The aroma side of the Maillard reaction

The Maillard reaction in food produces, among others, a diversity of sensory-active compounds (aroma, taste, color). The resulting key aroma compounds are often present only in trace concentrations of 1 microg/kg to 1 mg/kg. Nevertheless, they contribute to the respective flavor because of their low odor-perception thresholds. While Maillard intermediates, such as Amadori compounds and deoxyosones, are formed at percentage levels during model reactions, the yield of aroma compounds, in particular nitrogen and sulfur-containing ones, is often as low as 0.001-0.01 mol%, thus indicating their formation through chemical side reactions. The elucidation of the relevant precursors in food and the identification of previously unknown intermediates can throw light on these minor pathways. Also, model reactions with isotopically labeled precursors are of great value in gaining insight into the relevant formation mechanisms. Several examples of these studies are illustrated including work to elucidate the role of the solvent glycerol in the formation of pyrazines, trials to reveal the relative significance of 4-hydroxy-5-methyl-3(2H)-furanone as intermediate in the reaction between ribose and cysteine, and experiments to assess the proportional contribution of the precursors cysteine, xylose, and thiamine to the formation of the resulting aroma compounds in the thermal reaction.