Rosy or revolting? Ground-breaking research by Kiwi scientists has proven there's a reason why some people enjoy the smell of things that others might retch at - and it's got nothing to do with style or taste.
World-first studies by Plant and Food Research, published in the journal Current Biology today, effectively prove that subtle genetic mutations are the reason why we smell things differently, if even at all.
While this had been known anecdotally, the science behind the theory had not been properly investigated, co-author Dr Richard Newcomb said.
"We kind of had the idea that if there was a kind of genetic component to the ability to smell, that might under-pin or help explain why different people might like certain foods and not like others," he said.
"And then we thought this information might be useful for helping market New Zealand's food products internationally."
The 10 sample odorants the team used were all Kiwi export items, among them kiwifruit, wine and dairy.
"Unfortunately, what we found was the frequency of these genes are equally distributed around the world, so there was not so much power in helping our exporters in that way."
The researchers tested nearly 200 people for their sensitivity for 10 different chemical compounds that occur in foods and searched through the subjects' genomes for areas of the DNA that differed between people who could smell a given compound compared to people who could not.
This approach - known as a genome-wide association study - is widely used to identify genetic differences, such as between healthy and diseased individuals in the hope of identifying genes that underpin certain diseases, namely diabetes or cancers.
They found that for four of the 10 odours tested, there was indeed a genetic association, suggesting differences in the genetic make-up determined whether someone could or could not smell these compounds.
The familiar smelling odorants were malt (isobutyraldehyde), apple (b-damascenone), blue cheese (2-heptanone) and beta-ionone, which smelled floral to some and was particularly abundant in violets.
The results showed no differences in sensitivities between people in different parts of the world.
Researcher Dr Jeremy McRae said his team was surprised at how many odours had genes associated with them. "This might mean that when people sit down to eat a meal, they each experience it in their own personalised way."
The genetic variants that cause us to smell differently all lie within or near genes that encoded so-called odorant or olfactory receptors, of which humans have about 400.
The odorant receptor molecules sit on the surface of sensory nerve cells in our nose, and when they bind a chemical compound drifting through the air, the nerve cell sends an impulse to the brain, leading to the perception of a smell.
Dr Newcomb said knowing the compounds people can sense in foods, soaps, detergents and other goods would have an influence on the development of future products.
Stop and smell the ... violets?
Keen to see how you smell differently to others around you? Buy a bunch of violets.
"I predict there are some people that can smell violets and some that can't," Dr Richard Newcomb said.
In the case of b-ionone, the smell associated closely with violets, the Plant and Food Research team managed to pinpoint the mutation in the odorant receptor gene OR5A1 that underlay the sensitivity to smell the compound and perceive it as a floral note.
They found people who aren't as good at smelling b-ionone also describe the smell differently as sour or pungent, and are less likely to find it pleasant.
What's more, the genetic variant also affected whether people chose certain foods, such as chocolate or orange juice, that were spiked with b-ionone; there, the more sensitive group reacted differently to the floral smell in food.