Did you know that you’re walking around with an incredibly sensitive chemical detector, capable of detecting and identifying substances at levels as low as 0.2 parts per billion (I’m talking about gases here, but if you think about this ratio in another way it’s about half a second in a century), and possibly even lower? Capable of distinguishing between millions and very probably trillions of different substances and mixtures of substances?

Did you nose you were carrying around a very sensitive chemical detector?

Well you are. It’s your nose. Dogs, of course, can do even better than humans (bloodhounds can easily detect substances in the parts per trillion range) but our noses are still pretty impressive.

We are particularly good with nasty smells, for very sensible evolutionary reasons. If it smells bad it’s probably bad for you, stay away and definitely don’t eat it.

Which brings me to butyric acid, or butanoic acid (to give it its official IUPAC name, which literally no one uses outside of A-level chemistry). Butyric acid is a small molecule, early in the list of carboxylic acids, and you might imagine a chemistry student would meet it, well, if not frequently then at least once or twice during their studies. After all barely a week goes by when we don’t crack open the ethanoic acid (also known as acetic acid, the stuff that gives vinegar its pungent smell).

And yet I managed to go for years and years without ever knowingly coming across the stuff. I knew about its theoretical existence of course, and never really thought about it much beyond that. After all, you can’t use every chemical can you? I obediently followed my lab book instructions and then later specialised in physical chemistry, so the opportunity to fiddle around with cocktails of interesting organic molecules of my own choosing never really arose.

Butyric acid: it’s very stinky.

When I finally did get my hands on a bottle of butyric acid I quickly learned why it had never featured in an undergraduate practical task.

It stinks.

Of horrible things.

Everyone that smells it seems to identify it slightly differently, but descriptions fall out of the: ‘pooh, farts, sick, smelly feet, sweat, gone-off curry, sour milk’ general category of bad smells. Occasionally someone will generously suggest parmesan cheese, but really, it’s not that nice.

It’s not a smell that goes away, either. It’s a stench that just keeps on giving. One of my students managed to get a tiny drop of it on a lab bench and, despite trying to clean it up, the smell lingered for weeks. In fact it was quite interesting. Most people could smell it for about two weeks (as in, they walked into the room and immediately said “ugh, what’s that smell?!”) After that fewer and fewer people immediately reacted, but every now and then someone would walk in and complain of a horrible stink, which by then no one else was really noticing. I assume these were individuals with unfortunately (in this situation) sensitive noses, perhaps with great futures ahead of them as chefs, sommeliers and perfumers. Although some fairly recent research has suggested that ability to recognise smells has more to do with training than innate ability. Still, who nose? (Hehe)

So what is butyric acid and why is it so stinky? It’s name actually comes from the Latin word butyrum (or buturum) meaning butter, because it was first extracted from rancid butter by the French chemist Michel Eugène Chevreul (bet he loved his job). It’s a fatty acid, which means it’s one of the building blocks of fats. The fat molecule made from butyric acid makes up 3-4% of butter, and tied up in this form it’s completely innocuous. However once those fats start to break down, the evil butyric acid starts to be released.

Probably the least offensive thing associated with butyric acid. Probably.

It’s generally found in dairy products, and is a product of anaerobic fermentation (that is, fermentation that happens in the absence of oxygen), hence the links to butter and parmesan cheese. Anaerobic fermentation also happens in the colon. Hence, ahem, the pooh smell. Oh yes, and butyric acid is also what gives vomit that distinctive, smell-it-a-mile-off, odour.

And this, of course, is why we’re so good at detecting it. Humans can pick this stuff up at 10 parts per million (going back to those time analogies, that’s the equivalent of 32 seconds out of a year) which explains why the stench appeared to linger on and on – that single drop of pure butyric acid would have contained something like a thousand trillion molecules. Evolution has trained us to detect and avoid this stuff because it’s very probably a sign of disease and potential infection (gone-off food, vomit, faeces etc). This is stuff we need to steer well clear of to avoid getting ill, and so nature has given us a handy mechanism by which to detect and avoid it, of the “yuck! What is that smell?! I’m out of here!” variety.

From nasty to really quite nice: you can make pineapple scents from butyric acid.

Funnily enough though, it does have its uses. There are molecules called esters which can be made from butyric acid (that’s why we were experimenting with it in the first place) which actually smell rather nice. In particular there’s one that has a lovely apple-pineapple smell, and another that smells of apricots and pears. As a result, these much nicer-smelling substances are used as food and perfume additives.

The salts of butyric acid (butyrates, or butanoates) have interesting effects on the cells that might be in your colon. Butyrate actually slows down the growth of cancer cells in this area, while at the same time somehow managing to promote healthy, normal cells. Exactly how this works isn’t well understood, but it seems to be linked to dietary fibre. Yes, I’m afraid to say you can’t swap cheese for bran flakes and vegetables. You still need to eat your fibre.

Butyric acid also helps to prevent salmonella bacteria from taking hold in poultry, and as result it’s used as a chicken feed additive (lucky chickens). It’s also been used as a fishing bait additive, particularly for carp bait. And perhaps not surprisingly it’s been used, along with a cocktail of other stinky stuff, in stink bombs.

So even the stinkiest of molecules has it’s uses, and maybe it’s not so bad after all. Makes you wonder how anyone ever developed a taste for parmesan cheese though, doesn’t it?

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