Small Talk 13 - Smell

Hello! After a couple hectic months, it’s nice to be back at the desk sharing some small talk. I hope some of you are still interested, because there are many good topics to come.

Now, here’s a question: Do you think you can tell the difference between the smell of parmesan cheese and the smell of vomit? It might not be as easy as you think. In a 2001 study, people were asked to smell a jar containing butyric and valeric acid. If the jar was labeled ‘parmesan,’ 83% of them said it was parmesan cheese. But if it was labeled ‘vomit,’ the same number identified the smell as that.

But this doesn’t mean humans stink at smelling—quite the opposite, in fact. The study provides just a whiff of how complex the sense of smell is. I promise those will be the last puns, as we have a little chat about smell.

Smell

The human body has five senses—vision, hearing, touch, taste, and smell. Of these, smell is the least understood. The most significant scientific breakthrough in the science of smell didn’t come until 1991, when Linda Buck and Richard Axel identified the genes that control olfactory receptors. It’s hard to exaggerate just how important this discovery was in the study of smell. Before then, scientists knew that humans could identify tiny differences among molecules detected by our olfactory system. But they didn’t understand how our brain made sense of them, or why different individuals smell things differently. For this breakthrough, Buck and Axel won the 2004 Nobel Prize in Physiology.

Having read a couple books about smell recently, I can tell you that this science gets technical pretty fast—not great material for small talk. But it’s helpful to compare it to vision. Neurologically speaking, humans have three types of light receptors—one each for red, green, and blue. We use these signals to construct our perception of a huge array of colors. Thanks to Buck’s work, we now know that humans have almost 400 olfactory receptors! They work together, in an almost infinite number of possible combinations, to give us our perception of odors.

Let me share another example of how smelling is special among the senses. Color and sound can both be measured precisely. Color is expressed by wavelength on the electromagnetic spectrum. Sound is measured by waves of air pressure. There is no corollary for the measurement of scents! We know smells come from the chemical composition of things—volatile molecules come into our noses and are absorbed into our mucous, where they bind to a protein, which transports the odor signature to an olfactory receptor, which pass the signal to our epithelium, which passes it to olfactory neurons, and when enough of these neurons are activated (I’m hand-waving here), it creates an electrical charge that’s relayed to the olfactory bulb in your brain, where… odor perception happens. Like I said, it gets complicated fast.

What all this means is that we are capable of detecting millions of scents. And each of us perceives smells differently. What things smell like—or whether we can even smell certain things—depends on our genetic makeup, how we are feeling, whether we are smelling other things at the time, and whether we have encountered the smell before.

Let’s talk about coffee. Most people agree it smells pretty great. Olfactory scientists have a term for this—the hedonic value of the scent of coffee is high. But it is, in fact, a blend of smells—more than 600 aromatic compounds. One of these chemicals, a key part of the bouquet, is indole, which is usually described as having a ‘fecal quality.’ On its own, indole would have a very low hedonic value—the smell of poo repulses us. But when combined with many other scents, it helps make coffee special. And lest we write this off as a quirk of nature, consider this: one of the key ingredients of Eternity by Calvin Klein is indole. Perfumers know how to trick the nose.

Coffee does not have a particularly agreeable taste. If you pinch your nose and sip black coffee, you’ll mostly taste something bitter. This demonstrates how much smell contributes to the flavor of food and drink. Humans, it turns out, have a particularly keen perception of flavor. That’s because we smell food twice—once with our noses, as we sniff the food. And then again retronasally—that is, we smell the food from inside our mouths, as warm air comes up from our lungs and carries the scent molecules to our epithelium in the back of our nasal passages. I’m not making this up. Humans are lacking a bone, the transverse lamina, which prevents most animals from smelling inside their mouths.

Try this: Take a glass of red wine (or flavorful non-alcoholic drink). Swirl it in your glass, remark on what good legs it has. Then smell it and appreciate its bouquet. Say something about a clever cartoon you recently saw in the New Yorker. Then take a sip of the wine. Now stop! Consider what you taste. Okay now swallow and allow yourself to breathe normally. Hopefully, you will notice that the flavor of the wine becomes different and more interesting after you swallow. That’s because the air has pumped up from your lungs carrying the scent of the wine to your smeller. Same thing works with stinky cheese. Our perception of flavor is different between these two methods of smelling.

We have all experienced an association between a smell and a distinct memory. Frying bacon reminds me of winters at my grandparents’ cabin in Michigan. Lilacs take me back to a particular corner of my mom’s garden. It can be very powerful, feeling like you are transported in time and place, thanks to that fragrance. There is a physiological explanation for this. Smell is, in part, processed in the hippocampus in our brains. The hippocampus also plays a role in the consolidation of memory—and, it contains ‘place cells’ that activate at particular locations. So smell is part of the way we label experiences in particular places and times.

Have you noticed how hard it can be to identify a particular odor when you don’t know its source? I have, and for years thought I was just a bad smeller. But according to the science, this is typical. In one demonstration, they gave participants a strip of the chemical sulfurol and asked people to name the source of the smell. People really struggled. Then they showed them a picture of warm milk—most people said, ‘Of course! It's warm milk!’ But then they showed them a picture of ham, and people said, ‘No wait! It’s ham!’ Even when people know the trick, they have the same reaction. We struggle to name scents when they have no context, no comparison. (One exception is the chemical compound cadaverine, whose name speaks for itself, yikes.)

But we are incredibly good at relative discrimination—telling smells apart. We can distinguish between molecules that differ only slightly. Caraway and spearmint, for example, have the same atomic composition but different molecular structure. Most people can distinguish them easily. Wine experts can detect the chemical TCA, a marker of corked wine, in a few parts per trillion.

And here’s some good news! We can learn to smell better. Sommeliers don’t typically have a more acute sense of smell than the rest of us. What distinguishes these experts is that they have developed a vocabulary for the qualities of wine, especially their fragrances. Research shows that when we encode our experiences with scents in this way—when we give smells a verbal description—we recall them better.

We are all smelling things all the time. Our minds filter out most of it, otherwise it would be too distracting. But it can be nice to turn more of your attention to the scent of things. Despite its importance, smell is an underserved sense. Digital media has no way of translating smell. And as a student of literature, I’ve noticed how rarely authors use scent in their descriptions, which is a pity, because as we’ve seen, it can be so powerful in creating atmosphere. A lot of scholars refer to Proust’s memory of dipping cookies in tea with his aunt. I think I prefer Gabriel Garcia Marquez’s The Sea of Lost Time, when the main character smells the fragrance of roses from the sea.

Well that’s enough small talk for now. Let me just say happy Father’s Day to all the dads out there, and suggest you hug your kids and tell them you love them. No doubt you remember the scent of their little heads when you held them as infants, another gift of our remarkable senses.

Have a good one,

Kipling Knox

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Photo credit: Ziggy, © 2024 Haley Salay