After my Sideways post, I was reflecting on the statement, “the essence of wine tasting is not so much in the palate, as the bouquet.” And I was thinking that it’s pretty much common knowledge, that our ability to discern the flavor of wine – or anything else for that matter – relied principally upon our scent receptors, as our taste receptors are limited to salt, sweet, sour and bitter.

Well, I’ve learned with my movie trivia posts that presuming what others may or may not know is a pretty uncertain practice, so I thought I’d elaborate. But it’s been almost three decades since I studied any of this. So, rather than take my own knowledge for granted, I thought I’d best do a quick refresher. Boy, amazing the new things science has uncovered in the last three decades.

First, it’s “common knowledge” that different regions of the tongue are sensitive to different tastes. But it’s incorrect. I was skeptical apout this from the start, and tested it on myself by dabbing different areas of my tongue with cotton swabs dipped in different solutions. I didn’t notice any difference, save for that different regions had varying sensitivity to every taste. When I told my instructor about this, he called my little experiment “hardly scientific.” Now I feel vindicated. 🙂

Second, it seems we have taste buds not only on our tongues, but also our epiglottis and soft palate.

Finally, and this is the biggie: There is actually a FIFTH taste quality: umami (pronounced: oo-marmi). This was discovered in Japan almost a century ago, but only known of here in the West since 1996:

Umami is the taste of certain amino acids (e.g. glutamate, aspartate and related compounds). It was first identified by Kikunae Ikeda at the Imperial University of Tokyo in 1909. Recently it has been shown^1,2 that the metabotropic glutamate receptor (mGluR4) mediates umami taste. Binding to the receptor activates a G-protein and this may elevate intracellular Ca²⁺

Monosodium glutamate, added to many foods to enhance their taste (and the main ingredient of Soy sauce), may stimulate the umami receptors. But, in addition, there are ionotropic glutamate receptors (linked to ion channels), i.e. the NMDA-receptor, on the tongue. When activated by these umami compounds or soy sauce, non-selective cation channels open, thereby depolarizing the cell. Calcium enters, causing transmitter release and increased firing in the primary afferent nerve

¹Chaudhari et al, (1996) The taste of monosodium glutamate: membrane receptors in taste buds. J. Neurosci. 16, 3817-3826.
²Kurihara & Kashiwayanagi (1998) Introductory remarks on umami taste. Annals NY Acad Sci 855, 393-397.

Monosodium glutamate
Monosodium glutamate is the main ingredient of Soy sauce. This is added to foods to enhance their flavour. It probably works by activating NMDA receptors which are found in taste cells. NMDA receptors are integral receptor-ion channel complexes and when they open they allow an influx of Na⁺ and Ca²⁺ ions. This will depolarise the taste receptor cell and act as an excitatory influence. Then, far less of a particular taste will be required to cause the further depolarisation necessary to bring about transmitter release.