Coffee As A Source Of Antioxidants: An Epr Study

摘要

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-sul-fonic acid) diammonium salt (ABTS), 1,1-diphenyl-2-picrylhydrazyl (DPPH), 4-hydroxy-2,2,6,6-tetra-methylpiperidine N-oxyl (TEMPOL) and thermally decomposed K_2S_2O_8. 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 TEAC_(ABTS) = 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 H_2O_2). Caffeine is inert to ABTS~+ and DPPH oxidants, but effective in the scavenging of OH radicals, with a bimolecular rate constant K = 2.6 × 10~9 m~(-1) s~(-1). Caffeic acid is a very effective antioxidant in all oxidant systems, with k = 9.4 × 10~9 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 TEAC_(abts) and TEAC_(DPPH) 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).