Sniffing out the science of smells

Ever opened a backpack you haven’t used all summer to be hit by a face-scrunchingly bad smell? We’ve all done it. The stench usually emanates from a forgotten sandwich or piece of fruit that has been slowly rotting at the bottom of the bag.

If you think about it, the smells given off by rotten foods are useful, because they warn us not to eat things that will make us ill. But what are the chemical culprits behind these and other obnoxious odours? Can chemists guess whether something they’ve never smelled before is going to smell nasty or nice? And can you cover up a bad smell with a good one?

To answer these questions, we first have to remind ourselves what a smell – good or bad – actually is. As Jane Parker, a flavour chemist at the University of Reading explains, it’s all down to molecules ‘jumping’ off whatever it is you’re smelling (by evaporation), drifting through the air (by diffusion) and flying up your nose. These molecules trigger nerves at the top of the nose that send signals to the brain. Why some molecules smell strongly and others pass unnoticed is a mystery, though. ‘It’s pretty random,’ says Jane. ‘We can’t really explain it.’ She suggests that perhaps there is an evolutionary aspect to it – perhaps our ancestors’ noses became attuned to the molecules that could give them important information about their environment. Like whether a particular plant was good to eat.

Of course, the idea that these molecules liberate themselves and make their way into your nasal passages can make your nose wrinkle just thinking about it. But Jane says we shouldn’t worry too much about catching a whiff of smelly drains (or whatever it is in the bottom of your bag). ‘It’s a very, very tiny amount,’ she says. ‘Some of these bad smells from drains are equivalent to putting a fraction of a drop in a swimming pool and turning that whole swimming pool into a drain smell. So, it’s absolutely miniscule amounts that are required for you to perceive those smells.’

There are no rules

Rotten egg-type smells coming from drains or bins can be associated with hydrogen sulfide gas produced by microbes. In fact, many sulfur-containing molecules are responsible for what we consider to be bad smells, including bad breath (halitosis) and gas leaks. The sulfurous ethanethiol has been described as skunky. It’s hard to say that, as a rule, sulfur-containing compounds always smell bad, though – at lower levels, they may lend a tropical note, making you think of tropical-flavoured drinks or sweets, or contribute to meaty smells that would make plenty of people’s mouths water.

Whether the mixture smells good or bad is very much about how its molecules are balanced

It’s just as hard to come up with rules for ‘nice’ smells. Esters tend to smell fruity, as they’re the molecules given off by fruits like apples, strawberries and pears as they ripen. But it’s difficult to say that one group of chemicals always smells pleasant, especially as what we’re smelling is often a mixture of different molecules; whether the mixture smells good or bad is very much about how its molecules are balanced, as well as the context in which we smell it.

We tend to think that chocolate, for example, smells good but, as Jane points out, its smell comes from a whole bunch of different molecules, one of which is 3-methylbutanoic acid (also known as isovaleric acid), which smells like Parmesan cheese. In the context of sniffing a cheese you’re about to grate onto your pasta, this can be considered a good smell. ‘But if you give it to people to smell without any clues, [they’ll say] vomit or baby sick,’ says Jane. ‘It’s stinky.’

Another interesting example is the durian fruit, a spiky-skinned fruit hailing from south-east Asia that gives off an odour so offensive it has been banned from certain public spaces. When German researchers separated and analysed the chemical compounds that make up its revolting stench, they realised it couldn’t be pinned on one or two molecules – it was a combination of 50 or more. These included infamous sulfur compounds like ethanethiol, but also fruity compounds like acetaldehyde and others that, on their own, smell sweet or like roasted onion. And while most people agree that durians smell disgusting, there are those that find joy in its unique aroma.

Smells good to me

This brings us to the fascinating question of whether something that smells bad smells bad to everyone – universally – or whether it depends on who you are and what kinds of smells you are used to. Or, more specifically, if a bunch of different people smell the same compound, will they all find it similarly pleasant or unpleasant?

According to Asifa Majid, an experimental psychologist at the University of Oxford, scientists used to think that our sense of smell was rooted in our cultural background. ‘There are all of these reports of how, for example, Malaysian people eat durians and how each culture has some cuisine that seems really horrible to people outside that culture,’ she says. ‘So, all of this suggested that there were really large differences between what odours people prefer or not, and this had to do with people’s cultures and the things they have grown up with.’

Through her own research, though, she’s shown that whether we find a particular odour nasty or nice is pretty universal across cultures. In one study, her team asked people from all corners of the globe to rank a selection of smells by how pleasant they found them. ‘What we were trying to do was take people with very different smell experiences,’ says Asifa. ‘So, we tried to get people who lived in various environments where the “smellscapes” would be different.’

Those who took part ranged from hunter gatherers in coastal Mexico and Thai rainforest communities to New York City dwellers. Each participant was presented with 10 scented sticks, which they were allowed to sniff as often as they liked before arranging them in order from most to least pleasant – a task that was easy to explain despite the language barriers. Each stick was scented with a single compound.

People from two completely different cultural backgrounds make the same facial expressions when confronted with the same odours

Surprisingly, no matter where people came from, they had similar opinions about whether these compounds smelled pleasant or not. Most, for example, found vanillin (the compound in vanilla ice cream and many different perfumes) pleasant and isovaleric acid (the strong, cheesy or vomit smell mentioned above) unpleasant. Any differences could be accounted for more by personal preference than by cultural background. Personal preference is probably related to both genetics and individual experiences of, say, home-cooked foods.

The results from this study also complement those of a previous study, in which Asifa found that people from two completely different cultural backgrounds make the same facial expressions when confronted with the same odours, despite describing them in different ways.

Can you predict how something will smell?

Is there, then, some fundamental relationship between a chemical’s structure and whether it smells good or bad? Probably not – ‘good’ and ‘bad’ are not descriptive enough for chemists in any case. But Jane’s recent work suggests that, with the help of new technologies, we can use a compound’s chemical structure to predict quite accurately what it might smell like, using more precise terms like ‘toasty’ or ‘buttery’.

Jane’s team took the structures of thousands of molecules and fed them, along with information about how they smell, into a computer. They relied on AI to learn patterns in all this data so that they could predict how chemicals would smell, just based on their structure. ‘This worked in a surprising number of cases with molecules that, as a flavour chemist, I’d look at and think ‘that should be fruity’, but [turned out to be] meaty,’ says Jane.

When we mix different smells, we can’t guarantee that we’ll end up with what we expected

This clever way of predicting what a molecule might smell like could help with discovering new, more sustainable ingredients for fragrances used in all kinds of products, according to Jane. However, because it’s not clear what patterns the AI is picking up on, it can’t yet help chemists write new rules about the relationship between chemical structure and smell.

As humans, then, we have to admit that we don’t exactly know what makes a bad smell smell bad – not in a chemical sense anyway. Although we do know that our perception of a smell being ‘bad’ might depend on our own experiences. But do we, at least, know how to get rid of a bad smell when we smell it? Well, that might also prove a little tricky. We do, of course, have air fresheners, most of which work quite simply by spreading a ‘good’ smell that is designed to mask unpleasant odours.

This means, in the case of a stinky, rotten fruit smell emanating from the bottom of a forgotten backpack, you will still have stinky, rotten fruit particles invading your nostrils, but they will (hopefully) be overpowered by scents of summer flowers or forest pines. The potential problem is that when we mix different smells, we can’t guarantee that we’ll end up with what we expected. ‘Once you start mixing molecules, all sorts of things happen,’ notes Jane. Another option is to try an activated charcoal odour absorber, which is designed, instead, to mop up unpleasant smells.

Otherwise, the best way to eliminate a bad smell is to track down the source and deal with it. And for that, you’ll have to follow your nose.