The Six tastes.
Sour flavors are generally produced by acids, and different acids produce different acidic flavors. Some familiar sour flavors are vinegar (acetic acid), sour milk (lactic acid), citrus fruit (citric acid), apples (malic acid) and wine (tartaric acid).<ref name=“Reineccius”>Reineccuis, Gary. Flavor Chemistry and Technology. Taylor & Francis: 2006.</ref>
Salty flavors are most commonly associated with the mineral of the same name (sodium chloride), however most low molecular weight salts (ammonium chloride, potassium chloride, lithium chloride, sodium bromide, ammonium bromide, sodium iodide and lithium iodide) have a salty taste.<ref name=“Pyke” />
Less is known about sweet and bitter substances. There seems to be no relationship between organic functional groups and sweet flavor (as almost every class of compound has a member which tastes sweet), yet at the same time the chemistry is very specific. For example, saccharine is 300 times sweeter than table sugar (sucrose), however if the hydrogen bonded to the nitrogen is replaced with a methyl group, all flavor sensation disappears.<ref name=“Pyke” /> Likewise, bitter sensations are difficult to classify, though many plant-derived molecules which interact with the brain such as caffeine and nicotine,<ref name=“Reineccius” /> as well as large molecular weight ionic salts such as cesium chloride, rubidium bromide and potassium iodide<ref name=“Pyke” /> produce bitter tastes.
Umami (Japanese for “delicious”, “savory” or “meaty”) is a newcomer to the basic flavor group. Umami is associated with L-glutamate and is found in meat broths, sea weed broth and aged cheese. It can also be used in solid form, usually monosodium glutamate, MSG.<ref name=“Reineccius” /><ref name=“Pyke” /> Unlike the other four flavors, umami is not thought to have a flavor of its own, but rather it intensifies other flavors already present, hence the common use of MSG in many prepackaged food items.
Little is known about the mechanisms involved in tasting foods, but it is thought that sour and salty tastes utilize channels which allow inward proton transfer combined with H+-gated channels, while sweet, bitter and umami flavors utilize surface binding pathways to transmit flavor information. All five flavors depolarize the taste cell (which initially has a net negative internal charge) by causing the transfer of calcium ions (Ca2+) into the cell. This forms an electrical signal which is then sent to the brain and interpreted as flavor. When umami binds to a flavor cell, not only is there an influx of Ca2+ into the cell, but the cell also become more sensitive to the effects of other flavor compounds.<ref name=“Reineccius” />