Butyric acid, a very smelly molecule

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?

nose

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

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.

italian-parmesan-cheese

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.

pineapple

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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13 thoughts on “Butyric acid, a very smelly molecule

  1. Kat
    CLEAPSS comes across the chemical quite a lot in schools with the same issues as you. I learnt my lesson when I developed a scheme of work in organic chemistry (Ah life was creative in those days) based on butan-1-ol. We made butenes (great with molymod models), bromobutane, butananal and yes then we made butanoic acid. The odour was awful and I was disowned by the whole school (and at home) for several weeks.
    Our level of odour detection is lower that the Workplace Exposure Limit except for a few chemicals, hexane and chloroform, which is comforting to know. This is quite useful. We have to test filter fume cupboards a lot but odour detection is not an official test because people have a variable tolerances. But the unofficial line is that if you can smell the gases when using a fume cupboard something is wrong. You might like to turn your attention to hydrogen sulphide next.
    A good read as usual, Thanks.
    Bob

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  2. I can remember walking into the chemistry room at school one day, where the previous class had been making esters, including methyl butanoate. Unfortunately they’d messed it up somehow, so instead of a delicious pineapple smell the place reeked of vomit. That was not a fun lesson…

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  3. It was probably just a small spill, it only takes the tiniest smear to escape into the room and you’re doomed. It’s definitely a live-and-learn thing! After we had our little incident the bottle got locked away with several “VERY smelly, open with extreme care!!” labels.

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  4. Oh god…..I HATE this stuff, i did a practical today, making esters, and i used butanoate to make some esters and got some on my gloves, having never used it before i thought it’d would be fine boy was i wrong
    This little sucker seeped through the gloves and i didn’t realise the smell until a while after a left the classroom because the classroom was full of paint and banana smells, my friends were even kind enough to point out to me whilst standing 5 meters away from me so i end up spending 20 minutes in the toilets just washing my hands, i washed them around 10 times, very thoroughly, and the smell was still horrible. so i went to the science prep room to get some rubbing alcohol, but they refused claiming it wouldn’t work and made me soak my hands in detergent and rub them with lotion… i did this for a whole HOUR and the smell was still horrible, so i asked for some bicarbonate of soda and made a paste with it… that neutralized the smell drastically, though it sprang back up a few minutes later by this time i had to go to class and everyone was wondering what the smell was….

    Even when i got home i rubbed ethanol (Ethyl alcohol) on them, that didn’t do much. Though a face cleaning fluid containing glycerol did help a bit, though the smell did come back again soon after. Lastly i tried washing powder (with enzymes and sodium bicarbonate) made into a paste, which still only masked the smell.

    Any suggestions on how to neutralise the smell?

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    • Oh my goodness, poor you! I’m sorry, I don’t have a magic solution. When we had a spill we did bicarb, and then just left it. It dissipated eventually, but it did take a couple of weeks. The trouble is we’re so sensitive to these molecules that even the tiniest trace is noticeable. It will also go from your hands eventually, probably within a few days as the skin cell turnover is fairly rapid. In the meantime, my best suggestion would be the bicarb paste that you’ve already tried (just keep repeating it) and/or a good, old-fashioned, bar of perfumed soap (something cheap from the bottom shelf of the supermarket aisle). That’s usually fairly alkaline so it will help to neutralise any remaining acid molecules, and the perfume will help to mask the nasty smell. And maybe get a t-shirt printed with “I haven’t just farted, I’ve been using butyric acid” 😉

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      • Hahahaha those shirts should come in the materials list for the practical as a NECESSITY. I got rid of the smell by making a paste of sodium bicarbonate smearing it all over my hands and covering it with a bandage/dressing and leaving that on for a few hours when i took it off only a faint smell remained and was gone by the next day.

        Oh and i made a mistake in my previous comment didn’t use butanoate, it was butanoic acid, my brain wasn’t thinking straight after smelling that stuff for the entire day.

        Anyway thanks for replying 🙂

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  5. while in the Navy, our destroyer was in Boston Naval shipyard. Our XO was uniformly hated by all the enlisted men. I facilitated revenge by visiting the Boston U. lab where I acquired a vial of butyric acid. On the way back to the ship I took a bus. Standing room only, I worked my way toward the back of the bus. Popped the top off the vial for a second. The whole back of the bus moved to the front and I found lots of room to sit. Walked back to the ship and poured the contents down the exhaust stack from the XO’s head (bathroom). This head was adjoined to his stateroom. Needless to say, the stateroom was uninhabitable for months. Nobody ever figured out what was the cause of the stench, and most thought it was from the vile internal processes of that loathed XO.

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  7. hi, brothers, is there anybody know how to mask the ordor of Butyric acid? we have to use this bitch stuff, however i am really unwilling to open the bottle cap! is there any technologies to turn it into smellless liquid? Thanks

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    • Nothing I know of, well, apart from maybe putting Vicks under your nose! It’s vile stuff. Open it in the fume cupboard and wear disposable gloves that you seal into a plastic bag before throwing them away.

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