Bitter-Sweet
Sweetness
Sweetness is produced by the presence of sugars, some proteins and a few other substances. Sweetness is often connected to aldehydes and ketones which contain carbonyl group. Sweetness is detected by a variety of G protein coupled receptors coupled to the G protein gustducin found on the taste buds. At least two different variants of the "sweetness receptors" need to be activated for the brain to register sweetness. The compounds which the brain senses as sweet are thus compounds that can bind with varying bond strength to several different sweetness receptors. The differences between the different sweetness receptors is mainly in the binding site of the G protein coupled receptors. The average human detection threshold for sucrose is 10 millimoles per litre. For lactose it is 30 millimoles per liter, and 5-Nitro-2-propoxyaniline 0.002 millimoles per litre.
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Bitterness
The bitter taste is perceived by many to be unpleasant, sharp, or disagreeable. Evolutionary biologists have suggested that a distaste for bitter substances may have evolved as a defense mechanism against accidental poisoning. However, not all bitter substances are harmful; coffee and tonic water are both popular, bitter beverages. Other common bitter foods include bitter melon, arugula, watercress, and dandelion. Quinine, the anti-malarial prophylactic, is also known for its bitter taste and is found in tonic water.
The bitterest substance known is the synthetic chemical denatonium, discovered in 1958 and today marketed as the trademarked Bitrex [3]. It is a white, odourless solid used as an aversive agent that is often added to toxic substances to prevent accidental ingestion.
Research has shown that TAS2Rs (taste receptors, type 2) coupled to the G protein gustducin are responsible for the human ability to taste bitter substances. They are identified not only by their ability to taste for certain "bitter" ligands, but also by the morphology of the receptor itself (surface bound, monomeric)[3]. Researchers use two synthetic substances, phenylthiocarbamide (PTC) and 6-n-propylthiouracil (PROP) to study the genetics of bitter perception. These two substances taste bitter to some people, but are virtually tasteless to others. Among the tasters, some are so-called "supertasters" to whom PTC and PROP are extremely bitter. This genetic variation in the ability to taste a substance has been a source of great interest to those who study genetics. In addition, it is of interest to those who study evolution since PTC-tasting is associated with the ability to taste numerous natural bitter compounds, a large number of which are known to be toxic.
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