The identities of the specific chemicals that excite different taste receptors are not all known. Psychophysiological and neurophysiological studies have identified at least 13 probable chemical receptors in the taste cells, as follows: 2 sodium receptors, 2 potassium receptors, 1 chloride receptor, 1 adenosine receptor, 1 inosine receptor, 2 sweet receptors, 2 bitter receptors, 1 glutamate receptor, and 1 hydrogen ion receptor.
For practical analysis of taste, the aforementioned receptor capabilities have also been grouped into five general categories called the primary sensations of taste. They are sour, salty, sweet, bitter, and “umami.”
A person can perceive hundreds of different tastes. They are all thought to be combinations of the elementary taste sensations, just as all the colors we can see are combinations of the three primary colors.
Sour Taste. The sour taste is caused by acids—that is, by the hydrogen ion concentration—and the intensity of this taste sensation is approximately proportional to the logarithm of the hydrogen ion concentration (i.e., the more acidic the food, the stronger the sour sensation becomes).
Salty Taste. The salty taste is elicited by ionized salts, mainly by the sodium ion concentration. The quality of the taste varies somewhat from one salt to another because some salts elicit other taste sensations in addition to saltiness. The cations of the salts, especially sodium cations, are mainly responsible for the salty taste, but the anions also contribute to a lesser extent.
Sweet Taste. The sweet taste is not caused by any single class of chemicals. Some of the types of chemicals that cause this taste include sugars, glycols, alcohols, aldehydes, ketones, amides, esters, some amino acids, some small proteins, sulfonic acids, halogenated acids, and inorganic salts of lead and beryllium. Note specifically that most of the substances that cause a sweet taste are organic chemicals. It is especially interesting that slight changes in the chemical structure, such as the addition of a simple radical, can often change the substance from sweet to bitter.
Bitter Taste. The bitter taste, like the sweet taste, is not caused by any single type of chemical agent. Here again, the substances that give the bitter taste are almost entirely organic substances. Two particular classes of substances are especially likely to cause bitter taste sensations: (1) long-chain organic substances that contain nitrogen and (2) alkaloids. The alkaloids include many of the drugs used in medicines, such as quinine, caffeine, strychnine, and nicotine.
Some substances that initially taste sweet have a bitter aftertaste. This characteristic is true of saccharin, which makes this substance objectionable to some people.
The bitter taste, when it occurs in high intensity, usually causes the person or animal to reject the food. This reaction is undoubtedly an important function of the bitter taste sensation because many deadly toxins found in poisonous plants are alkaloids, and virtually all of these alkaloids cause an intensely bitter taste, usually followed by rejection of the food.
Umami Taste. Umami, a Japanese word meaning “delicious,” designates a pleasant taste sensation that is qualitatively different from sour, salty, sweet, or bitter. Umami is the dominant taste of food containing L-glutamate, such as meat extracts and aging cheese, and some physiologists consider it to be a separate, fifth category of primary taste stimuli.
A taste receptor for L-glutamate may be related to one of the glutamate receptors that are also expressed in neuronal synapses of the brain. However, the precise molecular mechanisms responsible for umami taste are still unclear.
