Primary Sensations of Taste
The identities of the specific chemicals that excite different taste receptors are not all known. Even so, psychophysiologic and neurophysiologic studies have identified at least 13 possible or 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 oftaste. They are sour, salty, sweet, bitter, and “umami.”
A person can perceive hundreds of different tastes. They are all supposed 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 con-centration, and the intensity of this taste sensation is approximately propor-tional to the logarithm of the hydrogen ion concentration. That is, 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 ionconcentration. 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. Someof 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 chem-icals. It is especially interesting that slight changes in the chemical structure, such as addition of a simple radical, can often change the substance from sweet to bitter.
Bitter Taste. The bitter taste, like the sweet taste, is notcaused 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 sub-stances 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 at first taste sweet have a bitter aftertaste. This 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 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 cause intensely bitter taste, usually followed by rejection of the food.
Umami Taste. Umamiis a Japanese word (meaning“delicious”) designating a pleasant taste sensation that is qualitatively different from sour, salty, sweet, or bitter. Umami is the dominant taste of food contain-ing L-glutamate, such as meat extracts and aging cheese, and some physiologists consider it to be a sep-arate, fifth category of primary taste stimuli.
A taste receptor for L-glutamate may be related to one of the glutamate receptors expressed in neuronal synapses of the brain. However, the precise molecular mechanisms responsible for umami taste are still unclear.
Threshold for Taste
The threshold for stimulation of the sour taste by hydrochloric acid averages 0.0009 N; for stimulation of the salty taste by sodium chloride, 0.01 M; for the sweet taste by sucrose, 0.01 M; and for the bitter taste by quinine, 0.000008 M. Note especially how much more sensitive is the bitter taste sense than all the others, which would be expected, because this sensation pro-vides an important protective function against many dangerous toxins in food.
Table 53–1 gives the relative taste indices (the recip-rocals of the taste thresholds) of different substances. In this table, the intensities of four of the primary sen-sations of taste are referred, respectively, to the inten-sities of the taste of hydrochloric acid, quinine, sucrose, and sodium chloride, each of which is arbitrarily chosen to have a taste index of 1.
Taste Blindness. Some people are taste blind for certainsubstances, especially for different types of thiourea compounds. A substance used frequently by psycholo-gists for demonstrating taste blindness is phenylthio-carbamide, for which about 15 to 30 per cent of allpeople exhibit taste blindness; the exact percentage depends on the method of testing and the concentra-tion of the substance.