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Why do some odors stink?

Mathematical model reveals odor map from natural environment.

Tim Lundeen 1

August 30, 2018

2 Min Read
Why do some odors stink?

Odor abatement in the livestock production industries is a significant issue, as farmers try to reduce malodors and government officials attempt to regulate and enforce odor standards. Increasingly, neighbors living near livestock operations look to the courts to litigate disputes over actual and perceived odors.

Even so, the science of odor, and what determines whether an odor is acceptable, is still very subjective.

According to an announcement from the Salk Institute, every smell -- from a rose to a smoky fire to a pungent fish -- is composed of a mixture of odorant molecules that bind to protein receptors inside a person's nose, but scientists have struggled to understand exactly what makes each combination of odorant molecules smell the way it does or predict from its structure whether a molecule is pleasant, noxious or has no smell at all.

Now, scientists from the Salk Institute and Arizona State University have discovered a new way to organize odor molecules based on how often they occur together in nature, which is the setting in which humans' sense of smell evolved. They were then able to map the data to discover regions of odor combinations people find most pleasing. The findings, published on Aug. 29 in the journal Science Advances, open new avenues for engineering smells and tastes.

"We can arrange sound by high frequency and low frequency [and] vision by a spectrum of wavelengths and colors, but when it comes to olfaction, it's been an unsolved problem whether there is a way to organize odors," said Tatyana Sharpee, an associate professor in the Salk Institute's Computational Neurobiology Laboratory and lead author of the new work.

Previously, scientists tried to classify odorant molecules strictly based on their chemical structures. "It turns out molecules with structures that look very similar can smell very different," Sharpee said.

Using existing data on the odorant molecules found in different samples of strawberries, tomatoes, blueberries and mouse urine, they used statistical methods to put odorant molecules on a map based on how frequently they occurred together in the four sets of samples.

Those molecules that occurred together more frequently were placed closer to each other.

Using this strategy, the scientists found that odor molecules could be mapped onto a curved surface in three dimensions, which turned out to be the shape of a Pringles potato chip — a shape mathematicians call a hyperboloid, the Salk Institute said.

When the team looked at how the molecules clustered on this surface, they found there were pleasant and unpleasant directions, as well as directions that correlated with acidity or how easily odors evaporate from surfaces. These observations now make it easier to construct pleasant odor mixtures to use, for example, in artificial environments, the announcement said.

Furthermore, by being able to better categorize odors, new and better abatement strategies may be possible on livestock operations as well as clearer regulatory thresholds.

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