I gave this talk in 5 minutes at LRUG (video) and in 10 minutes at GeekyConf (slides). Here’s a write-up with a little more detail and a little more science. I am a hobby-fucker¹ scientist, not a proper one, so I encourage you to go read the sources of this particularly where things get a bit deep at the end.

Fact 1

Humans have three chemical senses: Smell, taste and trigeminal. Your sense of smell detects light volatile compounds, taste detects a couple of heavier compounds and your trigeminal receptors react to irritants. These are things like minty coolness, chilli, pepper etc. Your trigeminal receptors are found all over your body, but the ones that effect taste are at the back of your tongue and down your throat.

Fact 2

1.5% of people have no sense of smell². This condition is known as anosmia. In 1987 1.5 million people were asked to smell six different compounds and report what they could smell. 1.2% of them reported not being able to smell anything. Some people can’t smell certain things, this is known as parnosmia.

I’m one of the 1.5%. I can’t smell anything and have never been able to. Until about seven I thought smelling things was a learned ability and so I didn’t tell anyone I couldn’t do it as I was embarrassed. It was only when I finally asked someone how to smell things that I realised that this wasn’t a learnt skill but a sense. Even then I didn’t tell people generally that I couldn’t smell for quite a while because of a lingering sense of shame.

Fact 3

Some smells are smellier than others and nobody knows why³. There are some compounds that humans can detect at vanishingly low concentrations and we have no explanation for this. The ability to smell these things presents no evolutionary advantage and does not seem to be related obviously to how the nose works.

FIVE UNUSUALLY SMELLY FLAVOUR COMPONENTS:

1. grapefruit juice
2. cork odour
3. butter
4. pepperoni
5. strawberry

Incidentally, I think the experiment for establishing these thresholds is pretty cool. You get your compound and dilute it in something odourless (water is often used) to a known concentration. You ask a bunch of people “Can you smell this?” and then if they say “yep” you keep diluting until they say “nope” and the lowest concentration at which a statistically significant number of people could tell there was something diluted in the water is your detection threshold. Isn’t that neat? There is another threshold often examined which is the recognition threshold, this is where people can tell there is something there and identify what it is.

Anyways, grapefruit is number 1 there on the list of smelly smells, but how smelly is grapefruit? How many more times smellier is a grapefruit than, say, a lemon? Time for some science. Citral is a compound that is kind of averagely smelly. It is found in Lemons. Citral has a detection threshold in water of 32 parts per billion(ppb)⁴. The less concisely named menthene-8-thiol is found in grapefruits and it’s detection threshold in water is 0.00001ppb⁵, 3.2 million times lower than citral. So grapefruits are 3.2 million times smellier than lemons⁶.

Fact 4

There are seven primary odours⁷. These odour classifications are used in perfumery and not rooted strictly in science. They are kind of analogous to primary colours. Chemicals in these primary categories often share similarities in chemical shape.

SEVEN PRIMARY ODOURS

1. camphoraceous (like moth balls)
2. floral
3. peppeminty (not to be confused with the trigeminal sense of mint —a coolness)
4. ethereal (like dry cleaning fluid)
5. pungent (like vinegar)
6. putrid (like bad eggs)
7. musky (angelica root oil)

I just want to point out right now that, as an anosmic person, I obviously have no idea what any of these things smell like, I’m fumbling around in the dark here. In doing the research for this post I learnt that a lot of things that I had assumed to be scentless actually had a recognisable odour.

Fact 5

This one really surprises me: we don’t really know how smell works. We know a bit, but you don’t have to look too far before you come across a whole lot of scientists shrugging. In 1914 Alexander Graham Bell wrote:

“if you are ambitious to find a new science, measure a smell”.

Though we’ve made leaps and bounds in the 100 years since, there are still some pretty important things we haven’t yet figured out about smell. For example: how many smells can the human nose detect?

You have 347 different types of smell receptor. We know this because they are coded for in the human genome. They exist on a 5x5cm slimy surface called your nasal epithelium, which is above and a bit behind your nose. Some people will have fewer types of receptors caused by mutations and this is one cause of parnosmia. One type of smell receptor will detect several molecules and react with differing intensities to them.

Your olfactory receptors seem to work in a combinatorial way. so instead of a 1:1 mapping of a smell to a receptor (meaning there would be 347 smells) it’s more like each receptor codes for a letter in the alphabet and in fact the nose could detect an infinite number of smells as there are an infinite number of combinations of letters in the alphabet (if you’re allowed to reuse letters). In actual fact nobody seems to know how many smells the human nose can detect, the latest research seems to suggest that humans can smell up to 1 trillion different scents.

Another thing we haven’t worked out is, what is actually happening at the reaction site in your nasal epithelium, when the compound triggers the olfactory receptor? There are two competing theories and neither have been disproven (yet).

Shape theory

Shape theory is also called “lock and key” theory and states that molecules fit into receptors like lock and key. As an aside, When we talk about shape in chemistry we mean more than just three dimensional shape, we mean vibration levels, composition, etc. Imagine you have two cuboids of the same size, but one is made of glass and one is made of steel. “chemical shape” would take into account the physical properties not just their dimensions in 3d space. Shape theory makes sense intuitively and has historically been the preferred theory.

However, there are problems with shape theory.

1. If it were true then you would expect to be able to isolate parts of a molecule (i.e. change it’s shape) and for it to smell different, but there are cases where it doesn’t.
2. Nobody has successfully managed to predict what a new molecule will smell like based on its shape.
3. Although most compounds with the same shape smell the same, there are a few that smell very different. Limonene is found in turpentine, lemon rind and probably some other stuff. It has two forms that are mirror images of one another, just like your hands are the same ‘shape’ but are in fact mirrors of one another too. The left hand molecule of limonene smells of lemons, but the right hand molecule smells like turpentine. There are a very small number of compounds that have these smell differences. Most ‘hands’ smell the same.

Vibration theory

The second theory is “vibration theory” and it states that it is not the shape but the vibration pattern of the molecule that determines its odour. This would allow molecules of the same shape to smell different, but there are molecules that seem to run counter to this theory too.

Shape theory was first suggested in 1949 and vibration theory was proposed in 1938 and people have been scientifically poking at them ever since but here we are some eighty years later still scratching our heads.

And neither of these theories offer an answer to why grapefruits are smellier than lemons.

References