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More than four taste qualities

by Mans H. Boelens

With this commentary¹ it will be proposed that there exist more than the generally accepted four taste qualities: sweet, bitter, sour and salt. Herewith it is suggested that there are five pairs of taste qualities, each with its opposite, namely: sweet with bitter, sour (acidic) with soapy (alkaline), salt with bland, savoury (Japanese: Umami) with nauseous, cool with hot (pungent).

Every living organism needs information for the maintenance of its life and species. To gather this information both animals and humans possess senses. By means of these senses they can communicate with their environment. With the senses of smell and taste the animal-world communicates via chemicals. Chemical communication plays for instance an important role in finding edible (non-toxic) food and in interindividual relations (social and sexual) and in detecting danger. With the chemical senses one may assume that a chemical substance (or mixture) interacts with a biological system resulting in a response. With the sense of taste it is known that the biological system may concern taste buds on the tongue. It is common knowledge that there should exist four taste qualities, e.g. salt, bitter, sour and sweet. In modern dictionaries it is stated that the sense of taste perceives and distinguishes between sweet, bitter, sour and salt substances through stimulation of the taste buds in the mouth by a solution of the substance in the salvia (see, for instance, New Webster's Dictionary and Thesaurus of the English Language). Of course, other biological systems than taste buds can be activated in the mouth during food consumption, for example interaction with trigeminus nerves, mouthfeeling nerves. For this proposal the overall taste impression of stimulation of all organs in the mouth is considered.

Chemical communication with the sense of smell and taste is an aspect of chemoreception. Chemoreception covers the whole field of effects resulting from stimulation of the senses of smell and taste by chemical substances. Chemoreception is a multidisciplinary science studied by chemists, physicists, biologists, physiologists, neurologists, entomologists and psychologists. Each year about 2,000 articles on chemoreception are published. Chemoreception may concern molecular processes at receptor sites, biochemical processes at membranes, transduction in and through cell, peripheral and CNS mechanisms and overt behavioural manifestations.

Below a scheme is shown of proposed taste qualities. For the savoury, appetising (in Japanese: Umami) taste quality a primary taste receptor has already been identified. American investigators Buck and Axel² identified a complete new family of odour receptors, which are proteins. With these receptors the smelling system can recognise and distinguish a large number of different odourant molecules. There may be as many as 1,000 different odour receptors in the human nose. If there exists an analogy between the tasting system and the smelling system it is unlikely that there are only four taste receptors. The identified proteins have acidic and basic endgroups; with these groups it is logical that they can interact with bases or alkaline (soapy taste quality) and acids (sour taste quality). The proposed taste qualities play an important role for finding and appreciating edible food and for the rejection of decomposed and toxic food by animals and humans.

Scheme of proposed taste qualities (reference material)

Sweet (sucrose)

Sour-Acidic (citric acid)

Salt (sodium chloride)

Savoury-Umami (monosodium glutamate)

Cool (menthol)

tasting opposites:

Bitter (quinine salt)

Soapy-Alkaline (sodium carbonate)

Bland (distilled water)

Nauseaous (sulphur-nitrogen compound)

Hot-Pungent (capsaicin)

1. This article was originally submitted as Commentary proprosal to Nature on August 11, 1999.
2. Linda Buck and Richard Axel, Cell, Vol. 65, 175-187, April 5, 1991.

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