The major non-volatile reaction products
formed from free amino
acids during the early stage of coffee roasting were investigated
using biomimetic in-bean experiments with labeled and unlabeled free
amino acids. Comprehensive untargeted screening by ultra-high performance
liquid chromatography-electrospray-ionization-quadrupole time-of-flight-tandem
mass spectrometry (UHPLC-ESI-QToF-MS) in data-independent acquisition
(DIA) mode was carried out and modeling by orthogonal partial least-squares
discriminant analysis (OPLS-DA) helped in revealing 11 pyrazine structures
identified in coffee for the first time. 2-(2′,3′,4′-Trihydroxybutyl)-(5/6)-methyl-pyrazine
(1) and 2,(5/6)-bis(2′,3′,4′-trihydroxybutyl)-pyrazine
(2) were the most prominent compounds, while 2-(3′,4′-dihydroxybutyl)-(5/6)-methyl-pyrazine
(5) and 2-(2′,3′,4′-trihydroxybutyl)-(5/6)-(2′-hydroxyethyl)-pyrazine
(10) were not even previously identified in other food
matrices. The structures could be verified by means of additional
biomimetic in-bean experiments with labeled sucrose leveraging the
carbon module labeling (CAMOLA) approach. Based on these results,
plausible formation pathways could be formulated fitting into the
known Maillard reaction mechanisms. Sucrose was highlighted as the
predominant precursor of the carbon backbone of all identified pyrazines
butonly 33–55% of the nitrogen atoms originated from free amino
acids.
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