Friday, September 9, 2016

Revealing Salient Features of the Smell Network

aka On Sweetness and Flowers

pdf screenshot: Aldrich's Flavors and Fragrances Catalog, 2013. "Organoleptic properties index", pp145-169. 
If you work in the Flavor and Fragrance Industry, chances are you use the Sigma Aldrich catalog to reference the chemicals, the raw materials, of your craft. Sigma Aldrich produces and procures about 1,000 ‘smellable’ chemicals for distribution among the Industry.

It is the nature of volatile organic compounds (‘smellable’ molecules) that the same units can be used to make a variety of aromas. Although there are only 1,000 chemicals, they are indexed by over 3,000 descriptors, such as smoky, anise, spicy, or rose. One does not search for the chemical Geraniol, but instead looks up “Rose”, under the super-category Floral, and there will be Geraniol, along with all other chemicals used to intimate the aromatic impression of a rose. Further collapsing these roughly 3,000 descriptors into more general categories, again there is redundancy. The aromatic descriptor “Lemon” is categorized under the Citrus as well as the Woody groups.

Of all these descriptors, “Sweet” is the most redundant; it is repeated in four different groups, more than any other descriptor. In network science parlance, this is called centrality, or betweenness. Thus, Sweet has a higher centrality value than any other descriptor. It is a kind of glue, a hub, which holds the network together. (This is a flavor and fragrance catalog, after all.)

Using Sweet as a locus, a modified aroma network can be configured. Within this “Sweet network”, every super-category that contains Sweet as a descriptor is present. (Note that the chemical indexed by “Sweet” in one grouping, such as Fruity, does not necessarily refer to the same chemical as the “Sweet” in the Balsamic group.) The related super-categories that share Sweet as one of their descriptors, they happen to be the most ‘copiously described’ groups. Fruity has 850 descriptors, the most of any group. Balsamic has 540.

This revelation is becoming somewhat anti-climatic, as it would be expected that the more descriptors a group has, the more likely it would share one of those descriptors with another group. But what is this? If we look down, at the third-most populated grouping – Floral, at 296 – we see that it is absent from the Sweet network. If groupings with many descriptors tend to contain Sweet, and Floral has many descriptors, then it should also contain Sweet. It does not.

In the first place, Sweet is not a smell, it is a taste. But this is a flavor and fragrance catalog, you retort. But what then about sour, salty, bitter, and savory? They do not exhibit the same centrality as sweetness. Next, we do not eat flowers. In fact, a major part of the profile of a “floral” note is the smell of shit – not the most gustatorily satisfying of aromas.

The corpus of the Sigma Aldrich smell network is highly complex in its structure and associations, with stable patterns found few and far between. No matter the corpus, throughout history such has been the same for smells. At least for one of its infinite configurations, it can be observed that Sweetness and Flowers are very distinct entities in the aromasphere: We do not eat Flowers, and we do not smell Sweet.

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