One of the most common myths about human taste perception is the existence of a “taste map”; stating there are regional differences in sensitivity across the human tongue for sweetness, sourness, bitterness, and saltiness (Fig. 1). Taste maps became popular in the early 20th century; however in the early 21st century, scientists confirmed that all taste qualities are found in all areas of the tongue (e.g. ). Different taste receptor cells are linked to the various taste perceptions, for example : i) Sweet stimuli trigger changes within taste cells by binding to T1R2 and T1R3 receptors on the taste cell’s surface; ii) Bitter stimuli interact with T2R receptors; iii) Umami tastes are stimulated by amino acids and bind to T1R1 and T1R3 receptors.
Fig. 1. A diagram of the outdated taste map showing where certain taste sensations can be detected by taste buds in specific areas of the tongue .
Humans sense taste via taste buds, which are clusters of taste cells that are bundled together. Each cluster contains about 50-150 taste cells . Most taste buds are located on small bumps on the tongue known as papillae. The tongue surface is covered in four kinds of papillae, as shown in Fig. 2. A detailed image of the circumvallate papillae is presented in Fig. 3. The tip of each taste cell forms a small taste pore (Fig. 4). Microvilli from the taste cells extend through these pores. A network of sensory nerves known as taste nerves are interwoven between the taste cells in a taste bud (Fig. 4). As food is chewed chemicals (tastants) from it are released and dissolve in saliva then enter the taste pores of taste buds. These chemicals interact with either the proteins on the surfaces of taste receptors or with ion channels (porelike proteins) . The interaction triggers chemical signals to be transmitted to the brain via the taste nerve fibres.
Fig. 2. A diagram of the human tongue showing all areas of the tongue are able to detect all taste sensations . The circumvallate, foliate, and fungiform papillae contain taste buds. The filiform papillae lack taste buds and are primarily for tactile sensation .
Each of the five common taste sensations: sweet; umami (ability to detect glutamate—an amino acid that makes up protein found in meat, fish and legumes); salty; sour; bitter can be triggered by various chemicals. The different threshold levels for detection are shown in Table 1 .
Table 1 Threshold for detecting various taste sensations .
|Taste||Substance||Threshold for tasting|
|Salty||NaCl||0.01 M (0.58g NaCl)|
|Sour||HCl||0.0009 M (0.033g HCl)|
|Sweet||Sucrose||0.01 M (3.42g sucrose)|
|Bitter||Quinine||0.000008 M (0.0026g quinine)|
|Umami||Glutamate||0.0007 M (0.10g glutamate)|
**If you recall from high school chemistry, Molarity (M) is a unit used to describe the “strength” of a solution. A “strong” would have a higher molarity than a “weak” solution. Molarity is determined by the weight of the substance (in moles) dissolved in 1 litre of solution. Mole is a unit which is used to compare different chemical elements and compounds and 1 mole of any substance is equal to the atomic or molecular mass in grams.**
Physiological implications of taste
Various digestive functions occur after the brain receives taste signals; for example, increased rapid salivation and low level secretory activity in the stomach . More importantly, sensory information provides triggers for the body to respond appropriately to nutrients. For instance, the taste of sugars leads to the ingestion of carbohydrates. Other physiological responses include the release of insulin to aid the body to use the nutrients effectively .
Furthermore, humans often avoid ingesting substances with strong bitter or sour tastes. Such reactions generally serve as protection against harmful substances . Strychnine and other plant alkaloids are toxic compounds, and often have a strong bitter taste. The sour taste of spoiled food is another example of how taste sensations are used to avoid ingestion of harmful compounds.
Smith, D., & Margolskee, R. (2001). Making Sense of Taste Scientific American, 284 (3), 32-39 DOI: 10.1038/scientificamerican0301-32
Adler, E., Hoon, M., Mueller, K., Chandrashekar, J., Ryba, N., & Zuker, C. (2000). A Novel Family of Mammalian Taste Receptors Cell, 100 (6), 693-702 DOI: 10.1016/S0092-8674(00)80705-9
 R. Bowen. (2006, Dec. 10). Physiology of Taste [Online]. Available: http://www.vivo.colostate.edu/hbooks/pathphys/digestion/pregastric/taste.html