The first weekend in December was a magical time in my childhood. Saturday morning we would load into the minivan, stop at Tim Hortons for hot chocolate, and take the half an hour drive to a local tree farm.
Walking through the forest, there would be an overwhelming smell of pine saturating the air. Even as an adult, whenever I smell the scent of a fresh pine tree, I am transported back to those idyllic childhood memories.
Recently, when I was visiting a friend, I had this memory tripped from a candle she was burning.
As a chemist, this immediately made me curious as to what chemicals I was actually smelling, but also: how do I smell, and why did it trigger this memory of Decembers past?
How do we smell?
You are constantly breathing in chemicals, like oxygen, every time you take a breath. Along with air, small amounts of other lightweight chemicals are floating around. If a chemical will trigger a smell, we call them odurants! When these odourants pass through the nose, they trigger some of the 40 million olfactory neurons in the human body. The specific neurons that are triggered depends on the chemical structure of the odorant, just like how only specific keys will fit specific locks. The specific combination of triggered neurons tells your brain what you're smelling. Everyone has a unique combination of these neurons, so how you smell a pine tree may be slightly different than the person beside you.
What was I smelling?
The distinct smell of a Christmas tree depends on which type of tree you have, but all conifers have 3 basic chemicals that contribute to their characteristic smells: a-pinene, B-pinene and bornyl acetate. These three odourants are part of the terpene family of chemicals. The two pinene molecules (C10 H16) may look similar, but they behave (or smell) quite differently. Pinene is the woody, fresh smell, whereas pinene smells a bit more like turpentine. The third chemical, bornyl acetate (C12 H2 0O2) is usually the most prominent smell, sometimes referred to as the “heart of pine.” The smell is described as balsamic or camphorous. This is the chemical I was likely smelling when my friend was burning her candle, since it is a common essential oil added to many products to achieve the “pine” or “woodsy” smell. Slight variations in trees are attributed to other members of the terpene family. These may include: terpenes limonene, myrcene, camphene, and alpha-phellandrene.
Why are smells so closely linked with our memories?
Scent is the first sense we develop as babies. Unlike our other senses, scent bypasses the relay station of the brain (thalamus), and connects directly on smell centre (olfactory bulb). The olfactory bulb is part of the limbic system, which also contains the memory (hippocampus) and emotional (amygdala) centres of our brains. Because of these close connection, when you smell something, especially as a young child with a developing brain, the scents gets tangled up with our emotional memories. Memories linked with scents are often stronger and more vivid. The recall of the specific memories when we smell a specific scent is called odour-evoked autobiographical memories.
Other scents of Christmas
For many people, December is especially full of these odour-evoked autobiographical memories. Whether it is the smell of fresh gingerbread, a turkey roasting in the oven, or a fresh pine tree, specific smells can transport you back in time. Some other common Christmas scents include:
Wood fires – guaiacol, isoegugenol and many more
The smoke from a wood fire contains hundreds of compounds, although not all of these contribute to their smell. Some of the more potent chemicals are phenolic compounds such as the smoky smell from guaiacol or the spicy smell of isoegugenol.
Gingerbread – gingerol and zingerone
There are two predominant chemicals that give gingerbread its unique smell: gingerol and zingerone. Gingerol is found in fresh ginger, and is it what gives it ginger its spicy zip since it’s a relative of capsaicin, (the spicy part of a hot pepper). Zingerone is not found in raw ginger but made during the cooking process to give the sweet flavour of cooked ginger.
Mint - menthol
Whether it was adding peppermint to hot chocolate or munching on a candy cane, mint is a staple in Christmas desserts. The predominant chemical that you are smelling (and tasting) is called menthol. Similar to the chemicals in ginger, menthol triggers similar responses in the body to capsaicin, but a cooling sensation instead of burning.
Roast turkey - thiophenes, furans and pyrazines
Any time you cook you are completing thousands of chemical reactions simultaneously! When you are cooking a turkey, some of the sugars and proteins are undergoing a Malliard reaction. These produce many chemicals, some of which you are smelling include thiophenes, furans and pyrazines.
Rebecca Yardley is a science communication student at Laurentian University.