Tastes are 6

First as on date the labeled basic tastes are 6 and not 5 as indicated in this article.

It is called as arusuvai in Tamil with the 4 famous ‘Ss’ of which one is missing in this article and which is predominant in Indian curries, though for want of appropriate vocabulary in English, the native English speakers use adjectives like pungent, acerbic, astringent etc though they could have coined an adjective out of these two major chemicals whose presence mainly contribute to those sensations namely ascorbic acid and Capsaicin.

So, tastes are basically six with the four Ss-Sweet, salty, sour and spicy along with  bitter and umami, though initially humans labeled it as only four leaving out umami and spicy.

There are innumerable factors that go to make taste and honestly even to restrict it to 6 is doing injustice to human biological sensations and psychological sensitivity.

On most topics of science I refer to the great writer who incidentally was not an academically anointed evolutionary biologist but a professional pilot. However his works are one of the best in evolutionary biology which every student, teacher and writer must read namely Guy Murchie, especially his work ‘THE SEVEN MYSTERIES OF LIFE’. In this work he writes about taste in chapter 8

“Taste is basically like smell except that it is less sensitive, requiring about 25,000 times as many molecules to elicit a sensation because it deals primarily with molecules in solid and liquid form rather than gaseous”

Chapter 8

“ Twenty-one Senses of Chemistry, Mind and Spirit

SO WE ARRIVE at what are often called the visceral or chemical senses, meaning those that enunciate the appetites for food, drink and, in some cases, physical love. And right off we encounter smell and taste, which work chemically and are thought to be the most experienced of all senses. If this is true, it is presumably because chemistry deals primarily with molecules, which are the material units composing the world, including all its organisms, and which therefore interact with creatures directly rather than indirectly through waves of radiation or compression, as in the cases of vision and hearing. Thus when organized life evolved on Earth several billion years ago, the first way it could sense anything almost inevitably had to be through direct contact, naturally at the molecular level, as earthly life had not yet organized larger units except such structures as crystals which, if they are definable as alive, may also be regarded as molecules or more accurately, supermolecules.

So did smell and taste (originally one sense) come into our world. At first the smell-taste organ (if it could be considered such) must have occupied practically the whole body, making the viruslike creature in effect a living nose or tongue. Then, as macroscopic life appeared, the organ retained its central forward position at the business end of its owner, while the snout took shape and, doing so, started to create the face.

TASTE

Taste is basically like smell except that it is less sensitive, requiring about 25,000 times as many molecules to elicit a sensation because it deals primarily with molecules in solid and liquid form rather than gaseous. And the primary tastes are but four in number: sweet, bitter, salty and sour; each key taste molecule having its own lock receptacle in what is called a taste bud on the human tongue, palate or throat. This means that each primary taste is tastable only in its own location: sweet at the tip of the tongue, bitter at the back, salty on the sides around the tip and in the throat, sour on the sides of the tongue farther back. Like smell, taste is very ancient and fundamental, and I feel no doubt that, when we know more about molecular structure, we will see geometric reasons why applesauce tastes good with pork, mint with lamb, cranberries with turkey, ketchup with baked beans and so on.

In the meantime the study of taste is increasingly bewildering. A couple of sample findings of research in the chemical senses indicate that the flavor of a common brand of coffee is a synthesis of about four hundred compounds (most of which are smelled more than tasted) while the formula for a different and more sophisticated artificial flavor "requires as many as 20,000 separate pieces of information." And this no doubt throws light on why man has been able to invent cameras, phonographs and associated techniques to record, amplify and transmit sights and sounds but has not yet devised any comparable method of recording, amplifying or transmitting a single smell or taste.

Relativity is another factor that makes smell and taste hard to comprehend. If, for example, you put on one side of your tongue a salt solution too dilute to taste noticeably salty but then add a little sugar on the other side, you will instantly begin to taste the salt along with the sugar. The opposite happens when you start with dilute, tasteless sugar on one side and add salt on the other. Furthermore, to most humans, a salt solution will begin to taste sweet when diluted down to .03 parts per million, particularly if it is cool, the amount of dilution needed to make this happen being roughly proportional to the temperature. On the other hand, although Epsom salts taste salty on the front of your tongue, they turn bitter when pushed back to the hind buds. And if you try many kinds of chemically graded salt, you will notice them tasting progressively bitterer as you get to salts of heavier molecular weight. The taste of salt of course is largely electrical (due to ionization of its constitutent atoms) and every sort of electric current has its own flavor: a gentle direct current savoring subtly sour when the positive terminal touches the tongue but "like burnt soap" when the flow is reversed, while an alternating current that smacks of astringent sourness at 50 cycles turns steadily more and more bitter as it escalates toward 1000 cycles.

Such relativity is not merely mental but rather an objective part of the natural and paradoxically complex simplicity of the chemical senses which, unlike senses that become electrical only in the final transmission of messages to the brain, may function electrically all the way from their first contact with whatever they perceive.

This is not to say that there aren't real differences of taste between individuals, for we all know such differences exist. Ordinary sugar, for example, is tasteless to a small percentage of children while saccharin tastes bitter to a few yet sweet to their brothers and sisters.

And there is a synthetic chemical called PTC (for phenyl-thio-carbamide) that tastes intensely bitter to an average of two people out of three all over the world but utterly tasteless to the third person. And the common food preservative sodium benzoate tastes like almost anything or nothing, depending on who tries it. But there is such conclusive evidence that these phenomena are objective that a chemist named Arthur L. Fox has formulated a genetic theory of taste on his finding that 26 percent of people consider PTC bitter but sodium benzoate salty and like almost every kind of food, while 17 percent with different taste genes register both these chemicals as bitter and dislike most sorts of food.

Indeed, of the four primary tastes, bitterness turns out to be the easiest for a human to detect, perhaps because it signals danger in the form of poison. But there always seem to be a few unlucky people who just can't taste anything at all, and them I would call "smumb." They aren't necessarily the same ones as those who are smuff and their deficiency is more serious - but fortunately medical researchers have discovered that most of them, even those who've been stone smumb for years, can be cured within a few days by taking small doses

of the trace metal zinc.

Animals naturally vary in sensitivity to taste, some insects going so far as to walk on their "tongues" and to taste with their feet.

Other creatures, notably fish, may be trillions of times more sensitive than man with taste buds so densely distributed over their body surfaces that they literally swim in a sapid sea. There is the uncanny account of a coho salmon raised in a California hatchery who, when a year old, was dumped into a strange stream several miles away and allowed to migrate with his fellows down to the ocean. But at spawning time the next year he appeared back in his original tank, having followed the familiar flavor into his home stream, threaded a particular culvert under U.S. Highway 101 which enabled him to enter the hatchery's flume and storm sewer, from which he finally wriggled up a four-inch drainpipe past 900 elbows, climactically knocking off its wire cap and even leaping over the screen

that surrounded the drain!

THE SENSES OF HUNGER AND THIRST

Close cousins of taste of course are the twin senses of hunger and thirst. Hunger has long been known to be turned on by rhythmic contractions of the empty stomach and, more recently, by a decline in the sugar content of the blood. Indeed a transfusion of low-sugar blood from a starving dog into a well-fed one will make the latter hungry for the same reason that high-sugar blood from a satiated dog will ease the pangs of a hungry one, Yet hunger can hardly be as simple as this. In fact certain researchers have recently found that the ratio of ions in the brain may regulate hunger and specifically that rats who have eaten to satiation can be induced to resume eating voraciously by injections of calcium ions in the cerebrum. Others predict that, when we fully understand it, hunger will turn out to involve a "hunger hormone," conveyable, if not normally from organism to organism, perhaps in some degree from organ to organ through lymph and blood.

Thirst is about equally mysterious but obviously different from hunger and much more compelling, at least to a water-dependent creature like man, as is proven by the fact that a man can live more than a year without food but, to the best of my knowledge, seventeen days is his world record without water. This record was made in 1821 when a prominent Frenchman named Antonio Viterbi committed suicide by refusing to drink, but of course he may have taken in a significant amount of moisture in whatever he ate. Doctors now say he probably would have survived if he had accepted water on the fourteenth or fifteenth day, but by the sixteenth it was almost certainly too late. There are cases on record of castaways deprived of fresh water for two weeks who were rescued just in time and managed to survive. Presumably all these sufferers were in humid environments, did not sweat and kept evaporation from their bodies to a minimum.

Something to consider also is that the thirst sensation is less influenced by the total amount of water in the body than by the amount of water relative to certain solids, particularly salts. And this accounts for the classic equation of thirst that assures the bartender he will sell in the end more than ten times as much in drinks as the cost of all the "free" salted pretzels, popcorn and potato chips he "gives" away to his customers to bolster their thirst. It also relates to the discovery in 1952 that a fraction of a drop of a salt solution injected into  the hypothalamus at the base of a goat's brain will immediately make the animal thirsty, which, in combination with later evidence, seems to have pretty well proved the site of thirst to be the hypothalamus. Strange as it may seem, drinking beyond a certain quantity of water hour after hour increases rather than decreases thirst because the body loses salt in urine and salt deficiency produces thirst, a kind of thirst, however, that can be relieved only by salt.

While on this subject, I should explain that the reason castaways are warned against drinking saltwater is not to deny that the watery   96.5 percent of it may immediately relieve them but only to avoid the ultimate problem of getting rid of their lethal excess of salt.

Most mammals, including man, cannot eliminate salt in urine at the rate of more than about 2 percent so, unless one can be sure of soon receiving enough fresh water to dilute one's urine that low, to drink seawater (averaging 3.5 percent salt) is to take a deadly chance.

The most dramatic thing about thirst has to be the kind of death it brings when a man is lost, say, in a hot desert, a grim dissolution witnesses describe as unfolding in five fairly distinct stages. First comes the protesting phase of increasing discomfort and querulous disbelief (with 3 to 5 percent of body water lost). Second, the feeling of having a mouth "dry as cotton," tongue sticking to teeth, a lump in the throat that no amount of swallowing can dislodge, a face tight from shrinking skin (water loss 5 to 10 percent). Third is the burning agony of having the tongue shrivel into a knot, eyelids stiffen, eyes staring as the victim irrationally tears at his clothing or scalp, bites his arm for blood or even laps up his last drops of urine (10 to 20 percent). Fourth is the stage of the skin cracking apart (more than 20 percent of water gone or too much for any chance of recovery), lymph and blood oozing out, eyes weeping blood, arms digging aimlessly into the sand. And fifth, the final or living-death stage of gentle writhing on the ground and, often, calm acceptance awaiting the end.

The third stage is usually the worst, for nature seems to have evolved pain to save life before it is too late. But after the blood begins to thicken and dry and the damage becomes irreversible, the pain eases progressively away - which suggests why those who die of thirst are so often reported to be joyous in their final hour - even as one of them was heard to murmur, "I'm melting, I'm sinking... I'm drowning in an ocean of unexplainable peace."