The 10 basic odor categories proposed in 2013 by Castro, Ramanathan and Chennubhotla are fragrant, woody–resinous, fruity (non-citrus), chemical, minty, sweet, popcorn, lemon, and two sickening odors: pungent and decayed. The researchers derived these fundamental categories of olfactive perception by applying a statistical method to Dravnieks’ atlas of 144 odorants (Castro et al., 2013).
Smell is perhaps our most archaic sense, and certainly one of the most elusive to describe. Taste is commonly organized around five basic qualities, but the basic perceptual dimensions of olfaction remain unclear (Castro et al., 2013). Most of the 10 basic odor categories are closely tied to the attractiveness or unpleasantness of a substance, consistent with the evolutionary importance of a sense of smell that helps us recognize the right foods (Castro et al., 2013).
Who identified the 10 basic odor categories?
The study on basic odor categories was published in PLOS ONE in September 2013 by three researchers (Castro et al., 2013):
Jason B. Castro, Bates College, Lewiston, Maine
Arvind Ramanathan, Oak Ridge National Laboratory
Chakra S. Chennubhotla, University of Pittsburgh
Together, they described the fundamental odor categories that can define our “olfactive space”, the perceptual space of smell.
How were the basic odor categories identified?
Castro and his colleagues started from Andrew Dravnieks’ Atlas of Odor Character Profiles (1985), a standard data set in odor research (Public Library of Science, 2013). The Dravnieks atlas describes 144 single-molecule odorants, each rated against 146 descriptors such as “sweet”, “floral” or “heavy” (Castro et al., 2013).
To group this information into basic odor categories, the researchers used non-negative matrix factorization (NMF), a mathematical technique that reduces a large data set to a small number of underlying components (Castro et al., 2013). The press release compared the process to compressing a digital audio or image file, which reduces its size without, ideally, compromising its usefulness (Public Library of Science, 2013).
What are the 10 basic odor categories?
The 10 basic odor categories identified by Castro et al. (2013; Public Library of Science, 2013) are:
fragrant
woody – resinous
fruity (not citrus)
chemical
mint / peppermint
sweet
popcorn
lemon
sickening – pungent
sickening – decayed (decomposed)
The last two are the two types of nauseating odors identified by the study.
What do the basic odor categories really mean?
The names of the 10 basic odor categories are only an aid to intuition, the researchers note: each category is really a “meta-descriptor” made of combinations of more elementary descriptors (Castro et al., 2013).
Most of the basic odor categories still relate to the potential palatability or desirability of a substance. “Fragrant” and “sickening” correspond to relative pleasantness, while fruity, woody, chemical, sweet and lemon appear to work as cues of potential palatability. This fits a broader ecological view of olfaction, which emphasizes the role of chemical senses in moving us towards safe food and away from potentially toxic substances (Castro et al., 2013).
Individual odors in the Dravnieks atlas tend to sit close to a single one of the 10 categories. In other words, odor space does not appear to be filled evenly, but in a discrete, clustered way (Castro et al., 2013).
What are the limits of the 10-category classification?
Castro and his colleagues warn that the 10-category classification is a first approximation of the “axes” of perceptual space, to be confirmed by expanding the data on which it was built. They describe the ten basic odor qualities as necessarily provisional for several reasons (Castro et al., 2013):
the Dravnieks data set covers only 144 odors, a small and limited sample compared with the number of possible odors;
ratings are bound to a fixed vocabulary of 146 descriptors, which may hide the true complexity of odor space;
odor profiles are based on verbal judgments and should be checked against discrimination tests;
the analysis cannot tell whether the categories reflect perception or learning and cognition.
Why do basic odor categories matter for perfumery?
After the 2013 study, the researchers planned to use the odor-category framework to predict how a molecule will smell from its chemical structure. Castro noted that nobody had yet done this with compelling accuracy, and that flavor and fragrance companies are very interested in doing it well (Public Library of Science, 2013).
References
Castro, J. B., Ramanathan, A., & Chennubhotla, C. S. (2013). Categorical dimensions of human odor descriptor space revealed by non-negative matrix factorization. PLOS ONE, 8(9), Article e73289. https://doi.org/10.1371/journal.pone.0073289
Public Library of Science. (2013, September 18). What’s that smell? Ten basic odor categories sniffed out with math. ScienceDaily. https://www.sciencedaily.com/releases/2013/09/130918180425.htm