The “white noise” of smells

Meet Laurax, a not-very-bold, not-that-exciting new fragrance. According to a panel of sniffers, it’s neither appealing nor revolting. It’s “intermediately pleasant”. People almost trip over themselves to describe it in non-descript terms—“fragrant”, “chemical” and “perfumery”.

Laurax isn’t going to set the perfume world ablaze in the near future, but its scientific implications are fascinating. This bizarre scent is actually a set of completely different fragrances that all smell roughly the same. It’s the odour version of “white”.

The colour that we call white is a blend of many different wavelengths of light. Add red and blue light together, and you get magenta. Add other colours and eventually, you converge on white. The same applies to sounds: if you combine tones of different frequencies, you eventually arrive at a perceptual hum called “white noise”. There’s no fixed formula for making white light or white noise. You don’t need to mix a specific set of colours or frequencies. As long as the individual ingredients are different enough, and roughly equal in intensity, whiteness emerges.

Now, Tali Weiss and Kobi Snitz from the Weizmann Institute of Science, Israel, have shown that this concept also applies to smells. Both work in the lab of Noam Sobel, who smell-based research includes creating sniff-steered wheelchairs, and unveiling the chemical signals hidden in tears.

The team’s latest discovery is no less fascinating. By mixing groups of 30 differently-scented molecules, they could create complex mixtures that all smelled the same, even if they didn’t have a single ingredient in common. The result isn’t a specific odour per se, but more like a unified perception. They called it “olfactory white”. Or, for the purposes of their experiments, Laurax.

It’s not obvious that olfactory white should exist. Think about the smell of coffee, the aroma of tobacco smoke, or the scent of a rose. All of these are complex mixtures of hundreds, if not thousands, of chemicals. And yet, they’re all distinctive and easy to recognise. It’s easy to think that large mixtures of smells retain their own identities, without merging into white like noises or colours.

Weiss and Snitz proved otherwise. They worked with 86 single-molecule odours that were different from each other in terms of both smell and chemical properties. In technical terms, they hailed from all corners of “perceptual space”.

The duo diluted the individual components so they all smelled equally intense, and mixed them into almost 200 groups, each containing between 4 and 43 ingredients. They dabbed the chemicals separately onto absorbent pads so they wouldn’t react with each other, but would release vapours that mingled in the air above. Fifty-nine volunteers then sniffed the mixtures in pairs, and rated how similar they were to each other.

Weiss and Snitz found that “the more components there were in each of two mixtures, the more similar the smell of those two mixtures became, even though the mixtures had no components in common”. As more and more distinct components are added to two brews, the more similar those brews would smell. And presumably, if you added enough chemicals, everything would eventually smell the same. That’s olfactory white.

Weiss and Snitz tested this concept further by creating four fragrances, each made from a different combo of 40 ingredients, and each labelled as “Laurax”. They asked a dozen volunteers to familiarise themselves with one of the four blends over three days.

On the fourth day, they had to assign labels to four new mixtures of 1 to 40 components; their choices were “Laurax” and three names concocted by a professional perfurmer. They volunteers were more likely to describe a fragrance as “Laurax” if it had a large number of ingredients, even if those ingredients were very different to the original blend that the volunteers had become acquainted with (below, left). And if they were given 21 blends of entirely new ingredients, they could easily tell that simple mixtures weren’t Laurax, but were basically guessing when it came to the complex 30-strong mixtures (below, right). Once blends hit that 30-ingredient mark, they all smelled roughly the same.

The various olfactory whites weren’t entirely identical. People could still distinguish between different versions of Laurax, but perhaps that’s to be expected. With vision and sound, it’s very easy to blend diverse ingredients using wavelengths and frequency, but it’s much harder to do the same with the traits of molecules. It’s also hard, and very labour-intensive, to balance the intensity of the different molecules.

This is a critical point: Laurax isn’t just any large mixture of molecules. It doesn’t smell like coffee, or wine, or tobacco. When Weiss and Snitz created blends of 30 chemically similar ingredients, or blends where ingredients were unevenly balanced, the volunteers were less likely to describe them as Laurax.

Does olfactory white actually exist in nature? It seems unlikely. Most natural aromas have ingredients that aren’t as diverse in chemistry, or balanced in intensity, as those in Laurax.  A rose, for example, may give off hundreds of molecules, but just one of these – phenylethyl alcohol – accounts for 70 percent of the mixture. If you spray this molecule alone, you’ll get an unmistakeable (if somewhat poor) smell of rose.

Nonetheless, Weiss and Snitz argue that both white light and white noise are also rather rare in nature, but both concepts have taught us a lot about how our senses work. Perhaps olfactory white will too. The team are now planning to study the brain activity of volunteers as they take a whiff of Laurax, and they’ve also submitted a patent “for a wide range of potential applications for olfactory white.”

And what does Laurax smell like? Not like anything in particular. When the team asked professional perfumer Danyel Gafsou to describe the various Laurax blends, he came up with a wide variety of terms. When they asked volunteers to rate the blends according to 147 different descriptions, they avoided specific ones like “leather” or “coffee”, and were most likely to choose generic terms like fragrant, chemical, perfumery, aromatic, floral, soapy, sweet, fruity and medicinal. Compared to many other smells, Laurax falls smack in the middle between pleasant and unpleasant, and between edible and poisonous. It’s as intermediate a smell as you can get.

“Finally,” Weiss and Snitz write, “the best way to appreciate the qualities of olfactory white is to smell it.” To that end, they’ve listed three different recipes for Laurax, two of which share no components at all. The numbers all relate to identifiers in the PubChem database. You can mix them yourself, and inhale away.

Reference: Weiss, Snitz, Yablonka, Khan, Gafsou, Schneidman & Sobel. 2012. Perceptual convergence of multi-component mixtures in olfaction implies an olfactory white. PNAS http://dx.doi.org/1073/pnas.1208110109

Photo by Waferboard

More from the Sobel lab:

• Sniff-detector allows paralysed people to write messages, surf the net and drive a wheelchair
• Tears as chemical signals – smell of female tears affects sexual behaviour of men