Protein-rich foods have a characteristicquality called “umami.” The Japanese chemist Kikunae Ikeda coined the term in1908 to describe the taste of meaty broths — “umai” means “delicious” in Japanese.
Umami is specifically the sensationproduced by glutamate; the form of the amino acid glutamine found in tea. Thistaste tells about the presence of protein, especially meat protein — 60% of theglutamate in muscle meat is free, that is, not attached to other amino acids toform a protein. You taste this free glutamate when you bite into a piece ofmeat.
It’s worth noting that the umami taste iselusive — in the West it is often confused with sweetness, or sometimessaltiness, or even sour. While umami isoften translated as savory, a more proper term might be ‘brothy,’ except thatwe don’t use the term ‘brothy’ for solid foods. It works for tea, though!
The confusion with “sweet” makes somesense: one of the many receptors for umami on the cells of human taste buds consistsof two coupled proteins. The two are known as TAS1R2 and TAS1R3. The sweet receptorin taste buds is also made up of TAS1R3, but in its case coupled with TAS1R1.Consequently, compounds that activate TAS1R3 may activate both receptors, andyou get both a sweet and an umami taste.
There are seven other proposed receptorsfor umami, indicating how important our ability to detect umami tastes, i.e.the presence of protein, is to our survival and well-being. Many of theseproposed receptors respond specifically to glutamate, while at least one isbroadly tuned to respond to amino acids more generally.
The most studied receptor for umami tasteis the TAS1R2+TAS1R3 pairing.
It is activated by a large number ofcompounds in tea. Foremost among these is theanine, the characteristic aminoacid of tea. Interestingly, theanine contributes both sweetness and umami to atea, which may explain why teas high in theanine are considered more delicious.They are sometimes described as having sweetness that is not sugary.
Other compounds that give an umamiimpression include breakdown products of DNA and RNA, and derivatives of aminoacids, in particular those produced during the Maillard reaction. This reactioninvolves the linking of sugars to amino acids — it’s the reaction that givesthe lovely brown color to a loaf of bread. It occurs when tea leaves are heatedduring processing, and through the actions of microorganisms during puer fermentation.
Our umami response to amino acids otherthan theanine and glutamate is relatively weak but can be enhanced bymethional. Methional is a derivative of methionine, a sulfur-containing aminoacid that breaks down during the early steps of tea leaf processing anddisappears with more prolonged processing. At high levels, methional smellslike raw potatoes (and is in raw potatoes). It can contribute to the sulfurousmarine qualities of some green and oolong teas. However, adding methional atlevels below detection to a solution of amino acids strengthens the umamiresponse — at these levels it may contribute to the pleasantness of a green oroolong tea!
In addition to their taste properties,umami compounds give a pleasant “full” mouthfeel. Whether it is activation ofumami receptors by themselves or activation of trigeminal (chemesthesis) nerveendings simultaneously with umami receptors is not known.
We cannot leave the discussion withoutmentioning another phenomenon observed by the Japanese, namely kokumi, a wordthat has been translated as "heartiness" or "full flavor.”
The kokumi sensation is accomplished byshort protein-like compounds called peptides that have a γ-glutamyl group (provided by glutamate) onthe end of their amino acid chain. It’s not surprising, then, that thesecompounds are found in foods with protein, and that they enhance the umamitaste of proteins.
By themselves, these compounds have littleif any taste. But if you add a compound that confers kokumi to a sugarsolution, it will taste sweeter. The same goes for a salt solution, it willtaste saltier!
How do kokumi compounds do this?
As Kuroda and Miyamura discovered, thesecompounds activate a calcium-sensing receptor on a unique subset of taste budcells. When a kokumi compound binds to this receptor, it causes a reaction,still poorly understood, that leads taste bud cells responsive to sweet, umami,or salt to be activated more intensely, and for a longer time.
And guess what! Tea’s major amino acid,theanine, is, in chemistry parlance, γ-glutamyl-L-ethylamide!Kokumi indeed!
Sources: Motonaka Kuroda and Naohiro Miyamura, 2015. Mechanism of the perception of “kokumi” substances and the sensory characteristics of the “kokumi” peptide, γ-Glu-Val-Gly. Flavour 4:11. doi: 10.1186/2044-7248-4-11.
Jianan Zhang, et al, 2019. New insight intoumami receptor, umami/umami-enhancing peptides and their derivatives: a review.Trends in Food Science & Technology, 88: 429-438.doi:10.1016/j.tifs.2019.04.008.
Motonaka Kuroda and Naohiro Miyamura, 2015. Mechanism of the perception of “kokumi” substances and the sensory characteristics of the “kokumi” peptide, γ-Glu-Val-Gly. Flavour 4:11. doi: 10.1186/2044-7248-4-11.
Yasuka Toda et al. 2018. Positive/negativeallosteric modulation switching in an umami taste receptor (T1R1/T1R3) by anatural flavor compound, methional. Scientific Reports 8,Article number: 11796.
FOOTNOTE: The density of the dots in the original illustration by Harvard psychologist Dirk P. Hänig corresponds to the intensity of the tastes in each of these locations sweet, 1-bitter, 2-sour, 3-salty and 4-sweet. This illustration has been interpreted to mean that we taste sweet only at the tip of the tongue through the fungiform papillae, bitter at the back through the circumvallate papillae, sour at the foliate papillae at the sides of the tongue, and salty at the sides as well. However, what Hänig intended was to show differences in the perceived intensity of the tastes at different parts of the tongue. The more closely set the dots, the more intense the sensation, but each taste can be perceived wherever there is a dot. And that is the case. — Virginia
- *Hänig, David P. 1901. Zur Psychophysik des Geschmackssinnes. Philosophische Studien 17: 576-623.
- **Feeney, E. L., & Hayes, J. E. (2014). Regional differences in suprathreshold intensity for bitter and umami stimuli. Chemosensory Perception, 7(3-4), 147-157. doi:http://dx.doi.org.proxy.library.cornell.edu/10.1007/s12078-014-9166-3