Online measurement of volatile organic compounds released during roasting of cocoa beans

• A photo-ionization-time of flight mass spectrometer was successfully applied for the analysis of cocoa roast gases.
• Roasting gases evolving inside and outside a single cocoa bean were investigated.
• A lot of aroma- and Maillard reaction products were identified.
• Slight changes between the measurements in- and outside the bean were observed.

The gaseous species evolving during the roasting of cocoa beans were analyzed by a newly developed technique, a micro-probe–photo-ionization–time-of-flight mass spectrometer system. A lot of volatile roasting products and typical aroma compounds, such as methylbutanal, phenylacetaldehyde and pyrazines, could be online detected. The differences between the roast gas composition inside and outside the bean were pointed out. Two additional signals at m/z 266 and 294 with resonance-enhanced multi-photon ionization (REMPI), which argues for an aromatic character, and a higher methanethiol signal with single photon ionization (SPI) could have been identified at first glance for the measurements inside the bean. Principal component analysis (PCA) and difference spectra revealed and confirmed these findings. Higher concentrations of theobromine and fatty acids are present outside and more caffeine inside the bean. With both ionization techniques, the changes in the PCA are more prominent at the beginning of the roast courses, which are mostly explained by caffeine and theobromine. Following the courses of these characteristic compounds showed the same results as obtained with the PCA and revealed that caffeine is always released before theobromine. The new analytical technique proved to be a fast, reliable and non-labor intensive method, which could therefore be a practical technique for the online analysis and monitoring of the cocoa roasting process.