Why You (Don’t) Like Parma Violets

Whilst some may unanimously agree on the perception of a specific flavour (or combination of flavours) most will dispute variances in its intensity and inclination to like it: but why? As it’s (partially) due to your genes, of course!

Before exploring such variations let’s first establish that our flavour perception is about 80% reliant on out perception of smells at how we actually smell. So how we do smell? Well firstly, molecules from the surrounding air are inhaled through the nostrils and filtered by hairs within it. They then reach the back of the nose where the olfactory epithelium is located. Here is where odour detection occurs.

This is due to the odour molecules dissolving in a layer of mucus which lines the olfactory epithelium, binding to certain receptors on olfactory sensory neurons as they do so.  When these neurons reach their necessary threshold, action potentials are created which travel up their axons, through the ethmoid bone to the olfactory bulb in the brain. Here, within the glomeruli (bundles which the olfactory receptor neurons converge into) the olfactory neurons meet with the dendrites of a mitral cell, passing the signal onto them and then through the olfactory tract to the olfactory cortex in the brain.

The brain then processes the smell in two ways: one is that the signal is then passed on to be consciously interpreted by the frontal cortex; the other is that the signal is transferred to the amygdala, parts of the neocortex and the rest of the limbic system, triggering an emotional response (this is partly why smells are so closely linked to our memories). This processing of the sense of smell differs from the processing of sound and sight in that the impulse stimulated by odour detection takes a more direct route through the brain as opposed to going to a relay centre in the middle of the cerebral hemisphere first.

But how can we distinguish between odours? This is due to the combination of chemicals that we smell creating an overall depiction of an odour, with different odorants activating different odorant receptors. Though the olfactory epithelium in humans is only about the size of a postage stamp (with a dog’s 20 times larger, explaining their better sense of smell), 396 unique scent receptors have been identified in reference human genome, hence allowing us to detect over 1 trillion different odours, despite each olfactory neuron having receptors for only one kind of smell, as a result of combinational coding.

However, variations in our genes can cause variations in the perceptions of odours. After all, 1 out of every 50 of our genes is dedicated to smell. Such variations in perception include that of β-Ionone, a chemical that gives violets their distinctive smell and as such are used in the sweets Parma Violets. Though studies (such as http://www.cell.com/current-biology/fulltext/S0960-9822(13)00853-1 ) have shown that the sensitivity in perception of this smell greatly varies in individuals, with some being able to detect the floral tone whilst others detect a smell that more pungent than flowery or are unable to detect the smell at all (known as specific anosmia).

When they sequenced the genomes of the individuals tested they found a correlation between their perception of β-Ionone and the variant of the gene OR5A1 that the individual had, with this difference accounting for 96% of the variation in perception. Furthermore, when the researchers gave the participants a choice of juices, the group of people who were more sensitive to β-Ionone chose the option without β-Ionone added whilst the less sensitive group preferred the option with β-Ionone added.

This, therefore, shows how our genes can influence our inclination to certain food and drinks. Various industries have acknowledged this and as such have changed their composition or production methods. Let’s look at the chemical androstenone, 5-α-androst-16-en-3-one, which to some smells like vanilla though to others smells like sweaty urine. With this chemical being a pheromone in boars, male pigs which are bred for pork production are castrated to prevent androstenone production, to account for those who would smell sweaty urine. Sorry, boars.

And with Christmas approaching, you’re probably wondering about those Brussels sprouts. Well this time taste has a big role to play in our perception of their flavour, with variations in the TAS2R38 gene influencing our sensitivity to bitter tastes caused by the chemical phenylthiocarbamide (PTC). With Brussels sprouts containing very similar chemicals and about 50% of people having the variation of the gene which makes them less sensitive to such chemicals, you’ve got a roughly 50-50 chance of liking them.

So next time your parents criticise your profound disliking of certain foods, just let them know that you can’t help it: it’s in your genes!