Discovery of Polyhydroxyalkyl Pyrazine Generation upon Coffee Roasting by In-Bean Labeling Experiments

Area of Science:

• Food Chemistry
• Organic Chemistry
• Analytical Chemistry

Background:

• Coffee roasting involves complex chemical reactions, including the Maillard reaction.
• Understanding non-volatile products is crucial for coffee flavor and aroma.
• Free amino acids and sugars are key precursors in coffee processing.

Purpose of the Study:

• To investigate non-volatile reaction products from free amino acids during early coffee roasting.
• To identify and characterize novel pyrazine compounds formed in situ.
• To elucidate the precursor origins (amino acids and sugars) of these pyrazines.

Main Methods:

• Biomimetic in-bean experiments with labeled and unlabeled amino acids and sucrose.
• Comprehensive untargeted screening using ultra-high performance liquid chromatography-electrospray-ionization-quadrupole time-of-flight-tandem mass spectrometry (UHPLC-ESI-QToF-MS) in data-independent acquisition (DIA) mode.
• Orthogonal partial least-squares discriminant analysis (OPLS-DA) for data modeling.
• Carbon module labeling (CAMOLA) approach for structural verification.

Main Results:

• Identification of 11 pyrazine structures previously unknown in coffee, with two (compounds 1 and 2) being most prominent.
• Discovery of two pyrazines (compounds 5 and 10) not previously identified in any food matrix.
• Confirmation that sucrose is the primary source of the carbon backbone for identified pyrazines.
• Quantification of nitrogen contribution from free amino acids (33-55%) to the pyrazines.

Conclusions:

• Plausible formation pathways for the identified pyrazines were proposed, consistent with Maillard reaction mechanisms.
• Sucrose plays a dominant role in pyrazine formation during early coffee roasting.
• Free amino acids are a significant, but not exclusive, source of nitrogen for pyrazines.