Keywords

Umami is now commonly identified as the fifth “basic” taste quality, joining sweet, salty, sour, and bitter. It must be emphasized that the term taste quality refers to human (and perhaps other species) psychological representations, not to the ligands that elicit those representations; for example, NaCl elicits a salty taste in humans, but sodium chloride itself is not “salty.” The four traditional basic taste qualities have a very long and deep history in human experience. As detailed in a recent review (Beauchamp, 2019), sweet, salty, sour, and bitter, along with pungency and astringency (these last two are tactile qualities, not taste qualities), were identified as the basic building blocks of perceived taste in Chinese, Indian, and Greek writings dating back several thousands of years. Moreover, these four taste qualities, as well as the two tactile qualities, also make up much of what is reported to be the current taste world in many independent, relatively isolated cultures around the world (Beauchamp, 2019).

These four basic taste qualities likely exist to provide vital information on the health and safety of potential foods. Sweet and salty substances are generally highly palatable, signaling vital nutrients: calories and sodium. Bitter compounds, with generally negative hedonic qualities, usually signal danger or poison. However, bitter has also been seen as a signal for medicinal value, both historically and currently, in many cultures around the world (Beauchamp, 2019). Functionally, sour remains a puzzle. Among several hypotheses to account for it, one is that it acts as an inhibitory signal for unripe fruit that could also injure the oral cavity, and another is that it could signal the presence of certain micronutrients (Breslin, 2019; Liman & Kinnamon, 2021). It is significant that, for sweet, salty, and bitter, the perceptual signal is generally identical with the actual function of the signaling molecules. That is, most sweet compounds in nature are calorie-rich, virtually all salty substances in nature contain sodium, and most molecules that are bitter act as poisons at least in high concentrations.

At the beginning of the twentieth century, the Japanese chemist Kikunae Ikeda identified the glutamate ion as inducing a novel taste quality that he proposed was a signal for protein, analogous to sweetness being a signal for energy or calories (see Chaps. 2 and 3). He described this novel quality as “the peculiar taste we feel as umai [meaning, according to the translators, meaty, brothy, or savory].” He called this taste umami. Subsequent studies further identified the ribonucleotides 5′-inosinate and 5′-guanylate as synergistic enhancers of this novel taste, although for humans and some other species, they may not produce a taste on their own. Beginning in the late 1960s and 1970s, additional researchers also proposed umami as a novel fifth basic taste. This idea gained momentum from a scientific meeting held in 1985 and the publication of the proceedings Umami: A Basic Taste (Kawamura & Kare, 1987). The evidence used to support the proposal that umami was the fifth basic taste at that time consisted primarily of human sensory studies and animal model studies of behavior and physiology.

The discovery in the 1990s and 2000s of specific taste receptors responsive to amino acids (see Chaps. 2 and 3 in this volume) gave substantial impetus to the idea that umami might be the fifth basic taste. Yet even the proponents of umami as a basic taste acknowledge that it differs significantly from the classic four (e.g., Hartley et al., 2019): it is more subtle, which may account for it not being identified historically or in most traditional cultures, and unlike the other appetitive taste qualities, sweet and salty, relatively pure solutions of monosodium glutamate (MSG) are very rare in nature and are not palatable for human adults or infants (see Chap. 2). Moreover, unlike sweet, salty, and bitter, the perceptual quality of umami for humans does not directly signal the presence of the purported nutrient, proteins (which generally have no taste), or even amino acids. Indeed, many umami-rich foods are not naturally high in protein (Breslin, 2013).

Although much current research on umami focuses on its apparently unique taste to humans, early descriptions of the umami percept, or perceptual characteristics, included a strong component that is best described as tactile rather than taste (Beauchamp, 2009). Whether this tactile percept, sometimes called mouthfeel, results from a true ability of MSG to engage somatosensory rather than taste pathways, or whether it is mediated by anatomically defined taste pathways, as has recently been argued by Yamamoto and Inui-Yamamoto (2023), remains to be determined. Nevertheless, mouthfeel is an extremely important and salient component of the umami percept, and it merits future research. For example, does this attribute have any causal relationship to the observation that umami sensations are involved in satiety and satiation?

1.1 Species Differences in Umami Perception

Do other species detect umami? Taste qualities are human-derived percepts, so it can be problematic to discuss umami in the context of other species—which of course is also true of the other basic taste qualities. The words salty, sweet, bitter, and sour refer to sensory properties that humans perceive. However, for these taste qualities, there are at least two reasons to believe that at least some other animal species perceive something similar to what humans perceive. First, many of the compounds we classify as sweet or salty and as bitter or sour elicit similar behaviors in several other well-studied species. For example, rats and mice tend to avoid bitter compounds and are attracted to many compounds humans describe as sweet—sweet is good and bitter is bad, for the most part. What is more, if a rodent is made ill by exposure to a simple sugar such as glucose (to which a tasteless purgative, e.g., has been associated), it will not only avoid this taste when presented again but also avoid many other simple sugars and even some nonnutritive compounds humans describe as sweet, such as saccharin. This suggests that all these compounds elicit a common percept—a common taste quality.

Is the umami taste similar across species? The apparent answer is no. Umami stimuli for humans are restricted to MSG and, to a lesser extent, aspartate and are synergistically enhanced by some ribonucleotides. However, this specificity is not evident for rodents (Nelson et al., 2002) and perhaps many other species. In some species, MSG appears to be sweet or salty, whereas in others, glutamate may elicit the same percept (neither sweet nor salty) as many other amino acids. That is, the putative receptor for umami in humans (the dimer T1R1/T1R3; see Chaps. 2 and 3) may be quite different in other species, due to molecular changes in its binding affinities and more central projections and thus different in the perceptual characteristics it elicits in many other species. Consequently, it is much more problematic to speak of the “umami receptor” in species other than humans than it is to speak of the “sweet receptor” or the “bitter receptors” in some other species.

1.2 Umami Perception in Humans

Why, from a functional and evolutionary perspective, is human perception of umami elicited almost exclusively by glutamate and, to a lesser degree, by aspartate? Several suggestions have been proposed to explain this—while none are definitive, each may have merit. Following the lead of Ikeda (2002) who was writing in 1909, Breslin (2013, 2019) has suggested that humans developed a preference (and presumably a specific receptor) for glutamate and ribonucleotides as markers for protein. But many high-protein foods do not have a strong umami taste. Breslin’s idea is that the specific umami taste quality is particularly human because it signals easily digested protein as formed during cooking (see Chaps. 5 and 9) and fermentation. He noted that fermented foods also have the nutritional advantage of providing easy access not only to amino acids but also to probiotic bacteria. Thus, he partially attributes specificity of human umami perception and preference for food manipulation by our human ancestors. As Breslin (2019, p. 15) summarizes: “We can presume that this taste, which we call savory or umami, was initially related to fermentation.” Although this explanation has merit, it fails to explain why other primate species as well as nonprimate mammals (which do not cook or ferment their foods) also appear to have a receptor system focused on glutamate and/or ribonucleotides.

A second recent approach toward understanding the human specificity of umami has been suggested in a comprehensive evaluation of the T1R1/T1R3 receptor in 17 species of primates (Toda et al., 2021). Each of these primate species has a functional T1R1/T1R3, but this receptor varies in which stimuli engage it most effectively. To evaluate these differences, Toda and colleagues tested the receptor responses to glutamate and to ribonucleotides 5′-inosinate and 5′-guanylate separately in each of these species and attempted to associate relative responsiveness to these compounds to the primate’s dietary habits. They concluded that for primates that consume primarily insects, this receptor is specialized for detection of unbound (free) ribonucleotides, which is consistent with the presence of large amounts of these molecules in insects. In contrast, unlike in humans, the T1R1/T1R3 receptor in these species does not respond well to glutamate. They proposed that ribonucleotide sensitivity was the ancestral response of all primates. Subsequently, the T1R1/T1R3 receptor evolved to respond specifically to glutamate in a variety of primate species, including human precursors. These species consume primarily leaves, which are low in free ribonucleotides but are relatively rich in free glutamate. They concluded that glutamate sensitivity could be useful for these species for detecting dietary protein in their plant-based diets. They further speculated that this responsiveness to the free glutamate in plant leaves also functions to mask or inhibit the bitterness of leaf-based secondary metabolites, thereby heightening leaf palatability. Although Toda and colleagues focused on primate analyses, they also investigated glutamate responsiveness in the isolated T1R1/T1R3 receptor in several other species of mammals, including mice, cats, dogs, horses, and pigs. The receptors of all these nonprimate species except pigs were relatively unresponsive to glutamate, but all were highly responsive to the ribonucleotides. This further emphasizes the novel specialized nature of the umami response in humans and closely related primates.

One additional aspect of umami in humans bears mentioning. Human milk is particularly rich in glutamate, as are the milks of several other closely related primates (gorillas, chimpanzees, and even rhesus macaques; Rassin et al., 1978; Davis et al., 1994; Sarwar et al., 1998). These species’ T1R1/T1R3 receptors are also highly responsive to glutamate (Toda et al., 2021). Could there be a causative relationship between specificity for glutamate and a high concentration of glutamate in breast milk? Thus, leaf eating may not be the only driving force toward high receptor specificity for free glutamate and thus for umami as a positive stimulus in primates that are more closely related to humans.

1.3 Conclusion

Although many mysteries about umami taste remain, we nevertheless know much more about this potent sensory percept and its health-related aspects now than we did even 20 years ago. Taste and associated oral sensations such as mouthfeel provide the last chance for an organism to decide whether to ingest or reject a particular food. Taste signals the potential worth and possible danger of foods. Umami perception in the oral cavity is now recognized as a significant force in nutrition and health, as are the associated physiological effects of umami stimulations of T1R1/T1R3 and perhaps other receptors in the oral cavity and elsewhere in the body. The chapters in this book dramatically illustrate this, describing what we know and calling attention to what we still do not know about mysterious umami.