Five basic tastes

There are five basic tastes: salty, sour, sweet, bitter, and savory (or umami). The perception of salty taste is caused by tastants that release sodium ions upon dissolution. Sour taste, by contrast, is produced by the release of hydrogen ions from dissolved acidic tastants. Salty and sour tastants produce a neural response by depolarizing the membrane directly (salty tastants) or via ion channel changes (sour tastants). Sweet, bitter, and savory compounds activate G-protein-coupled receptors. Chemicals perceived as sweet, like glucose and fructose, typically have hydroxyl and carbonyl groups. Numerous diverse molecules stimulate bitter receptors, and over 30 receptors for bitter taste have been identified. The amino acid glutamate activates savory receptors. The addition of monosodium glutamate, or MSG, to foods, enhances the savory flavor.

A 6th basic taste?

In 2015, CD36 is discovered as the fat-taste receptor. This receptor gives a neural response when free fatty acids bind to it. However, fat has not been fully accepted as the 6th basic taste, because it is still unclear whether free fatty acids can give a neural response on their own, or if sensations such as aroma or texture are needed to perceive a fatty taste.

The Tongue and Taste Buds

When eating something, the tastants in the food dissolve in saliva and binds to the taste receptors in the taste buds. These taste buds are nested in taste papillae.

These papillae can be seen as small bumps on the tongue. These papillae have different functions. They can either distribute the tastant over the tongue (filiform papillae) of contain the sensory taste (or gustatory) receptor cells (fungiform, circumvallate, and foliate papillae). As mentioned, embedded within each taste-related papilla, are the taste buds, clusters of 30 to 100 gustatory receptor cells.

Although in old textbook it is mentioned that distinct tastes can be located at specific areas, this is not true. It has been indicated that the five basic tastes can be perceived anywhere across the tongue, however the density of the different taste receptors may differ, hence explaining the observations that specific receptors are located at different places on the tongue. The latter is known as “the myth of the tongue’s taste map”.

There are 4 types of papillae, out of which 3 types contain taste buds and receptors:

•  Circumvallate papillae are the largest type of papillae and are located near the back of the tongue in a “V”-like formation and contain more than 100 taste buds per papilla
• Foliate papillae resemble folds on the side of the tongue and contain more than 100 taste buds per papilla
• Fungiform papillae are found across the front three-quarters of the tongue but are less concentrated in the middle of the tongue. These mushroom-shaped fungiform papillae are the most numerous and contain between one and eight taste buds each.
• Filiform papillae contain no taste buds. They assist in spreading tastants over the tongue.

Gustatory receptor cells extend finger-like projections called gustatory hairs (or microvilli) into a region known as the taste pore. Here, many of the cells contain receptors that detect different tastants. The average number of taste buds varies significantly among individuals, with estimates ranging from 2,000-10,000 taste buds. Taste cells have a lifespan of about 10-14 days and are continually replaced. Thus, each taste bud contains taste cells at different stages of development.

Neural processing of taste

When a tastant binds to its specific taste receptor in the taste bud, an electric signal is generated. Cranial nerves carry taste information from receptors in taste buds to the medulla in the brainstem. From there, the information travels to the thalamus and is then relayed to the primary gustatory cortex. This gustatory cortex also receives other sensory information from the tongue, like thermal and mechanical stimulation. The incorporation of the different sensory inputs provides a hedonic quality to flavor, allowing it to be perceived as pleasant or unpleasant.