The Science Behind Pumpkin Spice

Around this time of year, when the leaves descend in fiery flurries and dusk creeps into the early afternoon, the smell of pumpkin spice begins to accumulate. Suddenly pervasive by mid-October and worn like a protective charm against the darkening days of the spooky season, the warming allspice aroma is adopted as the signature scent of the masses. Whether in perfumes, candles, drinks, or food, you will inevitably encounter the warming fragrance immediately evocative of cinnamon, ginger, nutmeg, and cloves.

What is interesting in many of these situations, however, is that the smell sensations you experience are not a result of any of these actual spices. For example, many pumpkin-spiced reed diffusers and candles contain synthetic versions or chemical derivatives of the compounds found in the natural source. For cinnamon — which is derived from the dried bark of trees containing a naturally occurring chemical called cinnamaldehyde — the common replacement is a synthetic form of the same molecule. 

Synthetic cinnamaldehyde can be produced at an industrial scale through the reaction between benzaldehyde and acetaldehyde. Since these are cheap and readily available starting materials, this method is more economical and less wasteful of natural resources than it would be to distill the essential oil from harvested cinnamon bark. The same rationale is applied in making nutmeg- and clove-like fragrances, where synthetic versions of the key scent compounds in the natural product are readily used. 

These fragrance compounds possess key chemical features that confer their scent properties, highlighting an important concept in aroma chemistry. The scent molecules for nutmeg and clove, for example, belong to a class of chemicals called alkenylbenzenes, characterised by a six-carbon-atom ring with at least two extra carbon atoms added on. A similar motif is also found in cinnamon, though it differs slightly from the others by the presence of an aldehyde group, where a carbon atom has both a double-bonded oxygen atom and a single-bonded hydrogen atom. 

This tiny difference at the submicroscopic level amazingly allows the human nose to distinguish between the relatively similar aromatic profiles of cinnamon and nutmeg. The former gives sweet and woody notes, while the latter is more earthy. The science behind this subtle distinction is in how the unique scent molecules of the two spices bind to different combinations of olfactory receptors in the nose — of which there are around 400 different types — in different ways. These receptors then send electrical signals to the brain, which processes the scent information. 

However, the finer details of how this distinction is made are unclear and representative of a larger puzzle in olfactory science. Indeed, it was only in 2023 that the first three-dimensional structure of a human olfactory receptor bound to a volatile fragrance molecule (propionate, which, in its acid form, smells rancid and vinegary) was deduced. Solving the structures of other receptors and their bound aroma molecules will be necessary for further exploring the science behind human scent. 

All things considered, hopefully the next time you stroll into a cosy local gift shop, with pine reed diffusers by the door and candle wicks flickering atop pools of orange wax, you will better appreciate the warm comfort provided by artificial pumpkin spice — the quintessential smell of autumn.

Illustration by Isabelle Holloway