Chestnuts roasting on an open fire: Check out the science of Christmas aromas

The warmth of cinnamon, earthiness of nutmeg, and fresh scent of pine all add to the holiday spirit, at this time of year.

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Scents often intertwine with memories, making us nostalgic and bringing a sense of comfort. Why is that? Well, researchers have found that humans process smell very differently from other sensory inputs. Where all other sensory data passes through the thalamus, which functions as a traffic policeman, smell takes a different route, directly accessing the olfactory bulb (known as the brain’s smell centre).

This bulb has strong connections with the hippocampus and amygdala, which are centres of emotion and memory. Thus, we often end up processing a fragrance and recalling the memories associated with it, before we’ve even identified what it is.

Why would these pathways have developed in this way? Scientists believe that the sense of smell played a crucial role in our ability to survive. Certain smells could signal danger, food sources, or the presence of predators. As a result, the human brain developed to connect smell with memory at superspeed; and then built survival instincts based on those linkages.

Christmas, of course, is not about fight-or-flight (unless the family is being particularly problematic this year). But aromas, because of how they work on the brain, naturally play a key role in the good times too. Let’s take a look at why some of them evoke the feeling it’s Christmas.

In homes across much of the Western world, a fir tree installed and decorated indoors forms the heart of the celebration. The freshly chopped tree oozes a sticky resin, a defensive terpenoid meant to protect it against rot and insects. The scent of the resin and the fragrance of the pine from the leaves make for a fresh and woody whiff of the green outdoors, adding a celebratory air to a cold winter, and a reminder that it won’t be long before spring.

Here, the aromas of the tree are activated by chopping and enclosing it indoors. With the foods of the season, the fragrances are largely activated by heating aromatics and spices.

The cloves that flavour mulled wine and the dry fruit that is baked into plum cake contain volatile molecules, for instance, that are increasingly released when warmed. Some of these compounds, such as the cinnamaldehyde in cinnamon and the eugenol in cloves, are more soluble in steam and fats, in fact, than in water. So they would need to be in something precisely like a drink or dessert to work this well.

Another wintery benefit: Such spices can literally warm the skin. If gingerbread, fruitcake and classic Christmas pudding (with its nutmeg and cognac) are considered the perfect shots of joy in cold weather, it’s because the flavour compounds in some of the spices involved in each of these register as mild heat when they come in contact with our tastebuds.

The cinnamaldehyde in powdered cinnamon activates a receptor in the mouth called TRPA-1. Crushed black pepper, which contains piperine and also features in some recipes, triggers the TRPV-1 receptors (the same ones activated by the capsaicin in chillies) and makes the mouth and throat feel warm.

Ginger, meanwhile, acquires an entirely different flavour profile when dried. The drying process converts the compound gingerol into the compound shogaol, which is “spicier” in that it strikes the TRPV-1 receptors more strongly.

Spice route

Amid the gleam of decorations and the joy of the Christmas carols, it can be easy for aroma and flavour to get sidelined. Not so fast, says author Harold McGee. In his 2020 Nose Dive: A Field Guide to the World’s Smells, he explains that, of all the senses, it is only with taste and smell that we come in direct contact with molecules from an object.

Our ability to smell comes from a very small area of about one square inch, situated just below our eyes, he adds. Even though this surface area is less than one-tenth the surface area of the tongue, its 400 receptors are theoretically capable of detecting millions of molecules.

There are animals with a lot more receptors. Dogs have more than double the number. But we are capable of putting the receptors to nuanced use. Wines, perfumes, biscuits and biryani… our brains are connoisseurs, if not collectors. (It is the same with hearing; we do not have the keenest sense of it, but what we have is a unique affinity and appreciation for music.)

Interestingly, the aroma molecules carried to our noses are only those light enough to be volatile (ie, easily evaporate). They are sometimes catapulted and intensified, such as by a spray of perfume, or the steam rising off masala chai. In the absence of an intensifying or catapulting agent, we must get closer to the source, and so we bend for a whiff of a flower or a freshly baked loaf.

Ingredients that combine intense, volatile flavour compounds are the ones we call aromatics. It is aromatics such as spices, dried fruit and sugared flour that make a menu “Christmassy”. Combined with the memories they evoke of past celebrations, the aromas add up to form a large part of the magic of the season.

May you create and celebrate the sparkliest memories this year.

(To reach Swetha Sivakumar with questions or feedback, email [email protected])

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