Umami flavour makes for a fascinating story because of the molecular nature of this sensory sensation. For many years, it was thought to be an artefact of flavour combinations until sensory receptors specific for certain compounds was identified. Understanding the concept of umami with culinologists, product developers and flavour experts should help when devising reduced salt dishes or maximising flavour to boost dishes where sensory cells have been lost through ageing or cancer treatment.
History Behind Umami
Umami was recognised long ago in Japan as a flavour sensation associated with various seasonings, fermented foods and tuna fermented products. The sensation added more to taste than just sweet, sour, salty and sweet. At the molecular level, the most important compounds are glutamate based, mainly monosodium glutamate (MSG). It was also discovered that other food preparations releasing free amino acids like histidine, glutamic and aspartic acid, short chain peptides , various organic acids like fumarate and minerals. The most interesting were the 5’ nucleotides such as inosine monophosphate (IMP) and guanosine monophosphate (GMP) which were crucial to generating these flavour notes but to differing extents (Yamaguchi and Takahashi, 1984).
MSG is generated by fermentation of sugars and starches using the bacteria Corynebacterium glutamicum (Eggeling & Bott, 2010). To begin with, a crude glutamic acid preparation is generated, which is then filtered, purified and treated with sodium salts to generate monosodium glutamate. As for the 5′ nucleotides like inosine monophosphate (IMP), these occur naturally in a variety of animal derived sources such as Japanese bonito flakes, bouillion and aged beef products. GMP on the other hand comes mainly from plant sources, and fungi such as mushrooms.
Great strides have been made in developing yeast biomass from Saccharomyces cerevisiae and Candida utilis (which is now called Cyberlindnera jadinii).
The molecular interactions with sensory cells in the mouth have also been explored (Li et al., 2002; Zhang et al., 2008). We now know that there are specific receptors specific to the umami sensation. There is great interest in two specific receptor types – T1R1 and T1R3.
A variety of nucleotides based on derivatives of IMP and MSG have been investigated in various animal models. One important study looked at the chemical basis of synthesised derivatives of GMP based on N2-alkyl and N2-acyl derivatives (Cairoli et al., 2008). The synergies with MSG and these derivatives were investigated whereby these N2 derivatives enhanced the flavour of MSG by 20% and over 500% with IMP. The conclusion was that an exocyclic NHR group on the guanine moiety was important in creating the umami flavour synergy with the nucleotide derivatives.
The receptors that interact with the nucleotides are the G-protein-coupled receptors or GPCRs located in the cell membranes of cells mainly in the tongue (Zhao et al., 2003; Temussi, 2009). These receptors bind the various nucleotides to a G protein in the receptor known as gustducin. This switches on the ion channel TRPM5 within the receptor producing a neural signal that translates into ‘umami’.
Peptidomics
In recent years, peptidomics has become a the technique of choice in identifying a range of functional peptides for all sorts of applications. It has been recently exploited as a bioinformatic tool coupled to computer simulated digestion to identify and explain interactions with receptors at the molecular level (Yu et al., 2023). What is so interesting about the application of the technology is whether the creation of theoretical peptides fragments actually produce the taste sensations that they are meant to generate in these models (Qi et al., 2022).
Recent studies have used molecular docking methods alongside sensory evaluation and E-tongue to fully understand the mechanisms of the umami receptor. A study on an Agrocybe aegerita hydrolysate using uPLC-ESI-Q-TOF-MS found 11 possible umami peptides (Yang et al., 2024). Using molecular docking they found 4 peptides EY, EG, ECG, and DGPL could enter the T1R1 cavity of the umami receptor whilst 4 other peptides, EV, ENG, DEL, and EDCS could enter the binding pocket of the T1R3 cavity. They also showed that the primary binding sites of T1R1 were residues Ser109, Gln52 and Ser148, while the primary binding sites of T1R3 were residues Ser172, Arg277 and Ala170.
Why did umami develop as a separate sensory sensation? It is thought that the sensation is attenuated i.e. modified to encourage us as humans to eat protein, nucleic acid or amino-acid rich foods. The other taste sensation which some would describe as the sixth taste sensation is kokumi and this aspect of tasting a food is even more interesting given that so little is truly known about it.
References
Cairoli, P., Pieraccini, S., Sironi, M., Morelli, C. F., Speranza, G., & Manitto, P. (2008). Studies on umami taste. Synthesis of new guanosine 5′-phosphate derivatives and their synergistic effect with monosodium glutamate. J. Agric. Food Chem., 56(3), pp. 1043-1050.
Eggeling, L., & Bott, M. (Eds.). (2010). Handbook of Corynebacterium glutamicum. CRC press.
Li, X., Staszewski, L., Xu, H., Durick, K., Zoller, M., & Adler, E. (2002). Human receptors for sweet and umami taste. Proc. Nat. Acad. Sci., 99(7), pp. 4692-4696.
Temussi, P. A. (2009). Sweet, bitter and umami receptors: a complex relationship. Trends Biochem. Sci., 34(6), pp. 296-302.
Yamaguchi, S., Takahashi, C. (1984) Interactions of monosodium glutamate and sodium chloride on saltiness and palatability of a clear soup. J. Food Sci. 49 pp. 82–85
Yang, F., Cao, R., Fu, A., Liu, Y., Bi, S. (2024) Investigation of umami peptides and taste mechanisms in Agrocybe aegerita : based on sensory evaluation and molecular docking techniques , Food & Function, 10.1039/D4FO01369G, 15, 13, pp. 7081-7092.
Yu, X., Zhen, Y., Fang, T., Zhang, J., Xu, Y., Wu, F., … & Wang, D. (2023). Advances in the study of the role of extraoral bitter taste receptors. Authorea Preprints.
Zhang, F., Klebansky, B., Fine, R. M., Xu, H., Pronin, A., Liu, H., … & Li, X. (2008). Molecular mechanism for the umami taste synergism. Proc. Nat. Acad. Sci.,105(52), pp. 20930-20934 (Article)
Zhao, G. Q., Zhang, Y., Hoon, M. A., Chandrashekar, J., Erlenbach, I., Ryba, N. J., & Zuker, C. S. (2003). The receptors for mammalian sweet and umami taste. Cell, 115(3), pp. 255-266. (Article)
Updated article from 17th November 2014 with new details on the umami receptor and its biochemistry.
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