Thursday, 13 April 2017

Beeromatherapy: the science of hop aroma

We run the homebrew club at We Brought Beer and back in February we gave a presentation called Hoptimisation - The Science and Process of Hop Aroma. I was pleased to build on my existing knowledge of hops and learn more about why some of those hop aromas seem strangely familiar. In fact I found it so fascinating I thought I'd share my enthusiasm for it here.


What are hops used for?


We use hops to add bitterness, aroma and flavour to beer. They also have antimicrobial properties, i.e. they discourage spoilage of beer by undesirable microorganisms. We often think of smelling beer and tasting beer as two different experiences, but to say aroma and flavour are closely related is an understatement because flavour = aroma + taste. And taste is just those most basic descriptors: sweet, sour, salty, bitter, and umami. So when we talk about 'hop flavour' in beer we are generally referring to hop aroma.

Lupulin glands are the key component of hops

The 'functional unit' of the hop cone (as a biologist would say) is the lupulin gland. If you've ever brewed a beer or been on a brewery tour you'll have rubbed some hops and gotten sticky hands from the hop oils. The lupulin glands are those little yellow pockets you can see with the naked eye and they contain the active components of hops: resins, hop oils and polyphenols.

What's inside lupulin glands?

Lupulin glands contain hard and soft resins. The soft resins are the alpha (and beta) acids which contribute bitterness to a beer. You'll sometimes see an estimate of a beer's bitterness measured in international bittering units (IBU) given on the packaging. Hard resins are thought to be oxidised versions of soft resins; over time the ratio of hard to soft resins increases, this would make sense as hops gradually lose their bittering power as they go stale (i.e. become oxidised).

Hop oils consist of hydrocarbons, oxygenated hydrocarbons and sulphur compounds. These are the magical ingredients which create hop aroma.

There are also polyphenols present. These are proteins which are involved in haze formation - there's plenty more about them in my post on hazy beers from last year.

Hop Oils


These compounds have a variety of different characteristics, e.g. spice or fruit, which contribute to the aroma and flavour of the finished beer depending on when and how they are used in the brewing process. They can be subdivided into three groups - hydrocarbons (made from only hydrogen and carbon atoms), oxygenated hydrocarbons (made from hydrogen, carbon and oxygen), and sulphur compounds (made from hydrogen, carbon and sulphur). If you like organic chemistry you'll love the science of hop oils - it's well organised and follows predictable patterns based on the chemical composition of the compounds involved.

Hydrocarbons (also known as terpenes)


These are volatile compounds - when heated close to their boiling point their aroma evolves off into the local atmosphere, creating a delightful smell while you brew. This is very pleasant but it equals less aroma in your beer when it's ready to drink. If you want to be able to appreciate their characteristics in your beer you have to use them appropriately, i.e. at a temperature below their boiling point. That might be via a temperature-controlled hop stand or whirlpool, or via dry hopping.

Humulene is the traditional 'hoppy' flavour people try to describe in beer, it may also be described as herbal or noble. European hops feature high levels of Humulene. Myrcene which is responsible for the fruity and 'dank' qualities in impact hops such as Mosaic or Chinook is also found in cannabis, thyme, lemongrass and verbena. It has a low boiling point compared to the other hydrocarbons so if you want to taste those berry or resinous flavours in your IPA you'd better use your hops wisely. If you're curious to know more about the characteristics of specific hops then you can do some research by looking at the analysis of current hops available from online suppliers.

Oxygenated Hydrocarbons (also known as terpenoids)


The addition of oxygen makes these compounds both more aromatic and more soluble than the hydrocarbons described above. This means they are more likely to persist in the finished beer. There are many different oxygenated hydrocarbons present in hop oils, e.g. Geraniol (aroma of rose, flowers) and Linalool (aroma of orange, flowers). The suffix '-ol' in organic chemistry denotes an alcohol, they are created by the addition of a hydroxyl group (-OH) to a hydrocarbon (HC).

These compounds are not only found in hops but also plants, fruits, and spices. So when you grind coriander seeds and get that fiercely, citrussy fruity aroma? That's Linalool. It's interesting to note that you also get some Myrcene in coriander but it's far more subtle. That smell when you crush a fresh bay leaf? That'll be Linalool and Geraniol. Wiper & True released a beer in March which was brewed using bay (Citrus Bay Double IPA) and it has an intense, almost oily depth of lemon.

YCH HOPS Mosaic product description

Although they occur in nature, synthetic versions of these compounds are manufactured for use in perfumes, insect repellents, and cleaning products. Interestingly Myrcene is the intermediate for synthetic production of Nerol, Geraniol, Citronellol. Ever smelled a lemon toilet cleaner or insect repellent? That'll be Citronellol (aroma of citrus, fruit). Almost any perfume with a hint of rose will contain Geraniol and its very close relative Nerol. In fact, this family of alcohols all share the same chemical formula (C10H18O) and they only smell different because they are structural isomers of each other (same units but arranged differently).

Familiar aromas
By relative proportion these hop oils make up little of the total weight of a hop cone yet they pack a big punch. You'll find them interchangeably referred to as 'essential oils' - it's actually helpful to think of them in the same way you would aromatherapy oils. If you go for a massage only a couple of drops of a concentrated aromatherapy oil are diluted into a larger volume of an inert carrier oil. That's the essentially the same process which occurs with hops and beer. Of course, how good a beer smells and tastes by the time it reaches the customer depends on a great many variables, not least of which is oxidation.


Sulphur compounds 


Simcoe: cat or fruit?

The final component of lupulin glands is sulphur compounds, which comprise less than 1% of total hop oils. However, they can have a large effect on the finished beer in both a positive and negative sense. One of the reasons why they have such a profound effect in beer despite being such a minor component is that they have a low perception threshold, as little as 10 parts per billion in the case of skunk spray (trans-2-butene-1-thiol*).

These low molecular weight sulphur compounds are referred to as thiols or mercaptans. The latter has negative associations for me (thanks to repeated off-flavour tasting sessions) so I prefer to call them thiols. These compounds are the sulphur analogues of alcohol (SH-HC instead of OH-HC) and that's where they get their name from, the Greek 'theion' meaning sulphur + alcohol gives thi-ol.

When I think of sulphur compounds in beer I am reminded that most of the really unpleasant off-flavours have a sulphur component, e.g. Hydrogen Sulphide (rotten eggs) or good old 3-methyl-2-butene-1-thiol ('skunked' beer). So it's only natural to think that sulphur is going to be a bad thing, right? Well, yes and no.

Thiols have been studied extensively in the wine industry, especially due to a particular undesired reaction between yeast and sulphur, yielding Hydrogen Sulphide. Wine experts are way ahead of beer experts when it comes to flavour analysis, they've already studied the effect thiols have on flavour, how they work in conjunction with other compounds, and how they can be affected by ageing (oxidation).  

If I drink a single hop Simcoe beer I usually struggle with it because it is too overwhelmingly savoury for my tastes, almost to the point of being one dimensional. I must be especially sensitive to it because often people around me have no issue with the same beer and look at me blankly when I pull faces. When I try to explain what I dislike about it I usually say I find it 'catty'. Some people don't seem to understand my descriptor so for the record what I am talking about is cat urine. And guess what? Cat urine contains lots of 3-mercapto-3-methylbutan-1-ol (MMB). It's a sulphur compound. It's also a cat pheromone. Cats love it. I can take it or leave it.

I am careful with Mosaic too, because as derivative of Simcoe it can sometimes yield some of that off-putting cattiness or savoury quality too. Actually this was more of an issue in the earlier days of Mosaic, it's been far less of an issue in the past 18 months or so. Of course, every hop variety will vary year on year and probably none more so that the kaleidoscopic Mosaic. How predominant the catty/oniony flavour is in any given beer will depend on the beer style and other factors, such as serving temperature and age of the beer. Simcoe, Mosaic and Sorachi Ace are three hops that I will always try before I buy when it comes to single hop beers.

But where I get cat, some people get fruit. In fact, in small amounts some thiols will give fruity flavours, including grapefruit, passionfruit, and blackcurrant, but in larger amounts it can come across as savoury, skunky or catty. In red wines they are also responsible for some of the more extreme flavours, such as smoke, tar and chocolate. For low molecular weight compounds which make up hardly any of the overall proportion of hop oils they certainly make themselves known.

The science of hop aroma


Gas chromatography is a lab technique developed in the 1950's which is used to separate and analyse volatile organic compounds. Back then chemists believed they could analyse hops to identify all the different components present in hop oils and that from there it should be a simple process of selecting which hops to use based on what flavour profile you wanted - because chemistry is a hard science, right?

Well, the thing is, the science of hop aroma is not just chemistry; biological variation is involved too. Hop harvests vary on an annual basis and the quality of hops reaching your beer will vary based on how they are processed after harvesting (see also, oxidation). Plus there are at least two organisms playing a role in the perception of hop aroma - one is the yeast which ferments the beer, the other is the person who is smelling (and hopefully drinking) the beer. I'll return to yeast shortly, but just to touch very briefly on human aroma perception: it's incredibly complicated. The ability to recognise different odours, at different levels, in the presence of other masking odours varies enormously person to person at a genetic level. In a solution containing 20 different compounds most people can pick out three or, if they're lucky, four of them. There is also a subjective element to aroma - both memory and emotion have an effect on it, as I am sure you have experienced for yourself.

Take home message: it's not as easy as analysing the compounds in hop oils because there are hundreds of them occurring at varying levels, some of which are below the level of human perception. But more importantly they act in concert to produce a synergistic effect. It's not as simple as 2 + 2 = 4.

Oxygen is the enemy


Whilst I'm not covering the process side of things here, I find it impossible to talk about hops without mentioning oxidation (I've already mentioned it a few times in this post). If you begin to look in more detail at what is happening on a chemical level you will see how closely involved oxygen is in all things hop-related. As a biologist I never forget that whilst we need to breathe oxygen to live it is also responsible for causing cancer via free radicals.

The journey of hop-forward beer
There is a good reason that brewers want customers to drink their beers fresh. Ultimately, a beer begins to age and stale from the day it is packaged; it begins to deteriorate in the quality of its hop character. The aim of the brewery will be to protect their beer all the way through the process of production from the point where fermentation is finished to the point where their customer drinks it. This means minimising contact with oxygen because oxygen is the enemy of beer.

There are always people who say, 'oh but I prefer my IPA with some age on it' or similar. If you look around online it's quite easy to find evidence of people drinking IPA or DIPA when it's months or even years old and insisting it's still great. It's nice that they enjoy old beer but that's not what the brewer intended. Of course, depending on the size of the brewery, there are steps which can be taken to give their beer as long a shelf life as possible (filtering and cold chain distribution, for example). For smaller breweries there is a much simpler option: advise your customers to drink fresh by applying a short best before date to your hop-forward beers, e.g. three or four months.

The bottom line is that the flavour and aroma of a hop-forward beer goes on a journey from the day it is packaged. At the start of its journey the predominant feature will be the hops (and if it isn't, there's a big problem). Within the space of weeks (let's say 6-8 weeks) that hop character will begin to alter. During this transitional phase you might begin to get slightly less fruit, for instance, and slightly more cat. But the aroma will definitely alter. This depends on the relative compostion of hydrocarbons in the hops. Humulene tends to go grassy, whilst Myrcene tends to stay fruity, but the fruit will be fainter. A few months after packaging (depending on many factors) you can be into traditional oxidation territory - where the beer is often described as 'papery' in standard off-flavour terminology. Although for many of us oxidisation is perceived as a 'lack of hops' in a beer which should be 'hoppy'. As with all off-flavour descriptors you must find your own language based on your own perceptions.


The effect of yeast on hops


We all know that yeast converts sugar to ethanol and CO2. Yeast is also responsible for much of the aroma of beer as many of the by-products of fermentation are aromatic compounds, e.g. esters and phenols. In the context of brewing the term biotransformation refers to the ability of yeast to create new compounds from the ingredients added by the brewer. For example, it has been demonstrated that Geraniol can be transformed into Citronellol - leading to an increase in citrus lemon aroma/flavour in the finished beer.

As part of their evolving DIPA series Cloudwater conducted a large scale biotransformation experiment in 2016 to compare the effect of dry hopping at different stages during fermentation. You can read more about that here.  If you tried DIPA V4 and V5 you'll know they looked and tasted markedly different. Actually I found their DIPA V3 just as fascinating in terms of the interaction between yeast and hops. That beer used a Vermont ale yeast plus an enormous amount of hops, but when you smelled and tasted the beer the overriding peachy character was almost entirely from the yeast. And while the aroma generated by hops begins to fade from day one the aromas from yeast will persist.

Cloudwater DIPA v3

More recently it was interesting to compare a couple of Verdant beers which were released at the same time: Maybe One More PSI and 8 Mansions (a collaboration with Howling Hops). Both beers were DIPA but they used different yeasts. Maybe One More PSI used a Vermont ale yeast and in combination with lots of Mosaic it came out as a sweaty, dank, ripe fruitbomb. The 8 Mansions however, was sharper and cleaner with loads of sherbert and citrus. I loved them both and couldn't choose one over the other. But even with a beer style defined by its immense IN YOUR FACE hop character you can still perceive the personality of the yeast it was fermented with. And that makes me happy.



If you're serious about hop-forward beers and want to gain a deeper understanding of how they are made and how they age then I would recommend you get an A level chemistry book and learn some very basic organic chemistry.

If you enjoy tasting wine then you'll appreciate the crossover of knowledge between aroma and flavour in beer compared wine. I have come back to wine after beer and while I know barely anything about wine some knowledge of hops, specifically the characteristics of hops from different parts of the world has certainly been useful. For example, we recently managed to identify an unknown wine during a tasting menu based solely on our knowledge of hops and our ability to identify flavour characteristics in beer. It's really interesting stuff and I am excited to expand my knowledge and experience of delicious beverages.

Aroma is fascinating. Even though we can literally break it down into the precise chemical structures involved, we cannot escape the involvement of memory and the feelings associated with memory. Aroma is both objective and subjective at once. Maybe one of the reasons we enjoy the 'bouquet' of an IPA is related to an appreciation for a beautiful bunch of flowers, or the memory of cooking or eating an amazing meal?

A wise man once told me that the best way to become proficient at tasting beer was to sample as many different types of food as possible. He was right, but you should also get out and smell the flowers and the trees and the coffee and the wine...

*Please know that I've read a lot of papers on the chemical composition of skunk spray while writing this post. Hit me up if you want some good links.

NB: It was with sadness that I cut out the section on 4MMP, 3MH and 3MHA (thiols found particularly in Sauvignon blanc wines) because even though it was fascinating to me it made this post especially sulphur-heavy. Maybe next time!

  1. For the Love of Hops by Stan Hieronymus (Brewers Publications, 2012)
  2. ‘Thiols and Beyond - The Science of Sauvignon Blanc’ by Jamie Goode
  3. ‘History of Skunk Spray Research’ by William F. Wood

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