The Rarified Perfumes of San Miguel de Allende: Part Two of Four

By Aaron S. Reisfield

Previously, in this space, we took up the question of how we perceive plant perfumes at altitude. We figured that the floral bouquets of local vegetation around San Miguel de Allende are attenuated. But there are countervailing influences. Volatiles from all kinds of organic materials readily take flight, eagerly becoming airborne thanks to the reduced pressure of ambient air. So, as a matter of physics, the lower atmospheric pressure here enhances our perception of perfume. And, likewise, we can, in theory, perceive a greater variety of aromatic elements in the environment here. Yet there’s another countervailing factor that may be offsetting, in that the air is thinner, which is to say that there are fewer volatile molecules contained in any given parcel of air that we encounter.

So, all this begs the question: at this altitude, which is it—an olfactive upgrade or downgrade? And which is it—are scents here more or less intense? The right answer looks to be: it depends.

For there’s yet another detail, this one relating to human physiology. During periods of the year when ambient humidity is low, our nasal mucous membranes are prone to becoming dehydrated, in which event our olfactive and gustative senses may become compromised, on account that our olfactory receptors need a moist environment to function well. Bearing this in mind, we might better understand the airplane-cabin tomato-juice effect. Humidity in airplane cabins is uncommonly low, ten to twenty percent, which is sure to diminish our capacity to perceive scents, and hence our capacity to taste food. So some (I’m in this group) will crave tomato juice even though we rarely drink it on the ground. The idea is that the acidity and savory notes of the juice will serve to cut through our dulled senses. But this isn’t to compare San Miguel to a plane cabin. There isn’t much real correlation, and even if there were, our brains recalibrate when we live at this elevation. The psychology and biology of those who reside on the Mexican Altiplano is one of acclimation. And then again, plants acclimate to altitude too. Plants may compensate for the dampening of their redolence and become more fragrant. Or they may shift in the character of what they radiate. And since they join with pollen vectors in coevolution, we can expect that reciprocal adaptations are usual. 

An example comes to mind of aromatic plants that accommodate their surroundings as correlated with altitude: The herb, thyme, is known to comprise chemotypes, taxonomic groups of plants that differ from each other in chemical profile. There are distinct thyme populations that grow in hot and dry conditions, and are known for their aggressive, germicidal, or “hot” essential oils. We refer to these groups as phenolic chemotypes. They are distinguishable by the molecule, thymol. And there are populations that grow in cooler, milder environments, often on higher ground, and are known for their ethereal essential oils. We refer to these groups as terpene alcohol chemotypes. They are distinguishable by the molecule, linalol. There are countless such examples that relate to plant perfumes and altitude.

Let's revisit the differential process by which fragrant molecules volatilize. Following is an observation I find interesting: Scented products, as they age, will have the air of being less fresh at our elevation. This is due to the accelerated evaporation of low boiling point chemical principles, fleeting citrus monoterpenes for instance, which we associate with naturalness. Thus fragrances overall may transition quickly to seeming less bright and toppy. And should a vial of essential oil be left uncapped for a period, its profile will evolve readily at our elevation, soon giving the impression of being flat and heavy.

In the next installment of this series, we will shed some light on how our experience of cuisine is affected by altitude, so check this space in the coming edition of Atención News.

Aaron S. Reisfield is a long-time resident of San Miguel. He has collected plants throughout montane México and served as a Visiting Scholar in Botany at The University of Texas in Austin. He has published a book series, The Perfume of Life trilogy, premised on the idea that the story of life on Earth is best related in the language of molecules—the language of perfume. He distills plants for their fragrant oils and waters.