MELANOIDINS FROM GLUCOSE AND GLYCINE - COMPOSITION, CHARACTERISTICS AND REACTIVITY TOWARDS SULFITE ION

Melanoidins with M(r) > 12000 were prepared by reaction of glucose with glycine (molar ratios 1:9 to 9:1) at 55-degrees and 90-degrees-C, initial pH 5.5. Extinction coefficients (E1%) at 450 nm were 36.9 +/- 6.9 and 40.5 +/- 2.9 at the two temperatures, respectively, and do not depend on reaction time or the molar ratio of glucose to glycine used to prepare the polymer. All the melanoidins had similar degrees of dehydration (c. 3 mol H2O/mol glucose) and stoichiometry with respect to glucose and glycine, but the degree of decarboxylation of glycine varied in the range 0-38% of residues incorporated into the polymer. The melanoidin prepared when [glucose] > [glycine] was more reactive towards sulphite species than when [glucosel < [glycine], and the extent of bleaching of melanoidin at 450 nm was proportional to the number of sulphur atoms incorporated into the polymer. These observations are related to the nature of the chromophores in melanoidins and the mechanism of polymerisation.