The Fifth Tastebud
A New Flavor Has Been Added to The Sense of Taste
Most people remember learning about the basic tastes - sweet, salty, bitter, and sour. Savory was recently added to the list though it is perceived the same way.
One of the five senses, taste is determined by tastebuds on the tongue. These small, visible bumps are actually nerve endings within a protective envelope on a short stalk and are also called gustatory cells. These nerve endings relay taste sensations to the brain based on the information they receive as chemoreceptors; they are sensitive to chemicals. There are more than 10,000 taste buds on an average human tongue and they were first discovered in the 19th century.
How Taste is Perceived
Tastebud nerve endings conduct signals through ion channels or G-protein coupled receptors, depending on the type of chemical being detected.
The traditional flavors of salty and sour 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. This allows the two tastes to be different, though the signals are relayed in a similar manner.
The traditional flavors of 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. The bitter sensation has been found to have a genetic component; some people taste certain foods as bitter, broccoli for example, while others do not. This may explain why it was the last of the four to be added to the common list, by the Greek philosopher Democritus.
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.
Umami, the Meaty or Savory Taste
A French chef, Escoffier, became famous in the 1800s for creating dishes that tasted like none of the four taste sensations. His new taste came from his use of veal stock. Asian cooking has used this same flavoring as a fundamental taste in their dishes. A Japanese chemist, Kikunae Ikeda, published his findings about the key chemical in the journal of the Chemical Society of Tokyo. The chemical was glutamic acid.
Glutamic acid, or its basic form glutamate, activates G-protein coupled signaling. It is often found in fermented or aged foods. The taste is now known as meaty or savory, but the name given by Ikeda 100 years ago was umami. Umami is Japanese for “yummy” or “delicious”. In 2002, scientists found that there is indeed a fifth tastebud, one that senses L-glutamate. They gave the flavor the official name of umami.
Additional Reference:
Scanlon and Sanders, Essentials of Anatomy and Physiology, 4th edition, 2003.
