Taste
The gustatory system or sense of taste is the sensory system that is partially responsible for the perception of taste. Taste is the perception stimulated when a substance in the mouth reacts chemically with taste receptor cells located on taste buds in the oral cavity, mostly on the tongue. Taste, along with the sense of smell and trigeminal nerve stimulation, determines flavors of food and other substances. Humans have taste receptors on taste buds and other areas, including the upper surface of the tongue and the epiglottis. The gustatory cortex is responsible for the perception of taste.
The tongue is covered with thousands of small bumps called papillae, which are visible to the naked eye. Within each papilla are hundreds of taste buds. The exceptions 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. 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.
Taste receptors in the mouth sense the five basic tastes: sweetness, sourness, saltiness, bitterness, and savoriness. Scientific experiments have demonstrated that these five tastes exist and are distinct from one another. Taste buds are able to tell different tastes apart when they interact with different molecules or ions. Sweetness, savoriness, 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 metals or hydrogen ions meet taste buds, respectively.
The basic tastes contribute only partially to the sensation and flavor of food in the mouth—other factors include smell, detected by the olfactory epithelium of the nose; texture, detected through a variety of mechanoreceptors, muscle nerves, etc.; temperature, detected by temperature receptors; and "coolness" and "hotness", by chemesthesis.
As the gustatory system senses both harmful and beneficial things, all basic tastes bring either caution or craving depending upon the effect the things they sense have on the body. Sweetness helps to identify energy-rich foods, while bitterness warns people of poisons.
Among humans, taste perception begins to fade during ageing, tongue papillae are lost, and saliva production slowly decreases. Humans can also have distortion of tastes. Not all mammals share the same tastes: some rodents can taste starch, cats cannot taste sweetness, and several other carnivores, including hyenas, dolphins, and sea lions, have lost the ability to sense up to four of their ancestral five basic tastes.
Basic tastes
The gustatory system allows animals to distinguish between safe and harmful food and to gauge different foods' nutritional value. Digestive enzymes in saliva begin to dissolve food into base chemicals that are washed over the papillae and detected as tastes by the taste buds. The tongue is covered with thousands of small bumps called papillae, which are visible to the naked eye. Within each papilla are hundreds of taste buds. The exception to this are the filiform papillae, which do not contain taste buds. There are between 2,000 and 5,000 taste buds that are located on the back and front of the tongue. 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.The five specific tastes received by taste receptors are saltiness, sweetness, bitterness, sourness, and savoriness.
As of the early 20th century, Western physiologists and psychologists believed that there were four basic tastes: sweetness, sourness, saltiness, and bitterness. The concept of a "savory" taste was not present in Western science at that time, but was postulated in Japanese research.
One study found that salt and sour taste mechanisms both detect, in different ways, the presence of sodium chloride in the mouth. Acids are also detected and perceived as sour. The detection of salt is important to many organisms, but especially mammals, as it serves a critical role in ion and water homeostasis in the body. It is specifically needed in the mammalian kidney as an osmotically active compound that facilitates passive re-uptake of water into the blood. Because of this, salt elicits a pleasant taste in most humans.
Sour and salt tastes can be pleasant in small quantities, but in larger quantities become more and more unpleasant to taste. For sour taste, this presumably is because the sour taste can signal under-ripe fruit, rotten meat, and other spoiled foods, which can be dangerous to the body because of bacteria that grow in such media. Additionally, sour taste signals acids, which can cause serious tissue damage.
Sweet taste signals the presence of carbohydrates in solution. Since carbohydrates have a very high calorie count, they are essential to the human body, which evolved to seek out the highest-calorie-intake foods. They are used as direct energy and storage of energy. Many non-carbohydrate molecules trigger a sweet response, leading to the development of many artificial sweeteners, including saccharin, sucralose, and aspartame. It is still unclear how these substances activate the sweet receptors and what adaptative significance this has had.
The savory taste, identified by Japanese chemist Kikunae Ikeda, signals the presence of the amino acid L-glutamate. The amino acids in proteins are used in the body to build muscles and organs, and to transport molecules, antibodies, and the organic catalysts known as enzymes. These are all critical molecules, and it is important to have a steady supply of amino acids; consequently, savory tastes trigger a pleasurable response, encouraging the intake of peptides and proteins.
Pungency had traditionally been considered a sixth basic taste. In 2015, researchers suggested a new basic taste of fatty acids called "fat taste", although "oleogustus" and "pinguis" have both been proposed as alternate terms.
Sweetness
Sweetness, usually regarded as a pleasurable sensation, is produced by the presence of sugars and substances that mimic sugar. Sweetness may be 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 compounds that can bind with varying bond strength to two different sweetness receptors. These receptors are T1R2+3 and T1R3, which account for all sweet sensing in humans and animals.Taste detection thresholds for sweet substances are rated relative to sucrose, which has an index of 1. The average human detection threshold for sucrose is 10 millimoles per liter. For lactose it is 30 millimoles per liter, with a sweetness index of 0.3, and 5-nitro-2-propoxyaniline 0.002 millimoles per liter. "Natural" sweeteners such as saccharides activate the GPCR, which releases gustducin. The gustducin then activates the molecule adenylate cyclase, which catalyzes the production of the molecule cAMP, or adenosine 3', 5'-cyclic monophosphate. This molecule closes potassium ion channels, leading to depolarization and neurotransmitter release. Synthetic sweeteners such as saccharin activate different GPCRs and induce taste receptor cell depolarization by an alternate pathway.
Sourness
Sourness is the taste that describes 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.Sour taste is detected by a small subset of cells that are distributed across all taste buds called Type III taste receptor cells. H+ ions that are abundant in sour substances can directly enter the Type III taste cells through a proton channel. This channel was identified in 2018 as otopetrin 1. The transfer of positive charge into the cell can itself trigger an electrical response. Some weak acids such as acetic acid can also penetrate taste cells; intracellular hydrogen ions inhibit potassium channels, which normally function to hyperpolarize the cell. By a combination of direct intake of hydrogen ions through OTOP1 ion channels and the inhibition of the hyperpolarizing channel, sourness causes the taste cell to fire action potentials and release neurotransmitter.
The most common foods with natural sourness are fruits, such as lemon, lime, grape, orange, tamarind, and bitter melon. Fermented foods, such as wine, vinegar or yogurt, may have sour taste. Children show a greater enjoyment of sour flavors than adults, and sour candy containing citric acid or malic acid is common.
Saltiness
Saltiness taste seems to have two components: a low-salt signal and a high-salt signal. The low-salt signal causes a sensation of deliciousness, while the high-salt signal typically causes the sensation of "too salty".The low-salt signal is understood to be caused by the epithelial sodium channel, which is composed of three subunits. ENaC in the taste cells allow sodium cations to enter the cell. This on its own depolarizes the cell, and opens voltage-dependent calcium channels, flooding the cell with positive calcium ions and leading to neurotransmitter release. ENaC can be blocked by the drug amiloride in many mammals, especially rats. The sensitivity of the low-salt taste to amiloride in humans is much less pronounced, leading to conjecture that there may be additional low-salt receptors besides ENaC to be discovered.
A number of similar cations also trigger the low salt signal. The size of lithium and potassium ions most closely resemble those of sodium, and thus the saltiness is most similar. In contrast, rubidium and caesium ions are far larger, so their salty taste differs accordingly. The saltiness of substances is rated relative to sodium chloride, which has an index of 1. Potassium, as potassium chloride, is the principal ingredient in salt substitutes and has a saltiness index of 0.6.
Other monovalent cations, e.g. ammonium, and divalent cations of the alkali earth metal group of the periodic table, e.g. calcium, ions generally elicit a bitter rather than a salty taste even though they, too, can pass directly through ion channels in the tongue, generating an action potential. But the chloride of calcium is saltier and less bitter than potassium chloride, and is commonly used in pickle brine instead of KCl.
The high-salt signal is poorly understood. This signal is not blocked by amiloride in rodents. Sour and bitter cells trigger on high chloride levels, but the specific receptor is unidentified.