Sensory and receptor responses to umami: an overview of pioneering work.

This article provides a selective overview of the early studies of umami taste and outlines significant questions for further research. Umami compounds such as the amino acid glutamate [often in the form of the sodium salt monosodium glutamate (MSG)] and the nucleotide monophosphates 5'-inosinate and 5'-guanylate occur naturally in, and provide flavor for, many foods and cuisines around the world. Early researchers in the United States found that the flavor of pure MSG was difficult to describe. But they all agreed that, although humans found umami compounds, when tasted alone, to be unpalatable, subjects reported that these compounds improved the taste of foods. This taste "dichotomy" may be partly unlearned because it is also observed in very young infants. The uniqueness of umami perception is based on several lines of evidence. First, numerous perceptual studies have shown that the sensation aroused by MSG is distinct from that of the other 4 taste qualities. Second, biochemical studies that show the synergy of the binding of MSG and 5'-guanylate to tongue taste tissue mirror this hallmark perceptual effect. Third, several specific receptors that may mediate umami taste have recently been identified. There remain, however, a number of puzzles surrounding the umami concept, including the molecular basis for an apparent tactile component to umami perception, the reason for the unpalatability of pure umami, and the functional significance for human health and nutrition of umami detection. Future work aimed at understanding these and other open issues will profitably engage scientists in umami research well into the next century.