As the MR involves some of the most important food nutrients, its nutritional consequences must be carefully considered. Researcher attention has previously been focused mainly on milk and milk products, where thermal treatments are necessary for obtaining microbial stabilisation and the preservation of high nutritional quality. Vegetable products, which become edible only after thermal treatments, have been relatively neglected so far.
The degradation of sugars per se is never considered a problem because it is only rarely they are lacking in diet. However, free or protein-bound essential amino acids may be damaged irreversibly; the amount of free amino acids in food is always very low and they are important as constituents of proteins. This means that the most relevant nutritional effect of the Maillard reaction is non-enzymatic glycosylation of proteins which involves mostly lysine, whose bioavailability may be drastically impaired. This should be distinguished very clearly from enzymatic glycosylation, a normal step in the biosynthesis of glycoproteins, in which oligosaccharides are bound to serine or asparagine through a glycosidic bond.
The first glycation products are then converted to the Amadori product, fruc-tosyllysine, that eventually can cross-link with other amino groups intramolecu-larly or intermolecularly. The resulting polymeric aggregates are called advanced glycation end products (AGEs).
Lysine availability is an important nutritional parameter especially in foods for particular classes of consumers, such as infant formulas (Ferrer et al, 2000). Statistically significant losses of available lysine (about 20%) with respect to raw milk have been reported as a consequence of the thermal treatment applied in the preparation of these foods.
Because the reactions of lysine are so relevant in nutrition, over a period of time different MRPs have been proposed as markers of protein glycosylation.
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