Louis Camille Maillard (1878-1936) has described the major routes to non-enzymatic browning and flavour in heated food. Today, the Maillard reaction, sometimes described as exogenous glycation, is still subject of research by food scientists who are trying to identify the compounds that provide the flavour and colour characteristics of heated food. It is not a single reaction, but a complex pathway in which arnino compounds (especially free amino acids) and reducing sugars (in the case of potatoes especially glucose and fructose) are involved. The reaction is subdivided into three major stages. The early stage is initiated by the condensation of a carbonyl group with an amino compound producing a Schiff base. This cyclises to an aldosylamine if thesugar is an aldose (e.g., glucose). An acid-catalysed rearrangement forms an Amadori product. Otherwise, ketoses (e.g., fructose) generate a Heyns product. Both products are most unstable above ambient temperatures. Further reaction pathways give an increase in a collection of sugar dehydration and fragmentation products, m this advanced stage the amino acid also undergoes deamination and decarboxylation through a so-called Strecker degradation. This reaction is important for flavour generation, to provide routes by through which nitrogen and sulphur can be introduced into heterocyclic, compounds in the final stage of the Maillard reaction. The products of the early and advanced stage are colourless or pale yellow. Further condensation reactions between carbonyls and amines within the final stage give high molecular mass, coloured products known as melanoidins (advanced glycation end products, AGEs), containing heterocyclic ring systems such as pyrroles, pyridines, pyrazines and imidazoles.
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