Applications and Occurrences of Heterocycles in Everyday Life 189It is not only sugars that are the basis for heterocycle production, hot cooking oils also generate decomposition prod-
ucts that can react with amino acids. An analysis of the volatile products from heating peanut oil with cysteine showed
many thiazoles and thiophenes, together with a few pyridines and pyrroles.
Cysteine is the main source of sulfur for many of these products. Its decomposition to produce hydrogen sulfi de can be
explained by the decarboxylation of its condensation product with an -dicarbonyl compound, and it is also possible
that the sulfur could already be attached to another reactant before cleavage.
A number of other fl avour components are also the results of Maillard-type reactions, such as the baked bread aroma
acetyl pyrroline, although in this case, the product retains the ‘side-chain’ from the starting proline.
More complex heterocyclic compounds can also be produced (particularly on prolonged cooking at high tempera-
tures), including the potentially carcinogenic (and vaguely named) ‘heterocyclic amines’, such as PhIP.
Physiological aspects of the Maillard reaction
In addition to reactions during cooking, Maillard-type reactions can also occur after food is eaten, that is, in the body.
In this situation, reactions involving two basic amino acids (those with an additional -NH 2 : lysine and arginine) are
particularly important.
In addition to the normal Maillard reaction involving the -amino group, with decarboxylation and loss of the side-
chain, reaction at the -amino group retains the side-chain and further transformations lead to another group of het-
erocycles, known as Advanced Glycation End products (AGEs). These end products can be free small molecules, or the
equivalent products involving lysine or arginine residues in proteins, leading to cross-linking or other modifi cations,
thus damaging the physiological function of the protein. These occur normally and are not usually a problem, but
excessive formation has been implicated in a number of diseases such as cataracts and the serious sequelae of diabetes,
possibly related to the excess (blood) sugar and oxidative stress. The quantity of AGEs has been shown to correlate with
the seriousness of the disease. They are also thought to have a general role in aging.