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Wednesday September 25, 2013 MYT 12:00:00 AM
Wednesday September 25, 2013 MYT 7:46:16 AM
by melissa pandika
By analysing words used to describe 144 odours, scientists have classified the range of scents humans can detect into 10 basic categories. – EPA
IT might seem that the range of scents humans can detect is infinite, but American scientists have managed to sort them all into 10 basic categories, ranging from peppermint to pungent.
The classifications are meant to be the olfactory equivalent of the five basic tastes: sweet, sour, salty, bitter and umami.
To come up with the 10 scents, neuroscientists turned to a 30-year-old database that contained profiles of 144 odours. Each odour was assessed by human subjects, who were given a list of 146 words and asked to rate how well each word described the odour. The researchers wanted to see if they could look for patterns in those responses that would help them group the odours into distinct categories.
Using statistics, they analysed how the 146 words were used and how they were related to one another. Some words were almost always used together, like “fruity” and “honey”. Others were rarely or never paired, like “faecal” and “minty”. Words that were hardly used at all were ignored in the analysis.
By the end of the analysis, the researchers came up with total of 10 distinct groups of words that tended to be used together.
The researchers then identified the key word in each group that described the fundamental characteristic shared by all the group members. For example, “rose,” “floral,” “fragrant” and “violet” belong to the same group, but all of them can be described as “fragrant”.
“It’s sort of like what’s happening when you compress an image or audio file,” says Jason Castro, the neuroscientist at Bates College in Maine, United States, who led the effort. “You dump all the redundant stuff and keep only the most essential information.”
The result was a list of 10 key odour categories: fragrant, woody/resinous, minty/peppermint, sweet, chemical, popcorn, lemon, fruity (non-citrus), pungent, and decayed.
“For any given odour, we can assign it to one of 10 of these perceptual buckets,” says Castro, who reported the results this week in the journal PLOS ONE published by the Public Library of Science, an American nonprofit publisher and science advocacy organisation.
Odours in the fragrant category included lavender, soap and cologne, while freshly cut grass and mushrooms gave off a woody/resinous smell. Eucalyptus, camphor and tea leaves were considered minty/peppermint scents. Sweet odours included vanilla, almond and chocolate. Kerosene and ammonia fell into the chemical category.
Butter, molasses and fried chicken were lumped into the popcorn group. Oranges and other citrus fruits were grouped as lemon odours, while the other fruits went into their own category.
Rounding out the list were pungent odours like garlic and sour milk, and decaying smells such as rotten meat and manure.
Each odour in the database fell into only one category, but it’s possible that other odours – such as kettle corn or coffee – might belong to multiple categories, Castro notes.
The researchers have not yet tested these categories on people to see whether humans would make the same distinctions. But there is reason to believe they might, since the categories were not completely subjective. As part of the study, Castro and his colleagues examined the chemical structures of odours in the woody/resinous group using data from earlier studies. They found that several of these odours had similar chemical structures.
This surprising finding suggests it may be possible to predict how a chemical will smell based on how it’s built.
If so, that would have practical applications, Castro says. For instance, it could be useful for developing devices that “sniff out” cancer and other medical conditions.
In the meantime, scientists need to investigate whether the 10 categories can describe more odours, says Leslie Kay, a neuroscientist at the University of Chicago who wasn’t involved in the study. The 144-odour database “is still a small collection of the odours we encounter in our world,” she says.
What’s more, the odours tested consisted of only one type of molecule. In reality, most odours contain dozens of types of molecules.
“There could be a whole other level of complexity,” Castro says.
Still, the new scheme is “very good first step” according to Kay. “It’s like finding the colour wheel for odour. It’s immeasurable.” – Los Angeles Times/McClatchy-Tribune Information Services
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Science & Technology, biology, human, nose, smell, key odours
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