Vibrant Viburnum: the fascinating chemistry of fragrant flowers

There’s a Viburnum carlesii bush (sometimes called Koreanspice) near my front door and, right now, it smells amazing. It only flowers for a relatively short time each year and otherwise isn’t that spectacular – especially in the autumn when it drops its leaves all over the doorstop, and I’m constantly brushing them out of the house.

But it’s all worth it for these few weeks in April, when everyone who has any reason to come anywhere near our door says, ‘ooh, what is that smell? It’s gorgeous!’ We also rear butterflies at this time of year, and they love the flowers once they’ve emerged from their chrysalids. (No, of course this isn’t an excuse to include all my butterfly photos in a post. Painted lady, since you ask.)

But let’s talk chemistry – what is in the Viburnum carlesii’s fragrance? Well, it’s a bit complicated. Fragrances, as you might imagine, often are. We detect smells when volatile (things that vaporise easily) compounds find their way to our noses which are, believe it or not, great chemical detectors.

Well, I say great, many animals have far better smell detection: dogs, of course, are particularly known for it. Their noses have some 300 million scent receptors*, while humans “only” have 5-6 million but, and this is the really fantastic part, by some estimates we’re still able to detect a trillion or so smells. We (and other animals) inhale air that contains odour molecules, and those molecules bind to the receptors in our noses, triggering electrical impulses that our brains interpret as smell.

Most scents aren’t just one molecule, but are actually complex mixtures. Our brains learn to recognise combinations and to associate them with certain, familiar things. It’s not that different from recognising patterns of sound as speech, or patterns of light as images, it’s just that we often don’t think of smell in quite the same way.

Viburnum carlesii flowers have a fragrance often described as sweet and spicy.

So my Viburnum bush – and the flowers I’ve cut and put on my desk – is actually pumping out loads of different molecules right now. After a bit of hunting around, I tracked them down to (brace yourself for a list of chemical names) isoeugenol, eugenol, methyleugenol, 4-allylsyringol, vinyl-guaiacol and methyl nicotinate, plus the old favourites methyl salicylate (this stuff turns up everywhere), methyl benzoate (so does this), indole, cinnamic aldehyde and vanillin, and then some isovaleraldehyde, acetoin, hexanal, (Z)-3-hexen-1-ol and methional.


Don’t worry, I’m not going to talk about the chemistry of all of those. But just for a moment consider how wondrous it is that our noses and brains work together to detect all of those molecules, in their relevant quantities, and then send the thought to our conscious mind that oh, hey, the Viburnum is flowering! (It’s also pretty astonishing that, in 2021, I can just plug all those names into a search engine and, with only a couple of exceptions, get all sorts of information about them in seconds – back in the old days when I was studying chemistry, you had to use a book index, and half the time the name you wanted wasn’t there. You kids don’t know how good you’ve got it, I’m telling you.)

Anyway, if you glance at those names, you’ll see eugenol popping up quite a bit, so let’s talk about that. It’s a benzene ring with a few other groups attached, and lots of chemicals like this have distinctive smells. In fact, we refer to molecules with these sorts of ring structures as “aromatic” for this exact, historical reason – when early chemists first isolated them, they noticed their distinctive scents.

Eugenol is an aromatic compound, both in terms of chemistry and fragrance (image source)

In fact there are several groups of molecules in chemistry that we tend to think of as particularly fragrant. There are esters (think nail polish and pear drops), linear terpenes (citrus, floral), cyclic terpenes (minty, woody), amines (fishy, rot) and the aromatics I’ve just mentioned.

But back to eugenol: it’s a yellowish, oily liquid that can be extracted from plants such as nutmeg, cloves, cinnamon, basil and bay leaves. This might give you an idea of its scent, which is usually described as “spicy” and “clove-like”.

Not surprisingly, it turns up in perfumes, and also flavourings, since smell and flavour are closely linked. It’s also a local antiseptic and anaesthetic – you may have used some sort of eugenol-based paste, or perhaps just clove oil, if you’ve ever had a tooth extracted.

Plants, of course, don’t go to the trouble and biological expense of making these chemicals just so that humans can walk past and say, “ooh, that smells nice!” No, the benefit for the plant is in attracting insects, which (hopefully) help with pollination. Which explains why my butterflies like the flowers so much. (Another butterfly pic? Oh well, since you insist.) Eugenol, it turns out, is particularly attractive to various species of orchid bee, which use it to synthesise their own pheromones. Nature’s clever, isn’t she?

By the way, notice I mentioned anaesthetics back there? Eugenol turns out to be too toxic to use for this in large quantities, but the study of it did lead to the development of the widely-used drug propofol which, sadly, is pretty important right now – it’s used to sedate mechanically ventilated patients, such as those with severe COVID-19 symptoms. You may have seen some things in the news earlier this year about anaesthetic supply issues, precisely for this reason.

Isoeugenol has the same “backbone” as eugenol, with just a difference to the position of the C=C bond on the right. (image source)

Back in that list of chemical names, you’ll see “eugenol” forming parts of other names, for example isoeugenol. This points back to a time when chemicals tended to be named based on their origins. Eugenol took its name from the tree from which we get oil of cloves, Eugenia, which was in turn named after Prince Eugene of Savoy – a field marshal in the army of the Holy Roman Empire. And then other molecules with the same “backbone” were given the same name with prefixes and suffixes added on to describe their differences. As I said in my last post, this sort of naming system it was eventually replaced with more consistent rules, but a lot of these older substances have held onto their original names.

Still, regardless of what we call the chemicals, the flowers smell delightful. I’m off to replenish the vase on my desk while I still can. Roll on May, vaccines and (hopefully) lockdown easing!

Take care and stay safe.

*it’s even been suggested dogs’ super-powered sense of smell might be able to detect COVID-19 infections.

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Is acrylamide in your toast really going to give you cancer?

Acrylamide has been in the news today, and this might be the understatement of the year. Front page newspaper headlines have been yelling everything from “Brits officially warned off chips” to “Over-cooked potatoes and burnt toast could cause cancer” to the marginally more restrained “What is the real cancer risk from eating roast potatoes or toast?” All this has been accompanied by radio interviews with everyone from actual scientists to professional chefs to people keen to share their roast potato recipes. I expect there have been television interviews too – I haven’t had a chance to watch.

Hey, what could be more traditional, or more fun, than a food-health scare in January?



Never fear, the Chronicle Flask is here to sort out the science. Let’s get to the facts: what is acrylamide?

It’s actually a rather small molecule, and it falls into a group of substances which chemists call amides. Other well-known amides include paracetamol and penicillin, and nylon is a polyamide – that is, lots of amide molecules joined together. Amide linkages (the CO-NH bit) are a key feature of proteins, which means they appear in all kinds of naturally-occurring substances.

And this is where the food-acrylamide link comes in. Because acrylamide, or prop-2-enamide to give it its official name (the one only ever used by A-level chemistry students), forms when certain foods are cooked.

Acrylamide occurs naturally in fried, baked, and roasted starchy foods.

Acrylamide occurs naturally in fried, baked, and roasted starchy foods.

It begins with an amino acid called asparagine. If you’re wondering whether, with that name, it has anything to do with asparagus, you’d be on the right track. It was first isolated in the early 1800s from asparagus juice. It turns out to be very common: it’s found in dairy, meat, fish and shellfish, as well as potatoes, nuts, seeds and grains, amongst other things.

This is where the trouble begins. When asparagine is combined with sugars, particularly glucose, and heated, acrylamide is produced. The longer the food is heated for, the more acrylamide forms. This is a particular issue with anything wheat or potato-based thanks to the naturally-occurring sugars those foods also contain – hence all the histrionics over chips, roast potatoes and toast.

How dangerous is acrylamide? The International Agency for Research on Cancer have classified it as a Group 2A carcinogen, or a “probable” carcinogen. This means there’s “limited evidence” of carcinogenicity in humans, but “sufficient evidence” of carcinogenicity in experimental animals. In other words (usually) scientists know the thing in question causes cancer in rats – who’ve generally been fed huge amounts under strictly controlled conditions – but there isn’t any clear evidence that the same link exists in humans. It’s generally considered unethical to lock humans in cages and force feed them acrylamide by the kilo, so it’s tricky to prove.

screen-shot-2017-01-23-at-22-10-46At this point I will point out that alcoholic beverages are classified as Group 1 carcinogens, which means there is “sufficient evidence” of carcinogenicity in humans. Alcohol definitely causes cancer. If you’re genuinely concerned about your cancer risk, worry less about the roast potatoes in your Sunday roast and more about the glass of wine you’re drinking with them.

But back to acrylamide. In animals, it has been shown to cause tumours. It’s one of those substances which can be absorbed through the skin, and after exposure it spreads around the body, turning up in the blood, unexposed skin, the kidneys, the liver and so on. It’s also been shown to have neurotoxic effects in humans. BUT, the evidence that it causes cancer in humans under normal conditions isn’t conclusive. A meta-analysis published in 2014 concluded that “dietary acrylamide is not related to the risk of most common cancers. A modest association for kidney cancer, and for endometrial and ovarian cancers in never smokers only, cannot be excluded.” 

The dose makes the poison is an important principle in toxicology (image credit: Lindsay Labahn)

The dose makes the poison (image credit: Lindsay Labahn)

As I so often find myself saying in pieces like this: the dose makes the poison. The people who have suffered neurotoxic effects from acrylamide have been factory workers. In one case in the 1960s a patient was handling 10% solutions of the stuff, and “acknowledged that the acrylamide solution frequently had splashed on his unprotected hands, forearms and face.” The earliest symptom was contact dermatitis, followed by fatigue, weight loss and nerve damage.

Because of these very real risks, the Occupational Safety and Health Administration and the National Institute for Occupational Safety and Health have set occupational exposure limits at 0.03 mg/m3 over an eight-hour workday, or 0.00003 g/m3.

Let’s contrast that to the amount of acrylamide found in cooked food. The reason all this fuss erupted today is that the Food Standards Agency (FSA) published some work which estimated the amounts of acrylamide people are likely to be exposed to in their everyday diet.

The highest concentrations of acrylamide were found in snacks (potato crisps etc), and they were 360 μg/kg, or 0.00036 g/kg or, since even the most ardent crisp addict doesn’t usually consume their favoured snacks by the kilo, 0.000036 g/100g. (Remember that those occupational limits are based on continuous exposure over an eight-hour period.)

In other words, the amounts in even the most acrylamide-y of foodstuffs are really quite tiny, and the evidence that acrylamide causes cancer in humans is very limited anyway. There is some evidence that acrylamide accumulates in the body, though, so consuming these sorts of foods day in and day out over a lifetime could be a concern. It might be wise to think twice about eating burnt toast every day for breakfast.

Oh yes, and there’s quite a lot of acrylamide in cigarette smoke. But somehow I doubt that if you’re a dedicated smoker this particular piece of information is going to make much difference.

As the FSA say at the end of their report:

Your toast almost certainly isn't going to kill you.

Your toast almost certainly isn’t going to kill you.

“The dietary acrylamide exposure levels for all age classes are of possible concern for an increased lifetime risk of cancer. The results of the survey do not increase concern with respect to acrylamide in the UK diet but do reinforce FSA advice to consumers and our efforts to support the food industry in reducing acrylamide levels.”

This is not, I would suggest, QUITE the same as “Crunchy toast could give you cancer, FSA warns” but, I suppose, “FSA says risk hasn’t really changed” wouldn’t sell as many newspapers.

One last thing, there’s acrylamide in coffee – it forms when the beans are roasted. There’s more in instant coffee and, perhaps counterintuitively, in lighter-roasted beans. No one seems to have mentioned that today, possibly because having your coffee taken away in January is just too terrifying a prospect to even contemplate. And also perhaps because coffee seems to be associated with more health benefits than negatives. Coffee drinkers are less likely to develop type 2 diabetes, Parkinson’s disease, dementia, suffer fewer cases of some cancers and fewer incidences of stroke. Whether the link is causal or not isn’t clear, but coffee drinking certainly doesn’t seem to be a particularly bad thing, which just goes to show that when it comes to diet, things are rarely clearcut.

Pass the crisps, someone.

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