188 Applications and Occurrences of Heterocycles in Everyday Life
The key to the conversions of these open sugars is the equilibrium of -hydroxy-carbonyl compounds with ene-
1,2-diols. The ene-diols can then form the reverse -hydroxy-carbonyl system by tautomerism, or initiate further
reactions, such as loss of adjacent hydroxyls, resulting in formation of -dicarbonyl (1,2-dicarbonyl) compounds.
These, in turn, can then undergo cyclisation giving furans, but are also, together with smaller fragmentation products,
important in reactions with amino acids in the Maillard reaction (see below).
(Commercial caramel is made by heating sugars in the presence of a catalyst (acid or sulfi te), with or without ammonia,
those with ammonia showing darker colours. Being a type of product formed by normal cooking processes, it is con-
sidered a natural substance and therefore can be added to various food and drinks. It is by far the most common food
colour additive. The ‘ammoniated’ dark caramels are used especially for beers and cola-type drinks.)
Maillard reaction
The Maillard reaction can be considered as an extension of caramelisation. This reaction occurs when sugars and
amino compounds are heated together and results in a much more rapid and intense browning than simple cara-
melisation. The basic mechanism (and often the outcome) is similar to that shown above except that an amino-enol
intermediate, rather than an ene-diol is involved. Alternatively, later loss of the amino group (rather than an internal
hydroxy) results in formation of a methyl ketone and thence to compounds such as maltol and isomaltol.
When the amino compound is an -amino acid (an extremely common occurrence!), the process results in the cleav-
age of the amino acid side-chain and formation of a primary amino sugar. This type of reaction can also occur with
the smaller dicarbonyl fragments. Self-condensations of the amino-ketones lead to pyrazines: non-volatile (such as
2,6-Deoxyfructosazine) from sugars, and volatile, from the smaller fragments. (Note: 2,6-Deoxyfructosazine can be
prepared quite effi ciently by heating glucose with ammonium salts in water. It has been shown to produce volatile
simple alkyl pyrazines on heating above 220 °C.)