Umami – The 5th Taste

Close up of monosodium glutamate (msg), a flavor enhancer in many Asian foods and one of the main causes of the umami and kokumi flavour sensation.
Monosodium glutamate is one ingredient creating the sensation of umami. Copyright: topteen / 123RF Stock Photo

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.

The molecular interactions with sensory cells in the mouth have also been explored (Li et al., 2002; Zhang et al., 2008). 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’.

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

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)

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