Model melanoidins, pigments formed by nonenzymic browning reactions of amino acids with reducing sugars and glyoxal, behaved like anionic polymers that form moderately stable complexes with Ca*+ and, if incorporated into diets, could possibly influence calcium bioavailability. Hydrogen ions were released during complexation. The influence of pH on degree of binding by soluble and insoluble forms of mode1 melanoidins was determined. Pigments in coffee brew and toasted bread (milk-free) had no measurable calcium-binding ability, which suggested that there might be important structural differences between pigments formed in food systems and those formed from the interaction of simple amino acids with sugars. INTRODUCTIONNONENZYMIC BROWNING REACTIONS are of considerable importance to food chemistry and have been the subject of much research for many years (Nursten, 1980-81;Hodge, 1953). In foods the reaction of reducing sugars with proteins and other compounds containing free amino groups produces not only brown polymeric pigments (melanoidins) but also compounds that contribute to aroma and flavor. These products may be either desirable or undesirable, depending upon the food system in question. Browning reactions that incorporate proteins into the molecular structure of pigments can cause undesirable decreases in the nutritional availability of those proteins. This decrease is probably the result of a lowering of the ability of digestive enzymes to hydrolyze the protein, as the protein becomes chemically modified during the course of browning (Folk, 1956; Tu and Eskin, 1973).Although no published reports were found on the possible influence of browning on mineral bioavailability, observations by several investigators strongly indicate that melanoidins might behave as chelating agents for polyvalent metal cations and, therefore, influence the bioavailability of these metals. Hrdlicka (1976) reported that when salts of either Fe3+ or Cu2+ were heated with a glucose-glycine mixture or a fructose-glycine mixture, water-insoluble browning products that contained bound metal cations were formed. Copper was more strongly bound than iron. Adhikari and Tappel (1973) have provided flurometric evidence that Europium (Eu3+) is chelated by a water-soluble glucose-glytine melanoidin, and suggested that the chromophoric group responsible for the fluorescence is

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