[Excerpt from The Secret of Scent (2006) by Luca Turin, pgs 108-111]

Some Strange Clues

It has been said,* correctly in my opinion, that theories define facts as much as the other way around. Nowhere is this more true than in structure-odour relations, where all knowledge is anecdotal. Anecdotal evidence has a sort of slippery, jelly-like quality to it, and theories are needed to congeal the stuff together into single, solid facts. ‘Anecdotal’ is often used as a pejorative term in scientific circles, meaning unreliable. In practice it often means isolated, and therefore hard to assess. Think of a new field of science as a large jigsaw puzzle. Pieces are discovered one by one, and at first they are unlikely to fit together to make a picture. Things can look distinctly unpromising, sometimes for decades. But if you can bear the pain of feeling stupid and the humiliation of being wrong, anecdotal evidence is the call of the wild, the surest sign of the undiscovered. Columbus set sail on the basis of anecdotal evidence. The Mayan hieroglyphs were deciphered using anecdotal evidence. Life-saving remedies based on plants, such as aspirin and digitalis, were found by scientists who paid attention to anecdotal evidence.

Scientific problems typically go through three phases. In the first phase, a few bold explorers discover a new land and map out its basic features. In the second phase, boatloads of immigrant scientists arrive and colonize the land. In the third phase, statues are erected on town squares, sometimes to the original discoverers, more often to the able administrators who build the roads and railways. Smell, as it happens, did not follow this pattern. Scientific colonies never thrived on this particular island. Every few years, a new set of scientists claims to have cleared the jungle, but their cities are eventually overgrown and get lost in the weeds.

In smell, the difficulty is compounded by two additional factors, one obvious, the other more subtle. The first is the supposed untrustworthiness of the smell sensation I’ve mentioned earlier which makes strong men and women doubt their own noses. The second is that when facts, especially anecdotal ones, remain unexplained for long enough, a kind of question fatigue sets in, and they become accepted without being understood. The situation brings to mind a quintessentially British cartoon I saw once where a dinosaur strides past a terraced house, and a couple see it from their living room. Wife: “What was that?” Husband: “Oh, just one of those Things.” The fact that we can smell functional groups is just such a Thing.

Functional groups, as we have seen, are the specific structures of one or more atoms that are responsible for the chemical behaviour of a substance. Examples are thiols (-SH), nitriles (-CN), and aldehydes (-C(=O)H). The little hyphen indicates that these groups are, of course, attached to something and that the Something varies hugely. But the remarkable thing is that the Something matters little to the smell of the molecules. What gives the game away, especially to the casual observer, is the fact that types of smell are named after chemical groups: sulphuraceous, nitrilic, aldehydic, corresponding respectively to -SH, -CN, -(H)C=O. This is particularly clear in the case of -SH. All molecules which contain an -SH group smell (a) strong and (b) reminiscent of rotten eggs.

Kala Namak

Powdered Kala Namak (“black [really pink] salt”)

A word about the description ‘rotten eggs,’ since only a tiny minority of readers will be old enough to remember them. Eggs nowadays come with time stamps and serial numbers, so they seldom get a chance to rot. The rotten eggs smell is today more likely to be experienced in an oriental market (the durian fruit), by opening the gas tap on the stove (a small amount of an -SH compound is added to make sure we notice it), or best of all by going to an Indian store and asking for kala namak or ‘black salt’. Black salt, as its name does not indicate, is actually pink and is a type of rock salt that must come from Hell, as it contains ample amounts of Hell’s Kitchen smell, namely the HSH molecule. HSH is -SH repeated and smells bad twice over. Put some kala namak on your tongue and you will see what I mean. The first thing you will notice is that it reminds you mostly of a very intense hard-boiled egg smell. Clearly, eggs, even when fresh, are itching to fall apart. If you’ve done any chemistry at school, you will also recall the classroom when the teacher was making one of those stinks for which chemistry is famous. Beware though, the culinary satanism of kala namak is beguiling: a tiny amount in blackcurrant ice cream, strawberry daiquiris, coffee, and chocolate does wonders, as long as you don’t let anyone know you did it.

Do all -SH compounds smell identical then, i.e. of rotten eggs? Not a bit, actually: they smell of all manner of things, from grapefruit to garlic via blackcurrants,  but they all have this sulphuraceous (i.e. from Hell) character. The grapefruit compound is particularly instructive. It is called pinanethiol. Thiol means -SH, so pinanethiol means pinane-SH.

Pinane

2-pinanethiol

Remove the -SH and the rest of the molecule (pinane) smells like pine needles, as it should, since pinane is a major component of turpentine oil, itself extracted from pine. Add the -SH back and, having smelled the pinane by itself and familiarized yourself with kala namak, you can clearly smell the parts of the molecule. That is to say you smell both the pine needles and the sulphur. Smell another very strong -SH compound like H₃C-SH, or methanethiol, for a few seconds till the nose (mercifully) tires of the hideous -SH smell, then go back to pinane-SH. Surprise! The sulphur note is now almost gone and the molecule no longer smells of pinane-SH, but instead smells of pinane tout court. This means that this molecule smells like the sum of its parts. In other words, -SH is a primary, though the other smells are not. But how does that work? How do we know what parts it’s made of? This, as we shall see, is the greatest mystery of smell. Looking for an answer will take us amazingly far afield.

* Paul Feyerabend, among others, convincingly argued this view in Against Method, required reading for those who believe the scientific method is something which can be written down and followed like a recipe.

Comments:

On a recent conversation I had with Luca, I shared with him the fact that there are anti-tolerance drugs that can lessen (and even reverse) the physiological tolerance to drugs such as painkillers. He was seriously surprised by this fact. Despite spending a whole career studying biological regulatory systems, he had never in his life heard of anti-tolerance drugs in academia. Upon hearing this, he shared that in his experience, most of the innovation in science comes from people who work hands-on in the field, as this exposes them to a much broader evidential base than you would encounter when doing research in a strictly theoretical way.

Thus, he has learned far more about consciousness from psychonauts than he ever has from academic psychopharmacologists, and has learned more about electronics from radio amateurs than professional electrical engineers. In other words, the people who actually tinker with the inner mechanisms of the systems they’re interested in are the people to ask for “weird and novel phenomena”, rather than (only) those who study the field academically angling for a university post or a narrow job in the industry. Same, of course, with the science of smell: actually tinkering with aromachemicals can give rise to discoveries one may never stumble upon by merely studying scent receptors in a lab. Needless to say, the best outcomes will come from seamlessly blending both worlds; but for that to happen we will have to embrace phenomenological reports as acceptable leads for research in science.

See Luca Turin’s recent series on the science of smell on youtube: The Secret of Scent (including a video on the objections to the vibrational theory of olfaction).