Protein-rich foods have a characteristic quality called “umami.” The Japanese chemist Kikunae Ikeda coined the term in 1908 to describe the taste of meaty broths — “umai” means “delicious” in Japanese.
Umami is specifically the sensation produced by glutamate; the form of the amino acid glutamine found in tea. This taste tells about the presence of protein, especially meat protein — 60% of the glutamate in muscle meat is free, that is, not attached to other amino acids to form a protein. You taste this free glutamate when you bite into a piece of meat.
It’s worth noting that the umami taste is elusive — in the West it is often confused with sweetness, or sometimes saltiness, or even sour. While umami is often translated as savory, a more proper term might be ‘brothy,’ except that we don’t use the term ‘brothy’ for solid foods. It works for tea, though!
The confusion with “sweet” makes some sense: one of the many receptors for umami on the cells of human taste buds consists of two coupled proteins. The two are known as TAS1R2 and TAS1R3. The sweet receptor in taste buds is also made up of TAS1R3, but in its case coupled with TAS1R1. Consequently, compounds that activate TAS1R3 may activate both receptors, and you get both a sweet and an umami taste.
There are seven other proposed receptors for 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 these proposed receptors respond specifically to glutamate, while at least one is broadly tuned to respond to amino acids more generally.
The most studied receptor for umami taste is the TAS1R2+TAS1R3 pairing.
It is activated by a large number of compounds in tea. Foremost among these is theanine, the characteristic amino acid of tea. Interestingly, theanine contributes both sweetness and umami to a tea, 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 umami impression include breakdown products of DNA and RNA, and derivatives of amino acids, in particular those produced during the Maillard reaction. This reaction involves the linking of sugars to amino acids — it’s the reaction that gives the lovely brown color to a loaf of bread. It occurs when tea leaves are heated during processing, and through the actions of microorganisms during puer fermentation.
Our umami response to amino acids other than theanine and glutamate is relatively weak but can be enhanced by methional. Methional is a derivative of methionine, a sulfur-containing amino acid that breaks down during the early steps of tea leaf processing and disappears with more prolonged processing. At high levels, methional smells like raw potatoes (and is in raw potatoes). It can contribute to the sulfurous marine qualities of some green and oolong teas. However, adding methional at levels below detection to a solution of amino acids strengthens the umami response — at these levels it may contribute to the pleasantness of a green or oolong tea!
In addition to their taste properties, umami compounds give a pleasant “full” mouthfeel. Whether it is activation of umami receptors by themselves or activation of trigeminal (chemesthesis) nerve endings simultaneously with umami receptors is not known.
We cannot leave the discussion without mentioning another phenomenon observed by the Japanese, namely kokumi, a word that has been translated as “heartiness” or “full flavor.”
The kokumi sensation is accomplished by short protein-like compounds called peptides that have a γ-glutamyl group (provided by glutamate) on the end of their amino acid chain. It’s not surprising, then, that these compounds are found in foods with protein, and that they enhance the umami taste of proteins.
By themselves, these compounds have little if any taste. But if you add a compound that confers kokumi to a sugar solution, it will taste sweeter. The same goes for a salt solution, it will taste saltier!
How do kokumi compounds do this?
As Kuroda and Miyamura discovered, these compounds activate a calcium-sensing receptor on a unique subset of taste bud cells. 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 into umami 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/negative allosteric modulation switching in an umami taste receptor (T1R1/T1R3) by a natural 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