Friend of Pairteas and tea connoisseur Marzi Pecen has been directing my attention to olfactory training sets, and one suggestion in the literature is to include a sample for rancidity, which led me to wonder: why is rancidity so unpleasant?
Found a paper* about receptors for polyunsaturated fatty acids (PUFAs) in the mouth and gut, and saw that free PUFAs bind to TRPA1, one of the trigeminal receptors that causes disagreeable sensations including pain when activated.
Normally, PUFAs exist in foods primarily in the form of triglycerides. Such a triglyceride consists of three PUFA molecules bound to a glycerol backbone. When tied up in this way the PUFAs can’t activate TRPA1.
As food rots (and this is true of fish in particular), the triglycerides break down, so that PUFAs are freed. Free PUFAs become oxidized, and both oxidized and unoxidized versions have a distinct odor.
In addition, now they can activate TRPA1. Activation of TRPA1 associates the odor with a disagreeable sensation—the overall effect is unpleasant rancidity!
Not with this lot of fresh fish, though...
Image from Morguefile.
* Arianne L. Motter, Gerard P. Ahern. TRPA1 Is a Polyunsaturated Fatty Acid Sensor in Mammals. Published: June 19, 2012. http://dx.doi.org/10.1371/journal.pone.0038439.
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