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Humulus Lupulus: A 10-Minute Introduction to Hops



A Brief History


The humulus lupulus is a herbaceous, climbing plant which can rise up to 20ft growing as quick as a foot a day, producing cone shaped ‘fruits’ which we all know as hops. The female cones (technically ‘strobiles’ not cones, despite being commonly referred to as such) have been used to both flavour and stabilise beer since the 8th century, adding much needed bitterness to the sickly, sweet wort. Before this, beer was often flavoured with ‘gruit’, which was combination of herbs, spices and flowers which imparted bitterness, including heather, dandelions and ground ivy. It was not uncommon for ginger, aniseed, nutmeg and cinnamon to also be used to distract from the intense sweetness of unflavoured beer. Under the Holy Roman Emperor, the government would retain the right to sell gruit as a way of taxing breweries. This right to sell known as the ‘gruitrecht’ and those who held this right (known as gruiters) each had their own closely guarded recipe. Brewers were mandated by law to buy from these vendors. Gruit was pushed out by the hop’s growing popularity in Europe around the 11th century, in part for it’s more effective flavouring and preservative qualities, but also as a way to cut the revenue of the Roman Catholic Church during the Reformation, who held the monopoly on gruit.

Hops allowed brewers to make stronger beer which lasted longer, a discovery made by the Benedictine abbess Hildegard of Bingen. This allowed for breweries to sell past their local region, an activity which was before prohibited by unhopped beer’s quick spoilage. This gave way to the start of truly commercial breweries that could export their beer further and further. Germany was the first country to truly master hops and began exporting around the world, with Bremen in northwest Germany sending beer to northern Europe and Scandinavia. By the 14th century, larger German breweries were making more than a thousand barres a year. Brewing had truly become big business with large corporate interests and influential power, all thanks to the mighty hop.

The hop’s domination was resisted in Great Britain and England was the last country to adopt the plant until as late as the 16th century. In some counties there was even a legal distinction between ‘ale’ (unhopped) and beer (hopped beer, imported from Holland). Only in recent times have the two words become synonymous with each other. By the 18th century, though, hops had taken gruit’s place in Britain’s breweries.

Hops had changed the face of beer. Before, it was sweet, weak and had no shelf life. Even with herbs and spices added, it didn't have much balance and still spoiled quickly. With the arrival of the hop, beer could be made stronger, more consistent and could survive long journeys. Styles varied between locations but the unmistakable flavour of hops has become intrinsic to what we all think about beer.

Hops Around the World

In the 21st Century, hops are grown all around the world, although specifically do better in drier climates despite needing a lot of water to grow. The Hallertau in Bavaria, Germany, is the largest continuous hop planting area in the world, producing around one third of the worlds hops whilst Yakima in Washington, Willamette in Oregon and Conyon County in Idaho pushed the USA to the top of the world’s hop producers in 2015 accounting for 42% of the world’s hops. The cultivation of hops was introduced to the Maidstone area of Kent around the 15th century, which we know as the birthplace of the popular East Kent Golding hop.


Hop Farm in Yakima Valley, Washington

The amazing thing about hops around the world is that the different soils, topography and climates in each country give different hops completely unique flavour profiles, which allows for an diverse plethora of beers in the today’s market. Germany and the Czech Republic are home to the ‘noble hops’, a variety of hops which are low in bitterness but high in aroma. The noble hops comprise of Hallertauer Mittelfrüh (commonly known as Hallertau), Tettnanger, Spalt and Saaz (The mighty Czech Pilsner Urquell, a Bohemian Pilsener, is the ultimate showcase for Saaz). The low bitterness and strong aroma, combined with the unique earthy spiciness of the noble hops are distinguishing characteristics of European-style lagers like the Pilsner, Dunkel and Oktoberfest/Märzen.

Australia and New Zealand also produce their fair share of hops, despite perhaps being best known for their wine. Nelson Sauvin is probably the most well-known, imparting aromas similar to Sauvignon Blanc grapes, and with a flavour profile of lychee and melons. Galaxy is also full of fruit, with orange and passion fruit flavours, and Motueka is a combination of spiciness (as it is a descendent of the Saaz) along with citrus tones due to it’s New Zealand birthplace.

The USA has a far more distinct flavour profile, giving big citrus and zesty notes to beer. Because the country is so vast, American hops aren't all the same. They can range from the woody, pine-like notes of Chinook and Columbus, to the grapefruit tones of Cascade and Simcoe. Centennial offers a more floral profile, but the real star of the show is the relative newcomer Citra developed in 2009. Like it’s name suggests, Citra is full of citrus-imparting orange, pineapple, mango and other stone fruit. Mosaic and Amarillo are also big hitters in the world of fruity hop-forward beers. These hops are often described as ‘New World’ hops, and have quickly overtaken the more traditional hops as they offer big intense flavours, satisfying the new wave of craft beer drinkers who are happy to pay for large hop bills in the hunt for bigger, more flavourful beer.

English hops are far more grassy and floral, giving Britain’s traditional real ale it’s unmistakable earthy qualities. Fuggles and East Kent Golding are most popular in traditional bitters, and Challenger is used in many English pales. All have an earthy, woody quality which can often be drowned out in the new wave of American tropical hop-forward beers, which many find more exciting.


Breaking Down the Cone

The chemical compounds in the hop cone are essential to explaining its importance in the production of beer. Under each petal of the hop cone are little globules of yellow resin known as lupulin. This resin is filled with essential oils and vital acids which dictate which flavours and aromas that variety of hop will contribute to the beer.

The bitter value of hops comes from a collection of five different acids known as the alpha acids, or humulones. When boiled, these acids isomerize and release bitterness. The five acids are humulone, cohumulone, adhumulone, posthumulone and prehumulone. Whilst not a lot is known about the last three, humulone is the primary alpha acid and holds impressive anti-bacterial qualities. It is known for adding ‘soft’ bitterness, to beer. On the contrary, cohumulone has a much more harsh bitterness, but often found in much lower quantities than it’s softer counterpart. Alpha acids impart bitterness over time, so the timing of their addition in brewing is vital (although more on that later). In fact, the International Bitterness Unit (IBU) is simply a measure of the iso-alpha acids in a beer (one IBU is the same as one milligram of iso-alpha acid per litre of beer).

Another group of acids in the hop cone are beta acids, which are immediately soluble and don't need to be isomerized. The three main types of beta acids are lupulone, colupulone and adlupulone. Whilst alpha acids break down instantly when added to the boil, beta acids do the same over a much longer period of time. For this reason, beta acids only really make a difference in beers that have been layered or aged. This would explain why noble hops work better in lager styles as they have a higher level of beta acids than most other hops.

About 1 to 4% of lupulin resin is essential oils. Whilst scarce, these oils massively impact the flavour and aroma of a beer. The vast majority of the mixture is made up of just four oils; myrcene, farnesene, caryophyllene and humulene. Many of these are present in other botanicals, which explains why we may make associations between the aroma of a beer and certain spices and herbs. Myrcene can smell like bay leaves and thyme, caryophyllene is present in cloves and black pepper and humulene is part of the marijuana plant, to which hops are closely related.

Different Forms


Hops don’t just come in one form either, and any variety of hop can present itself in a number of different ways, all of which carry their own advantages and disadvantages.The most well known, or well represented, is the whole leaf. This is the entire hop bud which looks very similar to an artichoke (albeit a lot smaller). They are often used following the harvest, as they are hard to store and hard to transport, often going bad quicker than other forms. They also soak up more wort which leads to more loss when boiling. However, unlike other types, they float which makes them easier to strain and, when fresh, give off an amazing aroma.

An alternate type is hop pellets, which are dried hops pressed into pellet form, allowing for easier translation and storage. They are more concentrated which means they are more cost effective, making them the most common type of hop delivery method in brewing. However, unlike whole leaf hops which float, pellets form a thick hop sludge in the boil kettle which can block filters and make a beer very murky. This can be solved by dry hopping a muslin bag, although this does lower the surface area of the hops in the boil which makes the extraction of essential oils and alpha acids less efficient.


Hop Pellets

As a form of compromise between these two types, hop plugs are whole leaf hops compressed into small balls. They generally give a better flavour and aroma than the same hop variety in pellet form, making them good for dry hopping. However, plugs soak up beer in the same way as whole leafs (whereas pellets soak up very little) and have a poor surface area compared to pellets.

Hop extracts can also be used, which are basically a processed chemical form of hops. They are often used in large breweries due to their little storage space and relative cheapness. However, some craft breweries also use them as a way of increasing hop character in the beer without soaking up too much of the wort. For example, Russian River Brewing uses hop extract in conjunction with whole leaf hops in their world famous Triple IPA ‘Pliny the Younger’.

Whichever form is used, the freshness of the hop is the most important factor. Hops should be stored in bags which don't allow for any oxygen to permeate and should be kept in a fridge or freezer. Warm hops that have been exposed to oxygen can go off very quickly, and can lose most of their bittering qualities within a few months. The easiest way to tell if batch of hops has gone off is to smell; fresh hops have a wonderful fresh, grassy, herbal smell whereas oxidised, old hops have a unmistakable cheesy smell and often have started to go brown.


Uses in Brewing


Preservation The use of hops in beer is not just for flavouring, but they also significantly increase the shelf-life as they act as a natural preservative. By the start of the 18th century, hops were added directly to the cask after fermentation, allowing beer to stay fresher for longer. This gave birth to the India Pale Ale, a strongly hopped beer with a high ABV which could survive the voyage to India to satisfy the British soldiers. Sticky, sweet wort (unhopped, malty water that is rich in sugars from the boiled grain) is the perfect buffet for microbes and bacteria in the air to feast upon. Sometimes this is desired, with some brewers leaving their beer on large open trays known as ‘coolships’ in order for the natural bacteria in the air to ferment their beer, instead of physically adding yeast, a process known as ‘spontaneous fermentation’. This creates beers that are very sharp and sour, and the process is extremely hard given the lack of control the brewer has, and a very specific climate is needed. In most beer styles though, outside bacteria and oxygen is undesirable and can lead to infection and off-flavours. The alpha acids in beer not only provide bitterness but also an antibiotic characteristic. During ‘the boil’, where the hot wort is put on a rolling boil and the hops are submersed, the alpha acids in the lupulin glands in the cone are converted to soluble iso-alpha acids, a process known as ‘isomerization’. Boils can last anywhere between 45 to 90 minutes, although normally take an hour. The longer they are left to boil, the more alpha acid is released resulting in more bitterness along with more antibiotic characteristics. Different hop varieties have different levels of alpha acids in them, meaning that some hops left for the same amount of time in the boil could still produce different levels of bitterness (and in turn shelf-life) . For example, the hop Summit™ has an very high alpha content that is around 17% of it’s weight, compared to

Wai-iti with an alpha content of just over 3%.

Bitterness, Flavour and Aroma The extraction of alpha acids from hops is not only linked to the preservative nature of the beer, but also the taste. Both the shelf life and the bitterness level (IBU) of the beer is determined by how much alpha acid has been extracted from the hops, or in other words how long they have spent in the boil.

Hops that are added at the start of the boil and left for the duration will have had most if not all of their alpha acids extracted and the hop would only contribute a bitter quality to the beer. For this reason they are known as bittering hops, or kettle hops. They often have a high alpha acid content (around 10% of overall weight). The aromatic oils in these hops are boiled away, leaving little flavour or aroma but a lot of bitterness. High alpha hops often have little flavour or aromatic qualities, so it makes sense to use them to bitter a beer, instead of using double or triple the amount of a lower alpha hop variety.

If, however, hops are added at the end of the boil, with 10 minutes to go or even right at the end (known as ‘knockout’ or ‘flameout’), the alpha acids don’t have the time to be extracted and their essential oils don't have time to evaporate, retaining the pungent hop aroma. In some cases a ‘hopback’ can be used to run the wort through a chamber of fresh hops to maximise the aromatic qualities extracted from the hop. Essential oils which are responsible for aroma are extremely volatile and are very easy to boil away, so by adding hops at the end of the boil means the oils (along with their strong aromas) are kept within the beer.

A middle ground can also be found between these two. If hops are added halfway through the boil, at around 30 minutes for example, a compromise is found between complete isomerization of the alpha acids (bittering) and retention of the essential oils (aroma). These mid-way hop additions give flavour to the beer, giving characteristics unique to that hop variety, and both high and low alpha varieties can be used at this stage. A combination of different hop types at this stage can lead to a complex flavour profile of the beer.

Hops can even be added after the boil, when the beer is fermenting away. This is known as “dry hopping” and is added towards the end of the fermentation process when the beer is nearly ready. If added earlier, the aromatic qualities of the hop would be evaporated in the carbon dioxide that is produced from the yeast turning sugars into alcohol. The lack of heat means that the hops don't break down in the same way and no isomerization happens, giving the beer an even more intense aroma.



From their discovery right up until today, hops have completely changed the face of beer and brewing forever, and they are likely to never be replaced as the most vital and interesting part of the brewing process. The sheer variety of different types and the ability to conjure up flavours and smells of other fruits and spices is astounding, making them the perfect companion to breweries that are becoming more experimental and exploratory than ever.

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