I thought I’d write up some science behind hop aroma (and flavor), as I personally find it really interesting. This is part one (the introduction) of a multi-part mini-essay on hop science and factors influencing hop flavor and aroma in beer. I’m definitely no expert in the area, and have compiled most information from various scientific articles, books and web sources (list of sources will be found at the end of each post). Hope you enjoy it!

Hops are, along with malt, yeast and water, one of the main ingredients of beer, and they can, depending on beer style, have various influences on the aroma and flavor of beer. After tasting through various single hop beers (i.e. beers hopped solely with a single hop variety; see taste notes of Brewdog’s Bramling X, Citra, Nelson Sauvin and Sorachi Ace IPAs) and brewing some single hop beers myself, it has become evident (at least for me) that hops have an enormous influence on the flavor and aroma of beer. Through homebrewing I’ve also become aware that getting a strong hop aroma and flavor can be difficult, especially in lighter styles, since hops bring bitterness to the beer as well. Hop aroma also fades unfortunately fast. But how do hops contribute to the aroma and flavor of beer?

(Fresh hop cones. Source)

The hops used in beer are flower cones from the female Humulus lupulus L. plants. Over 100 different hop cultivars are grown globally today, and they all contribute various characteristics to the aroma and flavor of beer. Hop plants are usually harvested during the autumn/late summer, which means Northern Hemisphere harvest usually occurs between late August and early October, while Southern Hemisphere harvest usually occurs around March. Hops are rarely used fresh (though exceptions occur), and are dried after picking from around 80% to 10% moisture content to prevent molding. After drying, the whole hops are usually either vacuum-, CO2- or nitrogen-packed (to prevent oxidation), or processed into pellets (i.e. frozen, crushed with a hammer mill, homogenized, pelleted and packed under vacuum or inert gas), after which they are preferably stored cool before use (to maximize stability). Hops can also be processed into various non-isomerized and isomerized alpha acid extracts and hop oil extracts, and these are gaining popularity at breweries (because of less wort loss to hop absorption).

(Dried hop cones (above) and hop pellets (below). Source 1 and Source 2)

Hops produce and contain an aromatic essential oil (up to 3% by weight), and this oil is located in the yellow-colored lupulin glands of the hop cones together with the various acids (hop resins) that cause bitterness in beer. The essential oil and hop resin compositions are extremely complex and vary between different hop cultivars. Over 485 compounds have been identified in hop oil, while researchers predict over 1000 compounds are present. The hop resins and oil are transferred to the beer through the boil (or through post-boil contact with the wort). The essential oil is volatile, meaning most of it will be lost with steam when hops are boiled for a long time. Hence, hop aroma is added to beer by adding hops late to the boil (in the last 15 minutes of the boil), post-boil but pre-fermentation (e.g. whirlpool hops), and post-fermentation (e.g. by dry hopping or using a hop rocket). The hop compounds also undergo reactions during fermentation, meaning researching and analyzing the contribution to beer aroma from hops is complex.

(Lupulin glands. Source)

This is the end of the short introduction to the mini-essay. Next up will be a write-up on the major compounds found in hop oil and how they contribute to the flavor and aroma. The next part will hopefully be released later this week can be found here.

Sources:

• Briggs, D., Boulton, C., Brookes, P., Stevens, R., 2004. Brewing: Science and Practice. Cambridge: Woodhead Publishing.
• Kishimoto, T., 2008. Hop-Derived Odorants Contributing to the Aroma Characteristics of Beer. Doctoral Dissertation, Kyoto University.
• Schönberger, C., Kostelecky, T., 125th Anniversary Review: The Role of hops in Brewing. Journal of the Institute of Brewing 117 (2011) 259-267