Applications and Occurrences of Heterocycles in Everyday Life 187The biosynthesis of these ‘raw’ pyrazines probably involves condensation of -amino acid amides with 1,2-dicarbonyl
compounds, such as glyoxal, and/or are the products of dimerisation of -amino acids. The amino acid side-chains are
retained, unlike in the products of the Maillard reaction (see page 188).
Heterocyclic chemistry of cooking
The heterocycles produced during cooking include more pyrazines and also many furans and a range of sulfur-
containing compounds.
The details of chemical processes in cooking are very complex but the examples below will give an idea of the general
types of reaction.
The main components of food, apart from water, are carbohydrates, proteins, amino acids and fats, and all of these can
undergo considerable chemical change on heating. Some of the products may be formed from one of the class types
and others from combinations. Analytically, raw meat does not contain carbohydrates, but they can be formed from
glycogen and nucleotides, or they can be added in marinades and so on. The amino acid cysteine is the major source
of sulfur.
The most signifi cant processes are those that occur during baking or frying. Normal frying and baking temperatures
vary between about 170 and 250 °C, but, of course, this mainly affects the outer layer (of the food), with the inner
part being much cooler. The crust is where the interesting chemistry occurs! Two- to four-carbon -dicarbonyl or
-hydroxy-carbonyl compounds, which are important in the Maillard reaction (see page 188), can arise from the
breakdown of sugars or fats. These fragmentations can also occur slowly in water and can be found in unexpected
situations, such as heat sterilisation of glucose solutions for intravenous administrations, but fortunately only at low
levels (0.1%).
Caramelisation
When sugars are heated they turn to caramel, which shows shades of gold–brown due to the formation of polymeric
material of uncertain structure. In addition, various volatile monomeric aroma molecules are produced, including
hydroxymethylfurfural (HMF), and various furanones, and also a number of chain-cleavage products, such as those
shown above. The formation of the oxygen heterocycles is easily rationalized by invoking the open chain form of the
sugars, with reference to the standard ring syntheses of furans (page 105), via cyclocondensations of 1,4-diketones or
4-hydroxy-ketones.