The Chemistry of Nostalgia

by Hannah Winter, BSc Chemistry. Originally published in issue eight of Resonance.

TreesSo many aspects of our seasonal traditions are filled with nostalgia: the lights on the tree, the taste of mince pies or the heat of the fireplace. With the short days and cold nights, our senses lead us to reminisce. Out of our five senses, however, it is smell that really transports us back in time. Imagine the crisp smell of the air after snow, the smoky bonfire on the 5th November or your first pumpkin spiced latte of the year. It’s often astounding how vivid the memory within the scent of a slowly jading Christmas tree can be. But how is it that smell brings back such strong emotions that none of our other senses can compete with?

First, a little background about how the olfactory (relating to smell) system works. The olfactory sensors in the human nose are covered in a layer of mucus, so when we breathe in, air is passed over this mucus and volatile chemicals from our surroundings become dissolved in it. It is the following interaction of these dissolved chemicals with our receptor cells that cause ‘smells’. Hence, for a chemical to be smelled it is essential for it to not only be volatile, but also soluble in this mucus. The olfactory sensor cells have over 1000 different types of receptors, resulting in a much more complex sense compared to our other four, leading to the ability to differentiate between many types of stimuli. These cells also regenerate and change according to what a person frequently smells (this is why you cannot smell your own scent).

This complexity is one theory as to why odour stimulated memories are so sentimental. The other main theory is more closely related to neuroscience, but investigating the complex systems that are our own brains is still largely a work in process. However, it is known that the stimuli from the nose are processed in the olfactory bulb which is directly connected to our hypothalamus and amygdala; the two areas most associated with memory and emotion. So, the next time that certain smell brings back memories of home, remember that, really, it all comes down to chemistry.

CinnamaldehydeCinnamaldehyde (C9H8O) is the source of the aroma (and flavour) of most of your favourite autumn bakes and lattes. It is the major component of the essential oil found in the bark of cinnamon trees (cinnamon sticks).

MenthoneMenthone (C10H18O) is a monoterpene and ketone which is found in peppermint, and is often used in cosmetics due to its fresh minty aroma. It is one of 4 possible stereoisomers and is closely related to menthol.

PineneBornyl AcetateThe two Pinene (C10H16) isomers and Bornyl Acetate (C12H20O2) are responsible for the odour of pine trees.

Bornyl acetate has the typical fresh pine odour while the two isomers of pinene give the pines a smell particular to their region with the (-) isomer being more common in European pines and the (+) isomer in North America. Beta-pinene has a woodier, forest like smell in comparison to the alpha isomer.

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