Taste is the sensation produced when a substance in the mouth reacts chemically with taste receptor cells located on taste buds. Taste, along with smell and trigeminal nerve stimulation, determines flavors of food or other substances, write Arvind, B.D. Sharma, S. Talukder, Vivek Shukla, Tarun Pal Singh, and Lalchamliani
TASTE, gustatory perception, or gustation is the sensory impression of food or other substances on the tongue and is one of the five traditional senses. Taste is the sensation produced when a substance in the mouth reacts chemically with taste receptor cells located on taste buds. Taste, along with smell (olfaction) and trigeminal nerve stimulation (registering texture, pain, and temperature), determines flavors of food or other substances. Humans have taste receptors on taste buds (gustatory calyculi) and other areas including the upper surface of the tongue and the epiglottis (Jones and Bartlett Learning, 2005).
The tongue is covered with thousands of small bumps called papillae, which are easily visible to the naked eye. Within each papilla, hundreds of taste buds are present (Schechter and Daniel, 2009). The exception to this is the filiform papillae that do not contain taste buds. There are between 2000 and 5000 taste buds that are located on the back and front of the tongue (Boron, and Boulpaep, 2003) while others are located on the roof, sides, and back of the mouth, and in the throat. Each taste bud contains 50 to 100 taste receptor cells. It is proposed that the sensitivity of taste varies with age. It is thought that as we age, taste cell replacement slows, and thus the sense of taste diminishes (Schiffman, 2001).
Types of papillae are:
- Filiform (filum: rope)
- Foliate (folio: leaf)
- Fungiform (fungus-shaped)
- Circumvallate (circum-; around + vallum; wall)
The sensation of the taste can be categorized into five basic tastes sweetness, sourness, saltiness, bitterness and umami. Taste buds are able to differentiate among different tastes through detecting interaction with different molecules or ions. Sweet, umami and bitter tastes are triggered by the binding of molecules to G protein-coupled receptors on the cell membranes of taste buds. Saltiness and sourness are perceived when alkali metal or hydrogen ions enter taste buds, respectively (Human Physiology: Silverthorn).
Basic tastes and their perception mechanism
For a long period, it was commonly accepted that there is a finite and small number of “basic tastes” of which all seemingly complex tastes are ultimately composed. Just as with primary colors, the “basic” quality of those sensations derives chiefly from the nature of human perception, in this case, the different sorts of tastes the human tongue can identify. As of the early twentieth century, physiologists and psychologists believed there were four basic tastes: sweetness, sourness, saltiness, bitterness. At that time umami was not proposed as a fifth taste (Kikunae, I. 2002) but now a large number of authorities recognize it as the fifth taste. In Asian countries within the sphere of mainly Chinese and Indian cultural influence, pungency (piquancy or hotness) had traditionally been considered a sixth basic taste.
Taste buds are able to differentiate among different tastes through detecting interaction with different molecules or ions.
Sweetness, usually regarded as a pleasurable sensation, is produced by the presence of sugars and a few other substances. Sweetness is often connected to aldehydes and ketones, which contain a carbonyl group. Sweetness is detected by a variety of G protein coupled receptors coupled to the G protein gustducin found on the taste buds. At least two different variants of the “sweetness receptors” must be activated for the brain to register sweetness. Compounds the brain senses as sweet are thus compounds that can bind with varying bond strength to two different sweetness receptors. These receptors are T1R2+3 (heterodimer) and T1R3 (homodimer), which account for all sweet sensing in humans and animals (Grace et al., 2003).
Sourness is the taste that detects acidity. The sourness of substances is rated relative to dilute hydrochloric acid, which has a sourness index of 1. By comparison, tartaric acid has a sourness index of 0.7, citric acid an index of 0.46, and carbonic acid an index of 0.06 (McLaughlin and Margolskee, 1994). Sour taste is detected by a small subset of cells that are distributed across all taste buds in the tongue. According to this mechanism, intracellular hydrogen ions inhibit potassium channels, which normally function to hyperpolarize the cell. By a combination of direct intake of hydrogen ions (which itself depolarizes the cell) and the inhibition of the hyperpolarizing channel, sourness causes the taste cell to fire action potentials and release neurotransmitter. The mechanism by which animals detect sour is still not completely understood.
Saltiness is a taste produced primarily by the presence of sodium ions. Other ions of the alkali metals group also taste salty, but the further from sodium the less salty the sensation is. The size of lithium and potassium ions most closely resemble those of sodium and thus the saltiness is most similar. In contrast, rubidium and cesium ions are far larger so their salty taste differs accordingly. The saltiness index of sodium chloride is 1, while potassium chloride, principal ingredient in salt substitutes, has a saltiness index of 0.6 (McLaughlin and Margolskee, 1994).
Basis of salt taste perception has been studied for years; however, its molecular mechanism is still not fully elucidated. Taste receptors for salty stimuli include several candidates, consisting of specific and unspecific receptors, such as epithelial Na+ channels (ENaC) and taste variant of the vanilloid receptor-1 nonselective cation channel (TRPV1t) (Lyall, 2004). ENaC is hetero-oligomeric complex, comprised of three homologous subunits (, - and ), which together act as a specific salt-taste receptor by providing a specific pathway for sodium current into TRC, when Na+ ions are present in the environment in sufficient concentration. Na+ ions passively flow through these ion channels in the apical, as well as basolateral membrane of TRC according to the concentration gradient and trigger action potential. ENaC channels form adherent junctions on the apical surface of the membrane. With membrane depolarization Ca2+ ions enter through voltage-dependent Ca2+ channels, sensitive to calcium, which elicits neurotransmitter release and signal transmission on primary afferent fiber and eliciting salt taste response (Heck et al., 1984). ENaCs are distributed in dorsal lingual epithelium in vallate and fungiform papillae.
Bitterness is the most sensitive of the tastes, and many perceive it as unpleasant, sharp, or disagreeable, but it is sometimes desirable and intentionally added via various bittering agents. The taste thresholds of bitter substances are rated relative to quinine, which is thus given a reference index of 1. For example, Brucine has an index of 11, is thus perceived as intensely bitterer than quinine, and is detected at a much lower solution threshold (McLaughlin and Margolskee, 1994).The most bitter substance known is the synthetic chemical denatonium, which has an index of 1,000. Research has shown that TAS2Rs (taste receptors, type 2, also known as T2Rs) such as TAS2R38 coupled to the G protein gustducin are responsible for the human ability to taste bitter substances (Maehashi et al., 2008).
Umami is an appetitive taste and is described as a savory or meaty taste. Monosodium glutamate (MSG), developed as a food additive in 1908 by Kikunae Ikeda produces a strong umami taste. The glutamic amino acid is responsible for umami, but some nucleotides such as inosinic acid and guanylic acid can act as complements, enhancing the taste. Some umami taste buds respond specifically to glutamate in the same way that “sweet” ones respond to sugar. Glutamate binds to a variant of
G protein coupled glutamate receptors.
The sensation of the taste can be categorized into five basic tastes sweetness, sourness, saltiness, bitterness and umami.
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