The Maillard reaction between reducing sugars and free amino acids and proteins occurs readily during the preparation and storage of foods. This reaction, which evolves through several steps, leads to irreversible modification of the molecules involved. The subsequent nutritional and physiologic effects are due to both the loss of the availability of essential amino acids and to the presence of the new molecules formed.

In food proteins, the reaction involving lysine is the most important nutritionally. The deoxyketosyl-lysine molecule formed in the “early” stage of the reaction is not available biologically. In the “advanced” stage of the reaction, which produces browning, the digestibility of the protein is decreased and other amino acids are destroyed.

The metabolic transit and the nutritional and physiologic effects of the different groups of substances produced are described. The deoxyketosyl derivatives are partly absorbed and excreted in the urine without biochemical modification. The premelanoidins also are partly absorbed but are metabolized in the organism; the most representative groups of derivatives are biochemically transformed into acids (hydroxymethyl furfural, reductones) or glucurono or sulfo conjugates (pyrazines, maltol) before excretion in the urine; they have antiphysiologic effects. The melanoidins (molecules of high molecular weight) are not absorbed and their effects are negligible. The possible occurrence of Maillard-type reactions in living organisms is also discussed.

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