Disclosing the Nitrogen Sources via Isotope Labeling\nTechnique and the Formation Mechanism of Pyrazine and Alkylpyrazines\nduring the Heat Treatment of <i>N</i>‑(1-Deoxy‑d‑xylulos-1-yl)-alanine and Exogenous Alanine

The\nformation pathway and mechanism of various pyrazines were investigated\nduring the thermal treatment of the alanine-xylose Amadori compound\n(Ala-ARP) and exogenous alanine (Ala). <sup>15</sup><i>N</i>-labeled Ala was used to coheated with Ala-ARP to clarify the nitrogen\nsources and the respective contributions of exogenous Ala and the\nregenerated Ala released from Ala-ARP to different pyrazine formation.\nIt was found that exogenous Ala exhibited a priority in capturing\nglyoxal (GO) to form pyrazine during the thermal degradation of ARP.\nCompared to the Ala-methylglyoxal (MGO) model, a lower activation\nenergy was required for the Ala-GO reaction, where the reaction dynamics\nof Ala-GO followed a zero-order model. In addition to forming pyrazine,\nthe interaction between existing exogenous Ala and GO would accelerate\nthe thermal degradation of Ala-ARP and retro-aldolization reaction\nof deoxyxylosones (DXs) to α-dicarbonyls. During this process,\nthe release of regenerated Ala and MGO was promoted. Accordingly,\nas GO was expended by exogenous Ala during the initial stage of ARP-Ala\ndegradation, the condensation between regenerated Ala and MGO became\nintensified, leading to the generation of methylpyrazine and 2,5-dimethylpyrazine.\nAs a result, in the thermally treated mixture of Ala-ARP and exogenous\nAla, 55% of the formed pyrazine originated from exogenous Ala, while\n63% of the formed methylpyrazine and 57% of the formed 2,5-dimethylpyrazine\nwere derived from regenerated Ala (120 °C, 30 min).