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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,3 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 colliedescope 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?