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Learn & Discover

Hop Compounds 

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.

 

Charlotte Cook

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. 

 

Hop Compounds

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. 

Charlotte Cook

An experienced  brewer with an MSc. in brewing science. She has worked at some of the best breweries in the world.

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.



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.



dandelion pub Philedelphia

Resin Fraction:

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.

Essential Oil Fraction:

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.

Terpenes:  

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.



dandelion pub Philedelphia
dandelion pub Philedelphia

Resin Fraction:

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.

“…this is just a snapshot into the thousands of chemicals which go into making a delicious pint of beer, and researchers are constantly discovering newly significant flavours.”

Charlotte Cook

“You get soft, almost wine like aromatics, refeshing… acidity and apple driven, when you try Kent and Eastern style ciders”

— Alison Taffs

dandelion pub inside

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 though some not-yet-fully-understood mechanism) these compounds to 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.

 

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