Hops, Humulus Lupulus, are one of the most expressive ingredients available to a brewer when crafting a new brew. With hops, you can paint a malty canvas with flavours ranging from earthy spice, to citrusy pine, and even mango and soursop. The key for the brewer, when designing their beer, is to understand where these flavours come from and how to best utilise the hops available to them, and that requires an understanding of the chemistry of hops.
A brewer with 13 years experience in craft beer. She has an MSc. in brewing science and has worked at some of the best breweries on the planet. Most days she can be found at her R&D brewery in south London.
From a brewer’s perspective, one of the valuable parts of the hop flower is found in the Trichome and lupulin glands. This is essentially a sealed box of potential, where the hop plant stores its essential oils, alpha acids, and other compounds; all handily packaged up, as the concentration of compounds would kill the plant if they were stored more generally throughout its growth.
So, we all know that hops make beer taste nice, but what are the compounds in hops that lend them their unique flavour? Over 3000 flavour active compounds have been identified as playing a role in hop flavour and aroma. Many of these exist in such low quantities that, when standing alone, they could not be perceived by the human senses, but in a cascade of aroma, and synergistic amplifying, they help to build a complex sensory experience.
In general terms, hop derived flavour active compounds are separated as coming from the resin or essential oil fraction of the hop cone, and some of the most important compounds are listed below.
The bulk of the essential oils are formed of terpene hydrocarbons, namely myrcene, humulene, caryophyllene and farnesene. Terpenes are naturally produced by plants, and play a role in preventing disease, as well as making the raw plant unpalatable for herbivores, which explains why up to 80% of the essential oil content of hops can be made up of terpenes.
Caryophyllene has intense flavours of spice (namely black pepper), as well as cedar like woody notes, and is found in high concentrations in hops such as Cashmere. Humulene gives beer its characteristic “hoppy” character as well as herbal notes, seen in hops such as Goldings. Farnesene contributes a fresh, almost apple like character and is found in higher levels in Amarillo hops. Myrcene has an earthy character, seen in hops like Comet.
These are all very desirable characteristics, but, much like the essential oils used in a bath, these oils are volatile, and can be lost to the environment during hop processing, and indeed during the process of brewing beer. These compounds are also hydrophobic (imagine trying to dissolve a candle in cold water), so whilst they may be abundant in raw hops, their innate dislike of water means they are not very soluble, so do not survive into finished beer in large quantities when hops are used in the kettle, or as a late hop.
Alpha Acids:
Alpha acids are the principal contributors of hop derived bitterness in beer. They are found in the resin portion of the hop, and when boiled in wort the alpha acid isomerises into iso-alpha acids, which have a perceptively bitter taste. Isomerisation is essentially a reaction that occurs on the chemical level, where a molecule rearranges itself without interacting with other compounds. This transforms alpha acids from a relatively inert compound, into the one that gives beer its unique bitter taste, as the isomerisation reaction makes the iso-alpha acids considerably more soluble in wort. Iso-alpha acids have a double advantage, as they are potent antibacterial agents, helping protect wort and beer from bacterial contaminants that would damage flavour.
Bitterness from iso-alpha acids is measured in International Bittering Units or IBUs, which aim to present a standardised scale of measuring bitterness in beer that both professionals and consumers can understand and use to inform their choices.
The essential oils in hops are not directly analogous to the lavender oil you might add to a bath, but they do contain potent volatile aroma compounds that contribute to the hop aroma in the glass.
All is not lost, however, whilst oxygen may be a bit of a dirty word when it comes to beer quality, the oxidation of these essential oils, both in storage and wort boiling, means that flavour active oxygenated derivatives do survive the process, and into the pint glass. Collectively, these all contribute to kettle hop aroma.
Interestingly these compounds are found in levels below the sensory threshold for each individual compound, but in harmony with other hop compounds (and most likely through some not-yet-fully-understood mechanism) these compounds contribute to beer character.
Despite their hydrophobicity, and associated lack of solubility, some of these terpene flavours can be imparted to beer when hops are added as dry hops. This is when hops are added to fermenting or fermented beer, and the cooler temperatures help prevent some of the volatile losses seen when boiling hops.
There are other oxygenated terpene alcohols found in hops that contribute to flavour. These include Linalool, with powerful citrus and coriander seed notes. Linalool was thought to be one of the most potent contributors to beer aroma, but recent analysis suggests it is simply one of the easiest to detect, thus undue prominence was given its presence. The attention given to hop flavour chemistry has only truly begun to come into its own, and new compounds and their significance are being discovered each year, hop flavour chemistry is a truly fascinating and fast moving area of beer science.
Beta-Pinene gives notes of fresh pine, and freshness that is reminiscent of walking through a forest, with high concentrations found in newer hops such as Idaho 7. Geraniol is intensely floral, whilst Limonene conveys fresh citrus.
Sulphur compounds make up under 1% of the total flavour active compounds found in hops, but they do pack a mighty punch. Sulphur compounds don’t immediately strike inspiration into the heart of many, being responsible for unpleasant aromas in beer such as egg, garlic, and onion, which many find objectionable. Sulphur compounds are unique, in they have such a low sensory threshold, we can detect their presence in very small amounts, and many of these do contribute positive attributes to beer. Many of the pleasant tropical fruit notes we detect in hops have their origins in sulphur compounds.
One group of compounds that are making waves in beer circles are thiols. Whilst winemakers have known about, and utilised, thiols for decades, their presence was only detected in hops within the 21stcentury. Grapes themselves do not contain thiols, rather they must contain the precursors to thiols, and during fermentation these are liberated, giving some wines their distinctive tropical fruit or blackcurrant characters.
Hops contain thiols in very small amounts, which makes them hard to detect. Hops contain free thiols which are imparted into wort during the brewing process, and bound thiols, which like in wine are liberated during fermentation. Brewers must then decide when to use hops with different characteristics at different points in the production process to best utilise the thiols present.
This is a short introduction into the world of hop chemistry, a constantly evolving field, where brewers and researchers are working together to understand what exactly is going on when we use one of the most precious and expensive brewing ingredients, all with the aim of making better beer for the drinker.
One thing to understand about hops, and the chemicals that give them their flavours and aromas, is that these are organic and living things, and their composition is dependent on a myriad of factors. Hops need to be harvested during a golden window, and if that window is missed on either side by even a few days, the results can be a very different hop to what the farmers and brewers were hoping for. Environmental challenges can also vary hop chemistry, with unusually hot or wet growing seasons changing the make-up of hop. This means each harvest varies, and brewers need to adapt their hop usage and processing methods year-on-year to utilise what they have with minimal loss of consistency.
This is compounded by the fact that hop analyses do not include levels for each compound of interest, so brewers are often left with somewhat of a blind spot, to do with what they can. Brewers who use whole hops (rather than homogenised pellets) may have very different hops bail to bail, and must rely on stringent sensory testing and adaptation to give the consumer consistent beer.
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