Sniffing Out How We Smell
Many would argue that the
sense of smell is our far most important sense when it comes to
evaluating wine. This is because the olfactory sense plays a
fundamental role both in the perception of aroma and also flavour.
Without it all wines, rather than just a few, would appear to be
uninteresting alcoholic solutions of acid and sugar.
The sense of smell is by far
the most powerful of our senses. It can discriminate between tens of
thousands of different aromas, and is capable of detecting some
compounds found in wine at levels around a single part per trillion.
An example of this is the compound 2,4,6 Trichloroanisole, the
primary compound responsible for cork taint. Some people can detect
it at around 1 part per trillion which is analogous to a single
second in thirty two years. Not bad I suppose, but when you think
that an Alsatian or Beagle probably 'sees' a three dimensional
kaleidoscope-like scent world, our olfactory efforts seem pretty
pathetic in comparison.
The olfactory system consists
of three major components, each of which has a specific role in
converting the presence of an odorous stimulus to a perception.
These components are 1) the olfactory epithelium, 2) the olfactory
bulb and 3) higher order parts of the brain.
The epithelium is involved in
the sensation stage of odour processing. It is a small patch of
yellowish tissue, approximately 2cm2 located in the top rear of the
nasal cavity. It consists of three major cell types, however only
one of these, the receptor cell, is directly involved in odour
processing. A typical epithelium contains approximately 3 million
receptor cells. Incidentally, these cells only have a lifespan of
about 3 weeks. As they die, other underlying cells morph into new
receptor cells. Not surprisingly as we get older receptor cells die
and their replacements forget that it is their shift. This gradual
loss of receptor cells in part explains the typical loss of
olfactory acuity with advancing age. But don't worry, as it is
thought that this decline only begins at (wait for it), the age of
45!
The receptor cell is roughly
the shape of a bowling pin. The top portion of the receptor cell
gives rise to between 5 and 20 long filaments called cilia. These
cilia protrude into a mucus layer that protects the epithelium and
provides access for the aroma molecules to the epithelium. Embedded
into these cilia are molecular sized proteins that serve as the
receptor sites for the aroma molecules. The exact number of
different types of proteins types is unclear. However fairly recent
genetic studies have shown that each receptor cell is peppered with
only one of the estimated 100 to 1000 different types. As each of
the receptor proteins are large and complex molecules, it is likely
that each one can serve as a "docking site" for many hundreds of
different aroma molecules.
At the opposite end of the
receptor cell is an axon. You can think of an axon as a piece of
electrical conducting material that connects the receptor cell to
the next part of the olfactory system, the olfactory bulb which in
turn is connected to other parts of the brain. The olfactory bulb is
a small part of the brain located adjacent to the epithelium. Its
exact role is still being hotly debated, however inspection of how
the cells comprising the bulb are connected points to at least one
role; that of increased differentiation between similar smelling
odours.
The olfactory bulb connects to
other areas of the brain including the olfactory cortex, the
thalamus and the hippocampus. These areas are involved in odour
perception and interpretation. The cortex seems to be involved in
discrimination while the latter two areas are involved in long term
memory. The strong linkages between the epithelium and the parts of
the brain responsible for memory may explain why humans can recall
odours experienced in the distant past and why odours evoke such
strong feelings. However, sometimes the ability to recall odours
from childhood can be really annoying, like a bad tune that you
can't shake. Personally I have never been able to forget the smell
of my first newborn puppy, or the vomity smell of over-ripe bananas
in my school lunchbox. And for those who are interested the puppy
smelled of concrete dust, and exposure to the latter scent explains
why I can't come at over-ripe Sauvignon Blanc's, or wines made with
Lalvins ubiquitous wine yeast EC1118.
But how do we perceive these
odours? When we take a sniff of the wine in the glass, we suck up
the volatiles in the head-space of the glass. These molecules then
dissolve in the mucus and pass through it until they reach the
cilia. The next step is a crucial one. Each different type of odour
molecule has a different size and shape, which will allow it to
attach itself to some receptor proteins but not others. As the cilia
on each receptor cell is likely to contain only a single receptor
protein, then each odour molecule will have an affinity for some
receptor cells but not others. Think of it as a key being able to
only open a certain lock. When there are sufficient numbers of odour
molecules attached, the receptor cell will send out a burst of
electrical activity that will pass down the axon to be processed by
the olfactory bulb. This electrical activity signals to the brain
that odour molecules are present. We then perceive the odour. The
greater the total electrical activity, the stronger the smell, and
the pattern of activity across the epithelium determines whether
it's an afterbirth drenched puppy or 1118. Quite clever really. I
wonder whoever thought of it?
Richard Gawel
This article originally
appeared in The Wine Tutor Series: Winestate Magazine
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