Each human tongue contains an average of roughly 10,000 chemoreceptors. These microscopic hair-like nerve endings, called microvilli, are encapsulated in a protective envelope on a short stalk, called papillae. These buds sense chemicals in food, giving them their flavor. These “taste buds” are also called gustatory cells.
The chemoreceptors of the tongue were first discovered in the 19th century. Their nerve endings conduct signals two ways: ion channels and G-protein coupled receptors. The signal conduction is depending on the type of chemical that stimulated the receptor.
The chemoreceptors of the human tongue can detect five types of molecules: ions, acidic compounds, sugars and sugar-like molecules, glutamate/glutamic acid, and those that activate dedicated bitter receptors (the common component of bitter-tasting chemicals is still being determined).
These molecules correspond to the four traditional taste senses (salty, sweet, sour, and bitter), as well as a fifth, called savory (or umami), that was officially added to the roster of chemical signals recognizable by the human tongue in 2002. Each type of taste activates chemoreceptors in a particular manner.
Salty and sour tastes are transmitted by ion channels that generate an action potential. Saltiness is perceived when ions, including sodium, magnesium, potassium, and often calcium, are present in the saliva or on the tongue. The receptors respond more strongly to sodium, making it seem saltier. Sour is perceived when acidic compounds activate hydrogen ion channels, which depolarizes the gustatory cells. Activating different types of ion channels allows the two tastes to be different, even though the signals are relayed in a similar manner.
Sweet and bitter are relayed by G-protein coupled signaling. Sweetness is a response to sugars and other molecules, including aldehydes, ketones, and the amino acids glycine, alanine, and serine. Bitter was the fourth taste to be known, added by the Greek philosopher Democritus. Though it has been found to have a genetic component (some people taste certain foods as bitter, broccoli for example, while others do not), and it is known how bitter is transmitted once the chemoreceptor is activated, exactly how the molecules interact with the receptor and which molecules they may be is still being explored. Most naturally bitter compounds are toxic, which may indicate an evolutionary component to the taste. However, some medicines, such as the anti-malarial quinine, and common foods, such as coffee, beer, unsweetened chocolate, and citrus peel, are also bitter.
A Japanese chemist, Kikunae Ikeda, published his findings about the key chemical that is fundamental to the flavor in Asian cooking, called umami (meaning “yummy” or “delicious”) in the journal of the Chemical Society of Tokyo in 1908. The chemical was glutamic acid. In 2002, scientists identified the savory taste bud. Umami (i.e. savory or meaty) is relayed by the activation of G-protein signaling by glutamic acid, or its basic form glutamate (L-glutamate specifically).