Detection of Odor-Active Ethenylalkylpyrazines in Roasted Coffee

In roasted Robusta coffee, two potent earthy smelling compounds were identified as 2-ethenyl-3,5-dimethylpyrazine and 2-ethenyl-3-ethyl-5-methylpyrazine by comparison of gas chromatography and mass spectrometry data with those of the corresponding reference substances. The odor threshold values (0.014 ng/L, air) of the new pyrazines were as low as those of 2-ethyl-3,5-dimethyl- and 2,3-diethyl-5-methylpyrazine. 3-Ethenyl-2-ethyl-5-methylpyrazine was also detected in coffee, and its odor threshold value was 8000 times higher than that of the 2-ethenyl-3-ethyl-5-methylpyrazine. After addition of HBr, the two ethenylethylmethylpyrazine isomers were separated by capillary gas chromatography. The results indicate the presence of the two isomers in a 1:1 ratio in coffee. Keywords: Roasted coffee; aroma; 2-ethenyl-3,5-dimethylpyrazine; 2-ethenyl-3-ethyl-5-methylpyrazine; 3-ethenyl-2-ethyl-5-methylpyrazine; detection; synthesis; odor threshold