Contribution of volatile compounds to the antioxidant capacity of coffee

Heterocyclic volatile compounds present in coffee have been proposed as potent antioxidants, but their contribution to the antioxidant capacity of coffee is still unclear and controversial. The aim of this study was to assess the actual contribution of the main volatile compounds to the overall antioxidant capacity of coffee. A total of sixty-two and sixty-four volatile compounds were identified and quantified in Arabica and Robusta coffee, respectively, by static headspace-gas chromatography-mass spectrometry (SH-GC-MS). ABTS (2,2′-Azino-bi(3-ethylbenzo-thiazonile-6-sulfonic acid) diammonium salt) and DPPH (2,2-Diphenyl-1-picrylhydrazyl) antioxidant activity of the most abundant volatile heterocyclic compounds (7 furans (Fu), 3 pyrroles (Py) and 2 thiophenes (Th)), aldehydes (5) and diketones (2) was evaluated in model systems at different concentrations including those found in coffee. The model system with all the heterocyclic volatiles (Fu-Py-Th) was the most active followed by pyrroles and furans. Thiophenes were ineffective as radical scavengers at all concentrations including 100-fold, and aldehydes and ketones showed negligible activities in comparison to heterocyclic volatiles. In addition, only furans exhibited linear concentration dependent ABTS antioxidant activity and individual volatile model systems showed that only 2-methyl-tetrahydrofuran-3-one and pyrrole for ABTS, and also 1-methylpyrrole for DPPH, were the main volatile compounds responsible for the coffee antioxidant activity. However, the contribution of the heterocyclic volatile compounds to the overall antioxidant capacity of a filter coffee brew was almost insignificant, even at 100-fold concentrated Fu-Py-Th model system, accounting only for up to 3.3%.