Umami the Fifth Basic Taste: History of Studies on Receptor Mechanisms and Role as a Food Flavor.

Three umami substances (glutamate, 5'-inosinate, and 5'-guanylate) were found by Japanese scientists, but umami has not been recognized in Europe and America for a long time. In the late 1900s, umami was internationally recognized as the fifth basic taste based on psychophysical, electrophysiological, and biochemical studies. Three umami receptors (T1R1 + T1R3, mGluR4, and mGluR1) were identified. There is a synergism between glutamate and the 5'-nucleotides. Among the above receptors, only T1R1 + T1R3 receptor exhibits the synergism. In rats, the response to a mixture of glutamate and 5'-inosinate is about 1.7 times larger than that to glutamate alone. In human, the response to the mixture is about 8 times larger than that to glutamate alone. Since glutamate and 5'-inosinate are contained in various foods, we taste umami induced by the synergism in daily eating. Hence umami taste induced by the synergism is a main umami taste in human.

Pyrazine analogues are active components of wolf urine that induce avoidance and freezing behaviours in mice.

BACKGROUND: The common grey wolf (Canis lupus) is found throughout the entire Northern hemisphere and preys on many kinds of mammals. The urine of the wolf contains a number of volatile constituents that can potentially be used for predator-prey chemosignalling. Although wolf urine is put to practical use to keep rabbits, rodents, deer and so on at bay, we are unaware of any prior behavioural studies or chemical analyses regarding the fear-inducing impact of wolf urine on laboratory mice. METHODOLOGY/PRINCIPAL FINDINGS: Three wolf urine samples harvested at different times were used in this study. All of them induced stereotypical fear-associated behaviors (i.e., avoidance and freezing) in female mice. The levels of certain urinary volatiles varied widely among the samples. To identify the volatiles that provoked avoidance and freezing, behavioural, chemical, and immunohistochemical analyses were performed. One of the urine samples (sample C) had higher levels of 2,6-dimethylpyrazine (DMP), trimethylpyrazine (TMP), and 3-ethyl-2,5-dimethyl pyrazine (EDMP) compared with the other two urine samples (samples A and B). In addition, sample C induced avoidance and freezing behaviours more effectively than samples A and B. Moreover, only sample C led to pronounced expression of Fos-immunoreactive cells in the accessory olfactory bulb (AOB) of female mice. Freezing behaviour and Fos immunoreactivity were markedly enhanced when the mice were confronted with a mixture of purified DMP, TMP, and EDMP vs. any one pyrazine alone. CONCLUSIONS/SIGNIFICANCE: The current results suggest that wolf urinary volatiles can engender aversive and fear-related responses in mice. Pyrazine analogues were identified as the predominant active components among these volatiles to induce avoidance and freezing behaviours via stimulation of the murine AOB.

Glutamate: from discovery as a food flavor to role as a basic taste (umami).

In 1908 Kikunae Ikeda identified the unique taste component of konbu (kelp) as the salt of glutamic acid and coined the term umami to describe this taste. After Ikeda's discovery, other umami taste substances, such as inosinate and guanylate, were identified. Over the past several decades, the properties of these umami substances have been characterized. Recently, umami has been shown to be the fifth basic taste, in addition to sweet, sour, salty, and bitter.