Taste buds on the tongue can distinguish five basic tastes — salt, savouriness, sweet, bitter and sour — as well as having a less well-understood ability to taste fat. The ability to taste salt, sweetness, savouriness and fat permits detection of nutrients; the ability to taste sourness and bitterness permits avoidance of toxins in foods.
Salt (correctly the mineral sodium) is essential to life, and wild animals will travel great distances to a salt lick. Like other animals, human beings have evolved a pleasurable response to salty flavours — this ensures that physiological needs are met. There is evidence that sensitivity to salt changes in response to the state of sodium balance in the body, with an increased number of active salt receptors (see below) on the tongue at times of sodium depletion. However, there is no shortage of salt in developed countries and, as discussed in section 7.3.4, average intakes of salt are considerably greater than requirements, and may pose a hazard to health.
The sensation of savouriness is distinct from that of saltiness, and is sometimes called umami (the Japanese for savoury). It is largely due to the presence of free amino acids in foods, and hence permits detection of protein-rich foods. Stimulation of the umami receptors of the tongue is the basis of flavour enhancers such as monosodium glutamate, which is an important constituent of traditional oriental condiments, and is widely used in manufactured foods.
The other instinctively pleasurable taste is sweetness, which permits detection of carbohydrates, and hence energy sources. While it is only sugars (section 4.2.1) that have a sweet taste, human beings (and a few other animals) secrete the enzyme amylase in saliva (section 4.2.21); amylase catalyses the hydrolysis of starch, which is the major dietary carbohydrate, to sweet-tasting sugars while the food is being chewed.
The tongue is sensitive to the taste not of triacylglycerols, but rather of free fatty acids, and especially polyunsaturated fatty acids (section 18.104.22.168). This suggests that the lipase secreted by the tongue has a role in permitting the detection of fatty foods as an energy source, in addition to its role in fat digestion (section 4.3.2).
Sourness and bitterness are instinctively unpleasant sensations; many of the toxins that occur in foods have a bitter or sour flavour. Learned behaviour will overcome the instinctive aversion, but this is a process of learning or acquiring tastes, not an innate or instinctive response.
The receptors for salt, sourness and savouriness (umami) all act as ion channels, transporting sodium ions, protons or glutamate ions respectively into the cells of the taste buds.
The receptors for sweetness and bitterness act via cell-surface receptors linked to intracellular formation second messengers. There is evidence that both cyclic adenosine monophosphate (cAMP) (section 1.3.2) and inositol trisphosphate (section 10.3.3) mechanisms are involved, and more than one signal transduction pathway may be involved in the responses to sweetness or sourness of different compounds. Some compounds may activate more than one type of receptor.
In addition to the sensations of taste provided by the taste-buds on the tongue, a great many flavours can be distinguished by the sense of smell. Again some flavours and aromas (fruity flavours, fresh coffee and, at least to a non-vegetarian, the smell of roasting meat) are pleasurable, tempting people to eat and stimulating appetite. Other flavours and aromas are repulsive, warning us not to eat the food. Again this can be seen as a warning of possible danger — the smell of decaying meat or fish tells us that it is not safe to eat.
Like the acquisition of a taste for bitter or sour foods, a taste for foods with what would seem at first to be an unpleasant aroma or flavour can also be acquired. Here things become more complex — a pleasant smell to one person may be repulsive to another. Some people enjoy the smell of cooked cabbage and sprouts, whereas others can hardly bear to be in the same room. The durian fruit is a highly prized delicacy in South-East Asia, yet to the uninitiated it smells of sewage or faeces — hardly an appetizing aroma.
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