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±0.02mmolg−1 for ground and TEACABTS=0.71±0.07mmolg−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=2.6×109M−1s−1. Caffeic acid is a very effective antioxidant in all oxidant systems, with k=9.4×109M−1s−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 (rTEACABTS/TEACDPPH=0.859) and also between the phenolics content expressed in gallic acid equivalents (GAE) and TEAC antioxidant capacities (rGAE/TEACABTS=0.729andrGAE/TEACDPPH=0.922).

The antioxidant properties of 15 honey samples from different floral sources and various Slovak regions were investigated by means of electron paramagnetic resonance spectroscopy. Cation radical of ABTS (2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonate) diammonium salt), DPPH (1,1-diphenyl-2-picrylhydrazyl) and hydroxyl radicals generated by the photochemical decomposition of hydrogen peroxide were used as oxidants. The antioxidant activities found with ABTS(+), expressed as trolox equivalent antioxidant capacity (TEAC), ranged from 0.15 to 1.14mmolkg(-1), and those determined with DPPH, from 0.04 to 0.32mmolkg(-1). TEAC values correlated well with results found by elimination of DPPH, and both values revealed a linear relationship with the concentration of phenolics obtained with the Folin-Ciocalteu phenol test (expressed as gallic acid equivalents, GAE). The colour coordinates (CIE L(∗)a(∗)b(∗)), as well as reflectance spectra determined for original honeys using a white background, demonstrated that the colour difference (ΔE(∗)) and coordinate b(∗) interrelate with TEAC values. The radical-scavenging capacities (RSC) of the honey samples determined in the experiments with photochemically decomposed hydrogen peroxide, generating reactive OH radicals in the presence of spin trapping agent, differ from those found with ABTS(+) and DPPH. Here, probably, the reactive OH radicals, having higher redox potential, are scavenged by a variety of compounds not effective with ABTS(+) and DPPH (e.g., saccharides, proteins).