44 | New Scientist | 1 June 2019
because each of us has a different repertoire
of odour receptors, so we each play on our
own, unique keyboard. “Nobody smells things
the same way,” says Sell. And small genetic
differences between individuals can affect
how well a receptor latches on to its odorants.
That is important, because an odour’s
intensity has a lot to do with our perception of
pleasantness. At low concentrations, an odour
can be appealing, but as intensity goes up,
people begin to find it less and less pleasant,
says Joel Mainland, an olfaction researcher
at the Monell Chemical Senses Center in
Philadelphia, Pennsylvania. Individual
differences in receptors can affect perceived
intensity, and thus perceived pleasantness.
This might help explain, for instance, why
people with one variant of the odour receptor
OR6A2 are more likely to enjoy the smell of
coriander than those with a different variant.
Odour receptors may not be the whole
story, either. The mucus that lines the nasal
cavity is loaded with enzymes that can alter
odorant molecules before they even reach
the receptors. No one knows yet how much
difference these enzymes make to odour
perception, but there are hints that they
could be important. For example, some
people describe a particular molecule
as woody, whereas others describe it as
raspberry. On closer study, it turns out
that the two groups differ not in any odourreceptor, but in an enzyme called CYP2A13,
which converts the woody-smelling molecule
to one with a raspberry odour.
These individual differences haven’t
stopped olfactory scientists tackling the
big question of what exactly makes an odour
appealing. Given the molecular structure of an
odorant, can we predict how pleasant people
will find it? The first clue that this was possible
came more than a decade ago. Noam Sobel at
the Weizmann Institute of Science in Israel
and his colleagues asked 185 people to rate
the pleasantness of 90 anonymous odorants.
Then they turned to a database of physical
characteristics of molecules – everything
from number of atoms to various measures of
shape, more than 1500 attributes in all – and
used statistics to find out what combination of
features best predicted pleasantness ratings.
They found that better smells tend to come
from larger, more straggly molecules, while
smaller and more compact ones tend to smell
nastier. To provide an independent test of
this conclusion, Sobel’s team predicted the
pleasantness of 27 odours not used in the
earlier experiments, before getting people
to rate the smells. Sure enough, molecular
size and compactness turned out to be a
good – though not perfect – predictor of
pleasantness ratings (Journal of
Neuroscience, vol 27, p 10,015).
Since then, several other studies have
yielded more or less the same result. “But why
did the system evolve to extract that particular
chemical aspect of the world? I don’t have a
good explanation,” says Sobel. The answer,
others speculate, might be to do with how ourNice or nasty? Past
experiences play a
key role in how we
judge a smell“ Many cheeses
contain the
same odorous
molecule as
sweaty socks”
Not all new aromas are
prospected out of the
rainforest. Some are right
under our noses, so to
speak. “One of our biggest
achievements was to
extract apple. The fruits
are full of water, and in
perfumery we hate water,”
says Hervé Fretay.
He directs the natural
products division of
Givaudan, a fragrance
company that, along
with three other large
multinationals, controls
more than half the global
fragrance market.
Givaudan has also
succeeded in extracting
fragrance from ceps, theprized wild mushroom.
“It doesn’t seem exotic,
but for perfumery it was
something really unique,”
he says.
Once chemists find an
interesting molecule, they
play with it, making slight
changes to see how they
affect the odour. Givaudan
chemists, for instance,
took an extract of patchouli
oil and fermented it, using
enzymes to add peppery
notes to its earthy and
woody character. The
result, an ingredient called
akigalawood, is one of the
hottest new perfume
components.
Finding odour moleculesisn’t the biggest challenge,
though. “The problem
nowadays isn’t getting
molecules that smell like
such-and-such,” says
Charles Sell, a retired
fragrance chemist.
“The problem is getting a
molecule that smells that
way and doesn’t go off,
doesn’t interfere with
other ingredients or
biodegrade quickly, and is
totally safe on the skin.”
Many citrus-scented
molecules are unsuitable
for use in candles, for
example, because they
flare and smoke, or form
nasty, petrol-smelling
compounds when burning.SECRETS OF A FRAGRANCE LABORATORY
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