Hops are the flowers of a bine, Humulus lupulus, which are utilized in the brewing process to add bitterness and aroma to beer.
  • Hops contain resin with 𝛼-acids that are converted to iso-𝛼-acids during the boil in the brewing process, giving beer its bitterness. Oxidation products from β-acids, called hulupones, can also contribute to beer bitterness.
  • Essential oils are a small fraction of hop weight but are a highly aroma-active component of hops.
  • Hops are available in many different varieties with varying resin and essential oil compositions and can be broadly divided into bittering, aroma, and dual-purpose types.  
  • Brewers can purchase hops in different processed forms depending on the intended application, including whole cones, pellets, and extracts.

What are hops?

Hops are the flowers (often called cones) of Humulus lupulus, bines that grow from a perennial root called a rhizome each spring, which are native to the Northern Hemisphere and grow best between 30 and 52 degrees latitude (Oliver, 2012, pp. 459-461). Farmers cultivate hops for use in beer production, where they add bitterness, flavor, and aroma to the finished beer and some preservation properties. Hops grow rapidly during their growing season on trellises and typically grow to 4 to 9 meters high (Oliver, 2012, p. 460). At the end of the season, the bines are harvested, cones are separated and kilned, and are sold as whole cones or further processed into products for brewers like pellets or extracts. 

While beer has been around for more than 17,000 years, hops are a relatively new beer ingredient. Hops were likely first introduced to beer in Europe in about 550 BCE in Northern Italy but were not used regularly until around 1000 CE (Mosher, 2017, pp. 16-18). Before 1000 CE, most European beers contained gruit, a mixture of herbs and spices that was a mandatory beer ingredient used as a way to levy tax on brewers. Today, hops are now one of the core ingredients in beer and are in nearly every modern style of beer.

Hop Trellis
Hops growing on a trellis

Composition of hops

Hop cones contain a mix of carbohydrates, waxes, polyphenols, and proteins, but the resins and essential oil components are most interesting to brewers. The resins and essential oils are generated by the lupulin glands of the hop cones and form a yellow powder in hop cones.

Hop cone cross-section

The total resin content of hops is typically 15-30%, of which 5-13% is typically 𝛼-acids (alpha-acids), and 3-8% are β-acids (beta-acids) (Almaguer et al. 2014). Hops are added to the boil during the brewing process, where 𝛼-acids are converted to iso-𝛼-acids which are much more soluble in water and provide bitterness to the beer. 

β-acids do not contribute much to beer bitterness; however, their oxidation products, called hulupones, are also known to add bitterness to beer (Stevens & Wright, 1961).

Essential oils are a complex mixture of hydrocarbons (ex. β-myrcene), oxygen-containing compounds (ex. linalool), sulfur-containing compounds (ex. 4MMP), and more (Almaguer et al., 2014).

Essential oils are a complex mixture of hydrocarbons (ex. β-myrcene), oxygen-containing compounds (ex. linalool), sulfur-containing compounds (ex. 4MMP), and more (Almaguer et al., 2014). Essential oils are generally less than 3% of hop composition but significantly impact the aromas associated with hops in beer. Due to the volatility of hop oils, hop additions intended to impart aroma to the beer are added late in the boil or during or after fermentation (dry hopping). 

Hop varieties and products

Hops are available in many varieties with varying resin and essential oil compositions. Broadly, hops are classified by function in the brewing process as bittering, aroma, or dual-purpose. 

Hops grown for bittering are typically high in 𝛼-acids and intended for addition early in the boil to produce iso-𝛼-acids to add bitterness to the beer, and due to the long boil duration, the essential oils boil off. Aroma hops are generally lower in 𝛼-acids, and the focus is on the essential oils that will contribute to the beer’s aroma. Aroma hops are typically added late in the boil, whirlpool, or during dry hopping to preserve the essential oils. Dual-purpose hops are marketed as a hybrid of the other two styles that can be added for bittering, aroma, or both. 

Once harvested, hops are processed in many different ways for brewer’s use. Some common hop products are:

  • Whole cone hops – the hop cones are kept intact and kiln-dried, and the whole cones are added directly to the brewing process.
  • Wet hops/fresh hops – Hops can be used directly in the brewing process without kilning, however, the hops need to be used very quickly to prevent spoiling. In the fall, “fresh hop” or “wet hop” beers are brewed with hops immediately after harvest without kilning.
  • Pellets – Pellets are produced by first milling the hop cones into a powder, then extruding the powder through a die machine to make the pellets. T90-type pellets are the most common, which originally contained 90% of the original cone matter, but other varieties exist (Hieronymus, 2012, pp. 132-133).
  • Hop extracts – Typically, CO2 extracts of hops that contain high concentrations of resins and essential oils

Major hop-growing regions

Continental Europe, England, United States, Australia, and New Zealand are major hop-growing regions, however, hops are also grown in other locations. Below are some infographics on some of the significant hop-growing regions with examples of traditional hop varieties and common aroma descriptors of the traditional varieties. It should be noted that many regions grow hop varieties outside these traditional varieties with a wide range of characteristics.

US Hop Summary
European Hops
UK Hops

Additional resources to learn more about hops

For The Love of Hops: The Practical Guide to Aroma, Bitterness and Culture of Hops – This book by Stan Hieronymus is easily accessible to readers of all beer knowledge levels and is an excellent general resource for learning the history of hops, how hops are grown, and the fundamentals of how hops are used in brewing.

Yakima Chief Hops – A hop supplier in the US with a good overview of many of the hop varieties available to brewers in the US. They also have learning resources like webinars, sensory worksheets, and more.

References

Almaguer, C., Schönberger, C., Gastl, M., Arendt, E. K., & Becker, T. (2014). Humulus lupulus – a story that begs to be told. A review. Journal of the Institute of Brewing, 120(4). https://doi.org/10.1002/jib.160

European Commission (2021). Hop report for 2021 harvest year. European Commission – Agriculture and rural development. Retrieved October 18, 2022, from https://www.usahops.org/news/2021-usda-nass-national-hop-report

Hieronymus, S. (2012). For the Love of Hops: The Practical Guide to Aroma, Bitterness, and the Culture of Hops. Brewers Publications.

Mosher, R. (2017). Tasting Beer: An Insider’s Guide to the World’s Greatest Drink (2nd ed.). Storey Publishing.

Oliver, G. (2012). The Oxford Companion to Beer. Oxford University Press.

Stevens, R., & Wright, D. (1961). Evaluation of hops. X. Hulupones and the significance of β acids in brewing. Journal of the Institute of Brewing, 67(6). https://doi.org/10.1002/j.2050-0416.1961.tb01830.x

USDA National Agricultural Statistics Service (2021). Hop report 2021. USA Hops. Retrieved October 18, 2022, from https://www.usahops.org/news/2021-usda-nass-national-hop-report