Alkylpyrazines and Other Volatiles in Cocoa Liquors at pH 5 to 8, by Selected Ion Flow Tube-Mass Spectrometry (SIFT-MS)

ABSTRACT:  Cocoa beans were alkalized before or after roasting and made into cocoa liquor before analyzing by SIFT-MS. In both alkalized-before-roasting and alkalized-after-roasting samples, there were significantly higher concentrations of alkylpyrazines for the samples with pH above 7 than pH below 7. At pH 8, the concentrations of 2,3-, 2,5-, and 2,6-dimethylpyrazine (DMP), 2,3,5-trimethylpyrazine (TrMP), 2,3,5,6-tetramethylpyrazine (TMP), and 2,3-diethyl-5-methylpyrazine (EMP) in the samples alkalized-before-roasting were higher than those in the samples alkalized-after-roasting. Volatiles increased under conditions that promoted the Maillard reaction. The partition coefficient was not significantly affected by pH from 5.2 to 8. The ratios of TrMP/DMP and DMP/TMP increased while the ratio of TMP/TrMP decreased as the pH increased. The concentrations of Strecker aldehydes and other volatiles followed a similar pattern as that of the alkylpyrazines. High pH favors the production of alkylpyrazines and Strecker aldehydes.
Practical Applications: Cocoa beans alkalized-before-roasting had higher levels of many important chocolate aroma volatiles than those alkalized-after-roasting. Thus, alkalizing before roasting should produce a stronger cocoa aroma. The higher the pH, the higher the concentrations of these important volatiles. There was little change in volatile concentration at acid pH. Above pH 7, volatile levels increased rapidly with increasing pH. To maintain the recommended ratio of tetramethylpyrazine to trimethylpyrazine, a longer roasting time or higher temperature is needed for cocoa liquor with a pH below 6.5, while a shorter roasting time or lower temperature is needed for cocoa liquor with a pH above 7.