Free Penny le Couteur & Jay Burreson by Napoleon's Buttons: How 17 Molecules Changed History Page A

all these sugars so appealing is that they taste sweet, and humans like sweetness. Sweetness is one of the four principal tastes; the other three are sourness, bitterness, and saltiness. Achieving the ability to distinguish among these tastes was an important evolutionary step. Sweetness generally implies “good to eat.” A sweet taste indicates that fruit is ripe, whereas sour tells us there are still lots of acids present, and the unripe fruit may cause a stomachache. A bitter taste in plants often indicates the presence of a type of compound known as an alkaloid. Alkaloids are often poisonous, sometimes in only very small amounts, so the ability to detect traces of an alkaloid is a distinct advantage. It has even been suggested that the extinction of the dinosaurs might have been due to their inability to detect the poisonous alkaloids found in some of the flowering plants that evolved toward the end of the Cretaceous period, about the time the dinosaurs disappeared, although this is not the generally accepted theory of dinosaur extinction.
    Humans do not seem to have an inborn liking for bitterness. In fact, their preference is probably just the opposite. Bitterness invokes a response involving secretion of extra saliva. This is a useful reaction to something poisonous in the mouth, allowing one to spit it out as completely as possible. Many people do, however, learn to appreciate, if not like, the bitter taste. Caffeine in tea and coffee and quinine in tonic water are examples of this phenomenon, although many of us still rely on having sugar in these drinks. The term bittersweet, connoting pleasure mixed with sadness, conveys our ambivalence about bitter tastes.
    Our sense of taste is located in the taste buds, specialized groups of cells found mainly on the tongue. Not all parts of the tongue detect taste the same way or to the same degree. The front tip of the tongue is the most sensitive to sweetness, while sourness is detected most strongly on the sides of the tongue toward the back. You can test this easily for yourself by touching a sugar solution to the side of the tongue and then to the tip of the tongue. The tip of the tongue will definitely detect the sweet sensation more strongly. If you try the same thing with lemon juice, the result will be even more obvious. Lemon juice on the very tip of the tongue does not seem very sour, but put a freshly cut slice of lemon on the side of the tongue, and you will discover where the sourness reception area is the strongest. You can continue this experiment: bitterness is detected most strongly on the middle of the tongue but back from the tip, and the salty sensation is greatest just to each side of the tip.
    Sweetness has been investigated far more than any of the other tastes, no doubt because, as in the days of the slave trade, it is still big business. The relationship between chemical structure and sweetness is complicated. One simple model, known as the A-H,B Model, suggests that a sweet taste depends on an arrangement of a group of atoms within a molecule. These atoms (A and B in the diagram) have a particular geometry, allowing atom B to be attracted to the hydrogen atom attached to atom A. This results in the short-term binding of the sweet molecule to a protein molecule of a taste receptor, causing a generation of a signal (transmitted through nerves) informing the brain, “This is sweet.” A and B are usually oxygen or nitrogen atoms, although one of them may also be a sulfur atom.

    The A-H,B Model of Sweetness
    There are many sweet compounds other than sugar, and not all of them are good to eat. Ethylene glycol, for example, is the major component of antifreeze used in car radiators. The solubility and flexibility of the ethylene glycol molecule, as well as the distance between its oxygen atoms (similar to the distance between oxygen atoms in sugars), account for its sweet taste. But it is very poisonous. A dose of as little as one tablespoon