According to Peter Barham, “We taste things with out tongues. We can detect five basic tastes – four are very familiar: sweet, sour, bitter and salt. The fifth, while familiar in the East is less well known in Western cuisine – it is called Umami and is the taste of monosodium glutamate, MSG. MSG is used widely in Eastern cooking and that is probably why it is recognised as a separate taste sensation more readily by those familiar with that cuisine. However, many common western foods contain large amounts of MSG, notably tomatoes and parmesan cheese.” (p.29)
“There are many different molecules that trigger off each of the taste sensations. Taste buds that are receptors for salty taste react to many compounds besides common table salt (sodium chloride). Most sodium salts (that is most simple molecules that contain sodium) and most chlorides (that is most simple molecules that contain chlorine) will taste salty to greater or lesser extents.
Bitterness comes mostly from alkaloids (two common examples are quinine and caffeine). However, many alkaloids are poisonous, which may explain our general aversion to bitter flavours. Sourness comes from acids in our food – all acids provide a sour sensation, while sweetness comes from many other sources besides sugars.
There are many thousands of taste buds in the surface of the human tongue – exactly how these work and just what they respond to is still not fully understood. Indeed, researchers argue about how many different types of sensor there are for each taste.
The taste buds react to chemicals in food that manage to bind in some way to the surfaces of ‘cilia’, or fine hairs, that form a central part of each taste bud. Generally, a molecule has to be dissolved in water to reach the cilia of the taste buds.
When we put food in the mouth, flavour molecules that are already dissolved in water (e.g. those in a sauce, etc.) are likely to reach the taste buds first and provide an initial taste sensation in the mouth. As we chew the food, so we release new flavour molecules into our saliva, also enzymes can start to react with proteins, etc to produce new molecules through chemical reactions that actually take place in the saliva. Thus the taste sensation can change as we chew each mouthful.
Although we can only really detect the five distinct flavours, there remains a great deal of subtlety of taste in the mouth. We rarely taste foods that are purely bitter, sour, sweet or salty, so the different combinations of intensity of just these four tastes allow for a very wide range of tastes. If we consider only sweetness, different sugars taste more or less sweet; combinations of sugars can appear to be sweeter than either of the individual sugars. Most people find fructose to be sweeter than sucrose and glucose very much less sweet. So the actual sugar can affect the taste sensation.
When comparative tests are carried out different people have widely differing sensitivities to different sweeteners. So if you take two people both of whom like one teaspoon of sugar in their coffee, and ask them to use another sweetener instead, one may need only a tiny amount, while the other may need very much more. So we cannot be sure that any two people get the same taste sensation from any particular dish. The best any cook can hope for is that if they like the taste of a dish so will their guests.” (p.30)
“Our noses are much more discriminating than our tongues. We have 5 to 10 million olfactory cells sensing smells in our noses. We can detect the smell of some substances when only as few 250 molecules interact with just a few dozen cells. [-p.31] The limitation of smell is that we can only detect air borne molecules. This limits us to smelling ‘small’ molecules. Once there are more than a hundred or so atoms in a molecule it becomes too heavy to carry through the air in sufficient quantity for us to detect it by smell.
When we eat, most of the actual ‘flavour is sensed in the nose. Each time we breathe some breath comes from the back of the mouth up into the nasal passages where it is sampled by the olfactory cells. The resulting sensation is the major part of what we call ‘flavour’.
However, as with taste, the actual sensation varies with time. When we first put some food in our mouths only the most volatile molecules are carried in the air back into the nose where they are smelled. Generally it will be the smallest molecules that we smell first. Then as we chew the food more small molecules are released from the food and some of the larger molecules slowly evaporate into the nasal passages.” (pp.30-31)
Ref: (emphases in blue bold mine) Peter Barham (c2001) The Science of Cooking. Springer-Verlag: Berlin