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-tetramethylpipericline 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 TEAC(ABTS) = 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)-O-. radical (photochemically generated from H2O2). Caffeine is inert to ABTS(-+) and DPPH oxidants, but effective in the scavenging of (OH)-O-. radicals, with a bimolecular rate constant k = 2.6 x 10(9) m(-1) s(-1). caffeic acid is a very effective antioxidant in all oxidant systems, with k = 9.4 x 109 M-1 s(-1) in the bimolecular reaction with (OH)-O-center dot radicals. From the evaluated correlation matrix, a good linear relationship was found for the ground coffees between TEAC(ABTS) and TEAC(DPPH) values (r(TEACABTS)/TEAC(DPPH) = 0.859) and also between the phenolics content expressed in gallic acid equivalents (GAE) and TEAC antioxidant capacities (r(GAE/TEACABTS) = 0.729 and r(GAE/TEACDPPH) = 0.922). (C) 2008 Elsevier Ltd. All rights reserved.