Coffee as a source of antioxidants: An EPR study

Antioxidant properties of commercially-available ground and instant coffees were investigated by means of electron paramagnetic resonance (EPR) using as oxidants 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) diammonium salt (ABTS), 1,1-diphenyl-2-picrylhydrazyl (DPPH), 4-hydroxy-2,2,6,6-tetramethylpiperidine N-oxyl (TEMPOL) and thermally decomposed K2S2O8. Only moderate differences were found in the antioxidant capacities of individual samples (expressed as trolox equivalent antioxidant capacity TEAC) with TEACABTS = 0.22 ± 0.02 mmol g−1 for ground and TEACABTS = 0.71 ± 0.07 mmol g−1 for instant coffees. Two coffee components (caffeic acid and caffeine) were investigated using moderate hydrogen/electron scavengers ABTS+ and DPPH, and a powerful oxidant OH radical (photochemically generated from H2O2). Caffeine is inert to ABTS+ and DPPH oxidants, but effective in the scavenging of OH radicals, with a bimolecular rate constant k = 2.6 × 109 M−1 s−1. Caffeic acid is a very effective antioxidant in all oxidant systems, with k = 9.4 × 109 M−1 s−1 in the bimolecular reaction with OH radicals. From the evaluated correlation matrix, a good linear relationship was found for the ground coffees between TEACABTS and TEACDPPH values ( r TEAC ABTS / TEAC DPPH = 0.859 ) and also between the phenolics content expressed in gallic acid equivalents (GAE) and TEAC antioxidant capacities ( r GAE / TEAC ABTS = 0.729 and r GAE / TEAC DPPH = 0.922 ).