Bitterness is of interest to those who study evolution, as well as various health researchers since a large number of natural bitter compounds are known to be toxic. The ability to detect bitter-tasting, toxic compounds at low thresholds is considered to provide an important protective function. Plant leaves often contain toxic compounds, and among leaf-eating primates there is a tendency to prefer immature leaves, which tend to be higher in protein and lower in fiber and poisons than mature leaves. Amongst humans, various food processing techniques are used worldwide to detoxify otherwise inedible foods and make them palatable. Furthermore, the use of fire, changes in diet, and avoidance of toxins has led to neutral evolution in human bitter sensitivity. This has allowed several loss of function mutations that has led to a reduced sensory capacity towards bitterness in humans when compared to other species.The threshold for stimulation of bitter taste by quinine averages a concentration of 8 μM (8 micromolar). The taste thresholds of other 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 more bitter than quinine, and is detected at a much lower solution threshold. The most bitter natural substance is amarogentin, a compound present in the roots of the plant Gentiana lutea, and the most bitter substance known is the synthetic chemical denatonium, which has an index of 1,000.
SaltinessSaltiness is a taste produced best by the presence of cations (such as Na+, K+ or Li+) and is directly detected by cation influx into glial like cells via leak channels causing depolarisation of the cell.Other monovalent cations, e.g., ammonium, NH+4, and divalent cations of the alkali earth metal group of the periodic table, e.g., calcium, Ca2+, ions, in general, elicit a bitter rather than a salty taste even though they, too, can pass directly through ion channels in the tongue. SournessSourness is acidity, and, like salt, it is a taste sensed using ion channels. Undissociated acid diffuses across the plasma membrane of a presynaptic cell, where it dissociates in accordance with Le Chateliers principle. The protons that are released then block potassium channels, which depolarise the cell and cause calcium influx. In addition, the taste receptor PKD2L1 has been found to be involved in tasting sour. BitternessResearch has shown that TAS2Rs (taste receptors, type 2, also known as T2Rs) such as TAS2R38 are responsible for the ability to taste bitter substances in vertebrates. They are identified not only by their ability to taste certain bitter ligands, but also by the morphology of the receptor itself (surface bound, monomeric). SavorinessThe amino acid glutamic acid is responsible for savoriness, but some nucleotides (inosinic acid and guanylic acid) can act as complements, enhancing the taste.Glutamic acid binds to a variant of the G protein-coupled receptor, producing a savory taste. Further sensations and transmission
The tongue can also feel other sensations not generally included in the basic tastes.