 | Saccharin - the hydrogen which is important to the sweet taste is highlighted in red. |
Saccharin is familiar to many of us as the non-fattening sweetener that is used in place of sugar (or more precisely, sucrose) in many products. It was discovered in 1879 and went into commercial production in 1900. It is approximately 300-450 times sweeter than sugar but some people complain it has a bitter and metallic aftertaste.
| Saccharin - the hydrogen which is important to the sweet taste is highlighted in red. |
Saccharin is not very soluble in water, and so it is most commonly used in the form of its sodium or calcium salt. The reason why it tastes sweet is still unclear, but its shape must be correct to fit into specific receptors in the taste buds. Evidence for this comes from the fact that if the shape is modified slightly, say by changing the H on the nitrogen to a methyl, the new molecule no longer tastes sweet. It is also possible that the specific taste receptors it targets are peculiar to humans, since bees or butterflies, which normally crave the sweetness of nectar, do not treat it as a desirable substance.
There is some question as to the safety of saccharin because in some tests rats have developed cancer of the bladder when fed large quantities of this compound. However, other tests failed to confirm this finding. Moreover, unlike most other carcinogens, saccharin is not metabolised by rats or humans, and normally passes straight through the body unchanged. Rats, however, concentrate their urine far more than humans do, and so the waste products (including excreted saccharin) remains in their bladder for a very long time.
Because of the suspicions about the safety of saccharin, other non-fattening sweeteners have been developed. One of these is cyclamate, although, ironically, it too was banned from most countries in 1969 because of suspicions it was carcinogenic. The sweet character of cyclamate (it is about 30 times sweeter than sucrose) was discovered by accident in 1937 when a careless chemist smoked a cigarette that had absorbed some of it. The molecule resembles glucose slightly, in that it is composed of a cyclohexane ring, with polar groups attached to make it water soluble. In glucose, the polar groups are all -OH, whereas in this molecule it is the amine (-NH-) and the sulphite (SO3-) groups.
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| Cyclamate the polar sidechain is highlighted in red. | Aspartame The part derived from phenylalanine is shown in blue, and that from aspartic acid is shown in red. |
Another artificial sweetener is aspartame. It, too, was discovered (in 1965) by accident when a careless chemist licked his fingers after working on the compound. The aspartame molecule is a dipeptide, and is a combination of two naturally occurring amino acids. One of these is the almost tasteless aspartic acid and the other is the bitter-tasting phenylanaline - so it is a bit surprising that the combination of the two is sweet. In fact it is around 150 times sweeter than sucrose, and this means that less has to be incorporated into food to obtain the same degree of sweetness - which makes it less fattening. The reason for its sweetness is not properly established, but it is interesting to note that, like glucose and other sugars, it contains a ring structure and numerous polar groups (both -OH and -NH). Unlike the other artificial sweeteners mentioned above, so far tests have shown it to be safe, however, there are a number of drawbacks to its use. First, because it contains a peptide link (as do proteins), it decomposes on heating, and so cannot be used in foods that must be cooked. It also decomposes slowly in liquids, which means that drinks (lemonades and colas, for example) that contain it have a limited shelf life. However, for some reason a mixture of aspartame and saccharin is both sweeter, and more stable, than either substance on its own. Pure aspartame has the advantage though, that it does not have the metallic aftertaste of saccharin, and some people even prefer its taste to that of sucrose.