Scientists Just Divided the Sense of Smell Into 10 Types

Science is all about trying to explain and understand the apparent chaos in the world, and a big part of that is breaking things down into categories. We humans love categories. We've got the whole animal kingdom classified into genus, species, order, and so on; the seven colors of the rainbow; and do-re-mi-fa-so-la-ti-da. We've expanded on Aristotle's theory that taste can be broken down into two basic types—bitter or sweet—and since identified five categories that taste buds can recognize: sweet, sour, bitter, salty, and umami.

But until now, the science of smell—olfaction—was too complex for experts to group neatly into categories. Now researchers from Bates College and the University of Pittsburgh have used a mathematical algorithm to whittle hundreds of standard smells down to their basic essence, and determined that life's odors fall into some combination of 10 basic categories of smell: fragrant, woody/resinous, chemical, minty/peppermint, sweet, popcorn, lemon, pungent, and decayed.

Unsurprisingly, the smell groups have a lot to do with taste. In fact, the authors of the study,  published yesterday in the journal PLOS ONE, suggest that the scent receptors in the human nose are designed to help us sniff out the foods we need for a healthy body—and of course the sickening ones to avoid. "One interpretation we like is that these may reflect the essential portions of diet… Things you will actively seek in the environment,” study author Jason Castro, a neuroscience professor at Bates, told NBC News.

Several of the categories suggest how palatable a food is. For example, "lemon" covers the general citrus class of foods and "fruity" all the other, noncitrus fruits. "Minty" takes care of herbs and "woody" vegetables. "Popcorn" is an umbrella term for a range of nutty, fatty, smoky tastes, like cinnamon, chocolate, tobacco, burnt objects or toasted foods, even fried chicken.  "Pungent" and "decayed" naturally depict rotten, moldy food or drinks to stay away from.

Other categories speak to how good or bad people think a smell is. "Fragrant," for example, spans anything from flowers to soap. And on the other end of the spectrum are putrid, "sickening" smells like urine, and dangerous "chemical" smells like gas.

Most natural smells are more complex, like say a cup of coffee, so would be described as a combination of one or more categories, like a musical chord is made up of several notes, researchers explained.

The science of smell is more complex than its sister scents like taste, sight, and sound. We can arrange musical notes and pitches on a scale, and see colors based on light wavelengths. "In olfaction, by contrast, we lack a complete understanding of how odor perceptual space is organized," the study explains. "Indeed, it is still unclear whether olfaction even has fundamental perceptual axes that correspond to basic stimulus features."

To reach their conclusion, the Bates and University of Pittsburgh researchers started with 144 basic odors pulled from the "gold standard" of smells, the Atlas of Odor Character Profiles published in 1985 by Andrew Dravniek. They developed an algorithm to simplify the scents, by comparing them to each other based on the chemical that stimulates them. To test the model, they hope to expand future research to even more scents.