Characterization of the Pyrazines Formed during the Processing of Maple Syrup

Pyrazine formation in maple syrup was investigated during the boiling of maple sap at 105 °C for 220 min. In general terms, there was an induction period, characteristic of the type of pyrazine, associated with the formation of all identified pyrazines. No pyrazine was detected before 60 min of heating at 105 °C; 2,5-dimethyl- and trimethylpyrazine were formed after 60 min of heating, whereas methyl-, 2,6-dimethyl-, ethyl-, 2,3-dimethyl-, and 2-ethyl-3-methylpyrazine were detected after 120 min of heating. The total level of pyrazines increased from 3.42 ng/g after 60 min of heating to 72.32 ng/g in the final syrup. The formation rate constants (0.04−0.13 ng of pyrazines/min) were determined from the slopes of plots of concentrations versus time of heating. These plots were consistent with pseudo-zero- order reactions. The formation of these pyrazines was influenced by the heating time and by the pH of the boiling sap. The pH values of the sap increased from 7.2 to 9.2 during the first 40 min of boiling, then decreased to 7.3; the decrease in pH values was associated with an increase in the total soluble solids, mainly sugars, from 3% in the sap to 65% in the syrup. Consequently, the levels of sucrose, glucose, and fructose increased from 23.21, 0.09, and 0.09 mg/g, respectively, in the sap to 416.97, 3.25, and 1.82 mg/g in the syrup.