What makes all those gorgeous seasonal smells?
OUR sense of smell is probably the sense that most sparks our memories of days gone by. Smells most commonly associated with Christmas include those of a real Christmas tree, a fresh cold day, a log fire, freshly baked gingerbread, cinnamon and oranges.
Smells are caused by the different volatile chemical molecules that leave, for instance, the pine needles or oranges and move around the atmosphere. Sensitive cells located high up inside the nose contain a myriad of olfactory receptors that respond to these molecules and pass on this information to our brains. Different molecules interact with different combinations of receptors, allowing a relatively small number of receptors to identify a vast array of different molecules.
Recent research from America's Rockefeller University, in the prestigious journal 'Science', shows that we can identify at least a trillion different smells. In a small room the molecules will linger, resulting in increased amounts of molecules relative to outdoor locations and a larger response from our olfactory receptors.
On a cold winter's day there are fewer chemical molecules moving in the air due to the lower temperatures, while the olfactory receptors move a little more deeply into the nose for protection. Together, these mean there is less interaction of the molecules with the receptors, so it smells clearer.
Receptors can differentiate between molecules that are the same except they have a different orientation at one point in the molecule. The smell of lemons and oranges are both caused by the molecule limonene, but the form called S-limonone is associated with lemons, and R-limonone reminds us of oranges.
This Christmas, take a moment to enjoy the aromatic smells of terpenes (pine), cinnamaldehyde (in cinnamon), zingerone (gingerbread) and R-limonone as you consider our remarkable sense of smell.
Dr Simon Lawrence is a lecturer in University College Cork and a Principal Investigator with the Synthesis and Solid State Pharmaceutical Centre, based at UL.