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Hop, the essence of beer

Hop (Humulus lupulus L.) is an essential ingredient for brewing beer, and contributes a characteristic bitterness, aroma, and fullness. However, during the Middle-Ages, various other herbs including Rhodomyrtus tomentosa and Salix subfragilis, had also been used for brewing beer in Europe. Amongst these herbs, the female flower of hop (hop cone, Photo 1) was eventually selected to be the most suitable for making beer due to its noble bitterness, refreshing aroma, antibacterial activity, and fine foam. By 1516, Duke Wilhelm IV of Bavaria established the beer purity law, which stipulated that only water, barley, and hop could be used as ingredients for beer making.

Figure 1. Hop cone.
Photo 1. Hop cone. Courtesy of Hiroo Matsui and Eiichiro Ono.

Hop is a perennial, dioecious, and climbing plant belonging to the Cannabaceae family, which includes the medicinal plant, Cannabis sativa (hemp and marijuana). During springtime, hop plants adventitiously sprout from underground roots, and climb upwardly twining the wire and after just three months they are able to reach the top of the trellis (6-8 m height) (Photo 2). In early summer, they bloom and then develop hop cones, which are generally harvested within 1-2 months. It is the phytochemicals in hop cones that are crucial for beer character; the alpha acids, terpenes, and polyphenols contribute to bitterness, hoppy aroma, and fullness, respectively.

Figure 2. Hop field.
Photo 2. Hop field. Courtesy of Hiroo Matsui and Eiichiro Ono.

Various environmental and endogenous factors – such as temperature fluctuations, climate change, soil properties, and root age – affect crop quality as well as yields. Such quality and yield traits are essentially derived from the genetic information stored in a plant’s cell as genomic DNA.

Most recently, the draft genome of hop was revealed. By comparing genomes and RNAs between a wild hop and a cultivar, genes related to specialized metabolic processes, especially characteristic aroma and alpha acids, were enriched in the hop cultivar during the ancient long-term selection process by humans — probably reflecting our flavour preferences. For example, enhanced expression of a monoterpene synthase gene in the cultivar is clearly consistent with the drastically increased accumulation of monoterpene-class volatiles (responsible for aroma) in the cultivar, compared to the wild hop. This flavour trait is a likely consequence of continuous human selection for hops based on conventional sensory evaluation alone, prior to the development of any analytical instruments that are used in brewing today.

Figure 3. Beer.
Photo 3. Beer. Courtesy of Hiroo Matsui and Eiichiro Ono.

Today, there are approximately 200 commercial varieties of hop in the world and over 99% of them are used in brewing. In 2014, the world cultivation acreage and yield of hop cones was reported to be about 46,000 ha, and about 83,000 metric tons (alpha acid base was about 8,000 metric tons), respectively. Given that recent beer consumption in BRICs (Brazil, Russia, India and China) and developing countries is rapidly increasing, hop production is expected to gradually increase.

Concomitantly, the craft beer market is also expanding in the United States. The characteristic flavors required for craft brewing are distinguishable from established major brands, and their key qualities comprise unique aromas that are mainly derived from hop. Thus, the craft beer market eagerly awaits the development of new varieties with unique hoppy aromas.

Hop is the last of the three essential ingredients (lager brewing yeast, barley and hop) of beer (Photo 3) whose genomes have been sequenced. Knowledge of the genome sequence will certainly be useful for developing novel hop varieties that can be achieved via accelerated breeding to bring us fresh beers that we have not yet experienced.

Featured image: Hops. CC0 via Pixabay.

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