It's a matter of taste

"Nothing takes the taste out of peanut butter quite like unrequited love." - Charlie Brown (by Charles Schulz). Actually Charlie Brown is slightly mistaken here, but we can forgive him because we all know what unrequited love can do. I'm sure he can still taste his salty tears as he can the saltiness of his peanut butter. However, the apparent odour might be affected. Taste and smell are distinct senses, but they are easily entangled in common experience. Of course, it is our sense of smell that is impaired by our blocked nose, either from tears or from a cold, which affects the flavour. In this episode some background theory to the sense of taste. 

Additionally to smell the human sensory system has another perception for chemical substances (chemesthesis) and this is taste. The taste sensors are located on top of the tongue and on the inside of the mouth. (Indeed chemical receptors have been shown to exist on the insides of the cheeks.) With these receptors, human beings can perceive about five main tastes; Sweet, Sour, Salty, Bitter, and Umami. I say about five because recently additional receptors have been found in the mouth for fatty acids for example. The sense of taste seems less complex than the sense of smell, however also taste has not yet been completely elucidated. 

Sweetheart 

Table sugar or sucrose

The most direct and stimulating taste is sweetness. It is immediately linked to sugar, energy, and it's not strange that we have evolved a preference for sweet food. This love for sweet things is, however, not shared by all mammals. In recent research it was found that certain carnivores (cats, seals) are unable to perceive sweetness, an ability probably lost in time, while others still retain their sweet tooth like bears for example.

Most foods contain sugars. These sugars can be simple or more complex. The simple ones taste sweet and the complex ones do not, unless they are broken down into the simple sugars. The protein receptors for sweetness bind to glucose. Glucose (a.k.a dextrose) is the monosaccharide that our bodies can directly use without modification, other prevalent monosaccharides are fructose and galactose which are both sweet to the taste. Disaccharides and polysaccharides need to be broken down into monosaccharides by enzymes and they will taste sweeter, for example lactose and starch. The sucrose (table sugar) you use to sweeten your tea is a disaccharide made of glucose and fructose.

A notable exception to the "sweet means energy" rule are artificial sweeteners which are therefore used as low calorie sugar replacements. Most artificial sweeteners have slightly bitter aftertastes though. Some of them are not thermostable and hence not very useful in heated food. 

Stevia is a plant from which one can gain a natural sweetener, Steviol, but unlike other sugars it is not metabolised to glucose, hence Steviol can be used as a sugar replacement in certain products.

There is even a substance, obtained from the miracle berry, which is called Miraculin that binds to the sweetness receptors making normally sour food taste sweet. Eating a miracle berry first and then drinking vinegar yields a drink that is pleasantly sweet to the taste. It only works with sour products but not with bitter foods. It seems that Miraculin does not normally activate the sweetness receptors except in an acidic environment where the molecule changes shape slightly. 

A bitter note

"Bitter pills may have blessed effects" is a strange saying because humans have a natural aversion against bitter substances, and for good reason. Very many bitter chemicals in nature, e.g. alkaloids, are poisonous to humans when ingested. The "blessed effect" might just be death. Luckily the concentration threshold for detecting bitterness is very low. One can imagine that this sense developed with a rather high selection pressure. 

Coffee contains bitter substances like caffeine

There are so-called super tasters who can sense bitterness very well, and have a larger compared number of bitterness receptors (which are also protein receptors). Consequently these persons do not like coffee, grapefruit, or other (mildly) bitter foods. 

Combined with other tastes bitterness doesn't have to be unpleasant, although it is usually endured for a higher goal. Chocolate, coffee, or beer, all taste more or less bitter, though the pleasure in eating them is in all cases greater than our dislike for the bitterness. In other cases the alkaloids that cause it may be extracted and discarded, as in aubergine (eggplant), or one can prevent the extraction for example by not over extracting coffee.

Sour grapes

Citrus fruits like mandarins contain citric acid

The sour taste is caused by acids which have weakly bonded hydrogen ions (H+) or protons. It is not completely clear how the protons interact with ionic receptors on the tongue to bring a unique sour taste. Obviously, similar small ions like potassium (K+) do not cause a sour taste. Recent research has shown that there exists a specific receptor for protons. 

Sour is also a taste which is not very pleasant if it is not accompanied by something else, usually sweetness. The strange effect of miraculin seems also to completely block the acid receptors as there is no residual acidity left in the taste when eating sour things. It is not yet known how miraculin blocks the acid-receptors. Natural food contains different kinds of acids, citric acid in citrus fruits, maleic acid in apples, and oxalic acid in berries. Acids can also originate during fermentation, for example lactic acid and acetic acid.

Salty dog

Sodium ions (Na+) are responsible for the salty taste in food. One may wonder why salt tastes so nice. It seems salt, or at least sodium, suppresses bitter tastes somewhat and brings out the sweetness and sour taste more. It might be that this preference for salt originated in the old days when living on land meant there was not much salt available, except maybe in plants which take it from the soil. As sodium is difficult to retain in the body (it is soluble in water), a preference for salty food arose. Not surprisingly the receptors are ionotropic.

Table salt or Sodium Chloride (NaCl)

How do other cations taste? First of all it seems that sodium in combination with chloride ions tastes more salty than in other combinations, like sodium lactate. There are other cations that have a tast similar to sodium, a well known example being lithium. (Lithium salts are used as anti-depressants.) However also completely different compounds like ammonium chloride taste vaguely salty albeit with a different aftertaste. 

Salt replacements or sodium replacements are being sought in order to suppress the intake of high amounts of sodium which are detrimental to blood pressure, kidney function, and electrolyte balance. 

Delicious

The umami taste was discovered in 1908 by a Japanese professor, who called it after umai (delicious) mi (taste). It is actually a taste describing amino acids (glutamine, aspartic acid), and can be thought of as the savoriness or body of food. Monosodiumglutamate (MSG) is one of the best known umami flavoured substances. But the umami taste is also prominently found in for example fish, cured meat, tomatoes, aged products (cheese, soy sauce, fermented fish or shrimp paste) and, surprisingly, green tea.

Fat chance

There is some tentative evidence that suggests there is a sixth taste receptor which binds to free fatty acids. But it is not really clear whether test subjects cannot tell the fatty acids from the viscosity and lubricity. That being said, humans like the taste of fat very much, and saturated fatty acids (mostly from animals) are preferred over unsaturated ones (found in plants).

This is not the end of the story. Our mouth has more chemical senses than the mentioned six tastes. For example there is the phenomenon of having a metallic taste due to medicine. (And also blood tastes slightly metalic). Other chemical senses include spiciness, menthol-coolness and kokumi (or mouthfullness). There are more senses to be reckoned with.

Here I tried to give you a taste of this sense. There is slightly more than meets the mouth than just the four main tastes. On its own taste can be lonely, it has to be paired with the sense of smell to feel loved and appreciated for the complex perception it actually is.