We get calories from three categories of macronutrients: Carbohydrates (which include sugar and starch), Proteins and Fats. To consider how these might be detected, we must distinguish between taste and smell.
When food or beverages enter the mouth, they contact gustatory receptors on the tongue and palate. The sensations produced are sweet, salty, sour, or bitter. At the same time, volatiles from the foods and beverages rise through the oral and nasal cavities and ultimately reach the olfactory receptors located just under the eyes. The many qualitatively distinct olfactory sensations that can be produced are responsible for much of the sensory experience of eating. That is, while we eat, we both taste and smell foods. We call the composite sensation "flavor", and we perceptually localize it in the mouth.
We believe that this localization is produced by the sense of touch. Taste sensations are not localized to the location of taste buds, but rather to areas touched in the mouth. Thus, during drinking and eating, taste sensations seem to originate from the entire inner surface of the mouth even though the taste buds are found only on certain loci. This occurs because the brain uses the sense of touch to localize taste sensations.
MODALITIES THAT SENSE FOOD
Fox's discovery stimulated geneticists to evaluate families for the distribution of 'taste blindness' and the results of these and later studies led to the conclusion that tasting is produced by the dominant allele, T. [An allele is any of the group of genes from which a pair of genes occupying identical places on homologous chromosomes can be drawn]
Individuals with two recessive alleles, tt, are nontasters (the nontaster functions are lower at the lowest concentrations) and individuals with one dominant allele, Tt, as well as those with two dominant alleles, TT are tasters. There is evidence for three phenotypic groups in the threshold data. Nontasters were a very homogeneous group but tasters showed a great deal of variability.
WHAT MAKES SUPERTASTERS DIFFERENT FROM MEDIUM AND NON-TASTERS?
They found that tasters had more taste buds than nontasters. In addition, they found that subjects with more taste buds perceived stronger tastes.
Since there are pain fibres associated with taste buds, supertasters are unusually responsive to the oral burn of spices. A recent extension of this work showed that supertasters have the largest number of taste buds, nontasters the smallest.
The differences in number of receptors are very large. For example, the average number of taste buds per square centimeter was 96, 184 and 425 for nontasters, medium tasters and supertasters, respectively. The supertasters' fungiform papillae were smaller and had rings of tissue around them that were not seen on the fungiform papillae of nontasters. These anatomical differences may prove to be a better indicator of genetic status than the taste differences.
SUPERTASTERS AND ETHYL ALCOHOL
THE TONGUE MAP
The tongue map with 'sweet' on the tip, 'bitter' on the back and so on dates back to the PhD thesis of Hänig which was published in Philosophische Studien in 1901.
He believed that if the thresholds for his four stimuli (sweet, acid, salt and bitter) could be shown to vary differentially around the perimeter of the tongue, then this would support the argument that these four tastes had distinct physiological mechanisms.
Hänig noted that the sensitivity for sweet was at its maximum on the tongue tip and its minimum on the base of the tongue. For bitter, the sensitivity was at its maximum on the base of the tongue and its minimum on the tip. Saltiness was perceived approximately equally on all loci. For sourness, the sensitivity was at its minimum on the tip and the base with two equidistant maxima at the centers of the tongue edges.
Edwin Boring, the great historian of psychology at Harvard, discussed Hänig's thesis in Sensation and Perception in the History of Experimental Psychology published in 1942.
Boring did not reproduce Hänig's summary sketch but rather calculated the actual sensitivities by taking the reciprocals of the average thresholds given in Hänig's tables.
On Boring's figure, there is no way to tell how meaningful the sizes of the variations are on the ordinate.
Boring's graph led other authors to conclude that there was virtually no sensation at the loci where the curves showed a minimum and that there was maximum sensation where the curves showed a maximum and so we have the familiar tongue maps labeled 'sweet' on the tip of the tongue, 'bitter' on the base of the tongue, etc.
Collings reexamined the threshold variation in 1974. Her results differed from those of Hänig in some regards (e.g. bitter thresholds are actually lower on the front of the tongue than on the back); however, in one very important particular Collings and Hänig agreed: there were variations in taste threshold around the perimeter of the tongue but those variations were small.
EFFECTS OF TEMPERATURE ON THE PERCEIVED SWEETNESS OF SUCROSE
On the other hand, the sweetness of a lower sucrose concentration like the sucrose equivalent of 2 teaspoons of sugar in a cup of coffee increases by 92 % (i.e., the sweetness nearly doubles) as the temperature increases from 4°C to 36°C.
WHY DO WE LOVE SWEETNESS?
We can even examine the human's reactions to sweet taste at an earlier point in development. De Snoo (1937) was intrigued by the fact that the fetus drinks amniotic fluid.
He succeeded in getting the fetus to drink more amniotic fluid by injecting saccharin into it. This remarkable feat demonstrated that taste receptors function before birth, a fact that has now been extensively studied in other species. It also demonstrated that sweetness is liked before birth.
Did our sweet systems evolve to ensure that those sugars that are useful to us produce intense sweet tastes, while those that are not are less sweet?
The sugar molecule that is most important biologically is glucose. This molecule serves an important energy source in the body and is the only energy source that can be utilized by the brain.
Decant wines, both young and old. It is a sign of respect for old
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