Role of the Solvent Glycerol in the Maillard Reaction of d-Fructose and l-Alanine

The volatiles in the headspace above a solution of [13C6]fructose and alanine in glycerol/water, heated in a closed vial at 130 °C for 2 h, were analyzed by solid-phase microextraction in tandem with GC−MS. Carbonyl compounds and pyrazines were among the detected components. The examination of their mass spectra showed that most of the 1-hydroxy-2-propanone and 2,3-pentanedione were 13C3-labeled, the majority of the 2-methylpyrazine and 2-ethyl-3-methylpyrazine were 13C5-labeled, and 2,5-dimethylpyrazine and 3-ethyl-2,5-dimethylpyrazine were mainly 13C6-labeled. This is in agreement with the literature, and corresponds to the incorporation of fructose carbons, and in the case of 2,3-pentanedione, 2-ethyl-3-methylpyrazine, and 3-ethyl-2,5-dimethylpyrazine alanine carbons, into the molecules. However, minority fractions of 1-hydroxy-2-propanone (10%) and 2,3-pentanedione (14%) were found unlabeled, 2-methylpyrazine (10%) and 2-ethyl-3-methylpyrazine (11%) only doubly labeled, and 2,5-dimethylpyrazine (20%) and 3-ethyl-2,5-dimethylpyrazine (27%) only triply labeled, suggesting they contain carbons originating from the solvent glycerol. This could be confirmed by reaction of fructose and alanine in [13C3]glycerol/water, which produced the same volatiles, with 11−27% existent in their 13C3-labeled form. Hence, glycerol participated not only as a solvent but also as a precursor in the reaction. Keywords: Maillard reaction; glycerol; fructose; alanine; solid-phase microextraction; mass spectrometry; 13C-labeling