Humulus lupulus: the plant beer brewers are hopping mad for

If you are a beer drinker you owe a lot to April’s organism Humulus lupulus (hop), because it is widely used to flavour and preserve beer.

Female flowers (hops) growing on Humulus lupulus. Image by Dr Hagen Graebner via Wikimedia Commons (CC BY-SA 2.5)

Humulus lupulus is a herbaceous climbing plant native to Europe, western Asia and North America. It is a perennial, sending up new shoots in the spring and dying back in the Autumn (1). The species has separate male and female individuals (known as dioecious). Only the flowers from the female plants (hops) are used for beer brewing.

The hops are harvested in late summer. Traditionally hops were hand-picked mostly by visiting labourers, and after the industrial revolution many of the labourers were working-class London families who would visit the countryside for a working holiday (2). For many of them it was their only opportunity to get out of the overcrowded and polluted city. The introduction of hop-picking machines in the 1960’s meant fewer workers were needed and now most hops are picked by machines.

Women from London’s East End picking hops in Kent, 1932. Image via Shellyb22 on http://www.sharehistory.org (CC BY-NC 3.0)

Today, 98% of hops are grown for beer brewing, but in the past hops have also been used for dyeing, paper-making, rope-making and even medicines. The first written record of hops being used in brewing was in 736 AD, in the records of a Benedictine Monastry near Munich in modern-day Germany (3). Before this, beer had a limited shelf life, especially in hotter climates and would quickly turn sour so, unsurprisingly, over the next few centuries the hop became a popular ingredient with brewers.

Hopgarden in Hallertau (Germany). Image via Wikimedia Commons (released into public domain).

After harvesting, hops are usually dried before being used in brewing.  The hops are added to the wort (sugar-rich liquid produced from malted cereals e.g. barley) and boiled prior to cooling and the addition of yeast to start fermentation (4). Hops contain humulones (α-acids), which have antibiotic properties that help preserve the beer. The boiling of the hops with the wort promotes the conversion of humulones into isohumulones, which give the beer its bitter taste. Other compounds in the hops called β-acids contribute to the aroma but not the bitter taste. However, these compounds can be broken down by the high temperature so hops containing high levels of β-acids are often added near the end of the boiling process. The different amounts of humulones and β-acids in hop varieties contribute to the wide variety of tastes and aromas in beers.

Next time you are drinking beer, raise your glass to the hop.

 

References:

(1) Wikipedia: Humulus lupulus http://en.wikipedia.org/wiki/Humulus_lupulus (retrieved 30/03/14)

(2) http://www.dailymail.co.uk/news/article-1163634/The-hoppiest-days-lives-Recalling-summers-spent-fields.html (retrieved 30/03/14)

(3) Laws (2010) Fifty plants that changed the course of history. David and Charles.

(4) Wikipedia: hops http://en.wikipedia.org/wiki/Hops (retrieved 30/03/14)