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Tuesday, November 27, 2012

The smell of white

You’ve heard of white noise, and you’re certainly familiar with the color white. Now meet ‘olfactory white’, courtesy of researchers from the Weizmann Institute of Science led by Tali Weiss. Yes, they’ve identified white odors.

First, what is meant by white? When it relates to color or noise, white is what you get when you mix equal proportions of many wavelengths or frequencies. Despite being individually extremely varied, the disparate elements cancel each other out to produce a sensation we refer to as ‘whiteness’. The scientists were interested in whether you could create the same effect with smells.

To that end, the researchers started with 86 monomolecular odorants (chemically pure single smells) and diluted them to be of equal intensity. They then mixed the smells in various combinations. Mixtures contained anywhere from one to forty-three components. 56 volunteer sniffers compared 191 pairs of mixtures. Except for identity controls (both sniff-jars contained the exact same components), there were no component scents in common between the pairs of mixtures. In other words, if jar one contained elements 5, 13, 27 and 76, jar two did not contain any of those odorants.

The scientists found that the more components in each mixture, the more alike the two smells were judged to be. Jars with thirty or more component smells were rated as nearly identical despite having no scents in common. This is a clear analog for the color or sound we call ‘white’.

So why don’t complex foods and perfumes all smell alike? After all, many things contain more than thirty separate scents. Apparently, you need two conditions to achieve white smell. First, all the components have to be equally intense. This clearly isn’t the case for items like coffee. Second, the thirty plus individual aromas have to be from very different regions of the olfactory spectrum. You can’t get to white by mixing a bunch of smells that are already fairly similar. Taken together, this may mean that unlike the color white, white smells are a purely manufactured phenomenon. 

Weiss, T., Snitz, K., Yablonka, A., Khan, R., Gafsou, D., Schneidman, E., & Sobel, N. (2012). Perceptual convergence of multi-component mixtures in olfaction implies an olfactory white Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1208110109