You may have heard that our perception of flavour is governed by our sense of smell, but did you realise quite how sophisticated a palate your nose really has?
One of the most bandied-about statistics regarding flavour perception is that it is 80% governed by smell. This often surprises people when they first hear it from a local know-it-all, as it really feels as if flavour is coming from our mouths, where the food or drink is. All becomes clear, however, when you try eating while holding your nose. Try it. I just ate a strawberry with my nostrils pinched and it was like water, with none of that blush sweetness. In fact, it's common when people first lose their ability to smell for them to think they have lost the use of their taste buds.
A friend of mine was devastated when he was told while recovering from a head injury that his sense of smell was gone for good. He read up on acquired anosmia, and learned that with only the five basic tastes (sweet, sour, salt, bitter and umami) now at his disposal, he might either lose his appetite or lean too heavily on salty, sweet and fatty foods.
Curiously, though, while many foods (such as herbs) are dead to him now, he still waxes lyrical about coffee beans, practically bathes in the umaminess of parmesan and anchovies, and worships at the altar of a great steak.
What's so great about olfaction anyway?
Unlike taste, which is limited, we can detect many scents - 10,000 was the standard estimate passed down for years through academic literature, but when smell scientist Avery Gilbert tracked down the source, it turns out the figure was pretty much plucked from thin air.
"If you want to put on your biologist's hat," says John Hayes from Pennsylvania State University, "there are about 350 different odour receptor genes in humans," but smell perception operates on a complex pattern-recognition model. Linda Buck and Richard Axel won the Nobel prize in 2004 for working this out (we'll overlook the fact that they cited the 10,000 smells figure), and showed that each receptor type reads different volatile molecules in different ways. So the odour of, say, sizzling bacon, will be the intricate pattern of how that set of chemicals drifting up our noses reacts with our various smell receptors.
Coming at it from another angle, Terry Acree from Cornell University is interested in the amount of chemicals human noses can pick up. "A seemingly infinite number of perceptions are invoked by less than 1,000 odorants found in the human sensory environment," he writes.
Taste, smell, memory and emotion
It was Marcel Proust who first coined the term involuntary memory - a theme he introduced in his mega-novel Remembrance of Things Past (no, I haven't read it either) where the dunking of a madeleine in tea thrust him back to his childhood home. Smell, it is often said, conjures memories more lucid than any other sense, but the madeleine moment was the whole eating experience, not just the whiff. So can taste compete with smell in this department?
Hayes thinks largely not because there has to be a "unique pairing" of the sensation with that experience in your memory. And since there aren't many basic tastes, and we experience each of them often, that unique pairing can't occur. On the other hand, he adds, "the whole flavour experience, which is the integration of taste, touch and smell, can certainly evoke those memories." Dana Small, a psychiatry professor at Yale, agrees: "taste no, flavour yes," she says, "because taste doesn't identify food ... it's really the olfactory component that identifies the flavour."
Odors also have the power to stir up intense feelings. Unlike taste nerves, says Gilbert, "the olfactory nerves go directly to the amygdalae, which are areas in the brain involved in emotional response - fight or flight, positive and negative emotion". Furthermore, he says, we're extra alert to smells because they are invisible, whereas when we taste something, we've usually made a conscious decision to put it in our mouths, so it is expected.
Smells lost but flavours retained
When my nasally challenged friend discusses coffee, he talks about chocolatey aftertastes. But aftertastes are generally caused by the smell of food being pushed back up our noses by swallowing - they're a flavour thing.
However, as Smalls points out, if one piece of a familiar item is missing, the brain is capable of colouring in that bit itself. When we're watching TV, for example, we link the sound from the speakers with the actors' moving lips. This is called a localisation illusion, and we do the same with taste. If you put something sweet on the unreactive middle of your tongue, you won't taste much. But if you first touch it on the tongue's sensitive tip, and return it to the centre, you'll get the sweetness. If you lose your sense of smell, says Small, "it's possible that the memory of [the flavour] is reactivated because you have enough components of that thing being sent to the brain."
Smell: it's personal
We all inhabit our own olfactory world. Mechanical issues such as nasal polyps can inhibit sensitivity, while our smell preferences are learned and vary between cultures. Women are usually a little more sensitive than men, says Gilbert. Smell sensitivities are also genetically determined. My OR6A2 gene is what makes me hate fresh coriander. If you're an OR7D4 carrier, you won't be able to tolerate pork with boar taint.
Taste: it's complicated
While taste cannot compete with smell in numbers, "five basic tastes" is a gross oversimplification. We can't see taste receptors through microscopes because they're molecules. So just how many different types exist remains a hot topic. Food developer Barb Stuckey asserts in her book Taste What You're Missing that there could be 25 different basic tastes. Of those, the Oxford taste professor Charles Spence reckons that there's "reasonably good evidence for "fat, calcium, carbonation, water, metallic [which, Hayes says, we can probably both taste and smell], and electric tastes".
"The reason I get so upset about the statistic that taste is 80% smell," says Hayes, "is because it ignores the millions of dollars we spend on touch sensations." A nose is all very well but it can't feel the dry tannins in a vintage Bordeaux wine or the creaminess of Guinness, the flakiness of pastry, or ice-cream melting. And don't forget the satisfying burns from pepper, ginger and chillies. With no sense of smell, the importance of these sensations skyrockets.
What's your olfactory world like? Do you have the coriander smell gene, or have you experienced boar taint? Has your sense of smell been lost or diminished with age?
Mmm ... coffee: 'There are about 350 different odour receptor genes in humans.' Photograph: Yuri Arcurs/Alamy