Elucidation of selected Maillard reaction pathways in alanine and phenylalanine model systems through isotope labelling and pyrolysis-GC/MS based techniques

Alanine and phenylalanine based model systems were utilized in this thesis to elucidate selected Maillard reaction pathways through isotope labelling, Fourier Transform Infrared Spectroscopy (FTIR) and Pyrolysis-Gas Chromatography/Mass Spectrometry (Py-GC/MS) based techniques. The formation of glycosylamines from reducing sugars and amino acids is a well-known process in the initial phase of the Maillard reaction. They play a critical role in both the initiation and propagation stages, however, little attention has been paid so far on the ability of these imines to undergo isomerization and thus contribute to the diversity of Maillard reaction products. In this study, imine isomerization through 5-oxazolidinone formation was explored in phenylalanine and alanine sugar models systems. Spectroscopic evidence was provided for its formation by taking advantage of the strong carbonyl absorption band centered at 1784 cm-1 in the phenylalanine/glyceraldehyde and at 1778 cm -1 in phenylalanine/glycolaldehyde model system. The importance of 5-oxazolidinone formation lies in its ability to decarboxylate to azomethine ylide and subsequently form two isomeric imines, each capable of producing distinct Maillard reaction products. Evidence for the formation of such ylides was also provided through their ability to undergo 1,3-dipolar cycloaddition with dipolarophiles. Regarding the role of oxygen in the Maillard reaction, it was found that molecular oxygen can influence carbon-carbon bond cleavage through the formation and degradation of 1,2-dioxetane moieties generated from enol structures abundantly formed in the Maillard reaction from their corresponding ketones and aldehydes such as phenylacetaldehyde the Strecker aldehyde of phenylalanine and subsequently can be oxidized into benzaldehyde. Furthermore, the alpha-dicarbonyl compounds generated during the Maillard reaction play a significant role as precursors of important flavour-active heterocyclic compounds. The origin of many such alpha-dicarbonyl compounds still remains unknown. Using glucose and glyoxal with labelled [13 C]- and [15N]-alanine, the mechanism of formation of 1,2-butanedione and 3,4-hexanedione were confirmed and proposed to proceed through amino acid-assisted chain elongation pathway. In addition, the role of alpha-dicarbonyl compounds in the formation of various heterocyclic compounds such as pyrazines, pyrazinones and imidazolidinones were also illustrated. The thesis further demonstrates the utility of pyrolysis-GC/MS as a powerful analytical tool especially if used in conjunction with isotope labelling techniques.