Characterization of Free Radicals in Soluble Coffee by Electron Paramagnetic Resonance Spectroscopy

Abstract

EPR spectra of soluble coffee display single-line free radical signals in both the solid state and aqueous solution, along with signals from the paramagnetic ions Fe(III) and Mn(II). The intensity of the free radical signal in the pure solid was estimated to be ca. 7.5 x 10(16) unpaired electrons/g, and there was no significant change on dissolution in water. In aqueous solutions, however, the free radical signal declined rapidly over ca. 10-15 min in the temperature range 20-65 degrees C, after which only slow changes were observed. This decline, which was essentially independent of atmosphere, was greatest for the lowest temperatures used, and the intensity after 1 h fitted well to an exponential curve with respect to temperature. The free radicals responsible for the single-peak EPR signal did not react with any of the spin traps tested in the present experiments, but unstable free radicals with parameters consistent with adducts of C-centered radicals were detected in coffee solutions in the presence of PBN and 4-POBN spin traps. The presence of oxygen in the solutions increased the initial rate of formation of these free radical adducts. No adducts were detected when DEPMPO was used as spin trap. However, *OH adducts of DEPMPO were shown to be unstable in the presence of coffee, a fact which illustrates the strong free radical scavenging ability of coffee solutions.

Free Radical Processes in Coffee II—Liquids

“…Decreases in EPR free radical signal intensities have also been observed during the storage of coffee concentrates , and this decrease was faster if O 2 was present in the extracting water, than if it was absent. In contrast to these observations, the results of Pascual et al (2002) indicate free radical formation in coffee solutions heated in air, and also that this increases with increasing temperature. The generation of this latter radical seems to correlate roughly with the pro-oxidant activity of coffee, as measured by its ability to destabilize ascorbic acid to produce the ascorbate radical.…”

“…The free radicals trapped in the measurements shown in Figure 68.4 are not those responsible for the temperature-dependent single peak signal in Figure 68.3. This was demonstrated by Pascual et al (2002), who showed that the single peak resonance is present along with that from the adduct, and as shown in Figure 68.5, its intensity is virtually identical to that seen in the absence of the spin trap.…”

“…The role of O 2 in the formation of unstable free radicals in coffee solutions is illustrated by the spin trapping data of Pascual et al (2002) for the heating of 5% instant coffee solutions in air in the presence of the spin trap PBN (Figure 68.4). The rate of adduct formation for fresh aerated solutions (Figure 68.4a) was ∼ 2.5-times faster than for solutions that were initially deoxygenated ( Figure 68.4b).…”

“…These results can be interpreted in terms of an initial conversion of O 2 to H 2 O 2 FIGURE 68.2 EPR free radical intensity in 5% soluble coffee solutions as a function of time at (a) 65°C, and (b) 21°C. Adapted from Pascual et al (2002). ity of coffee and its free radical concentration as a function of temperature.…”

“…ity of coffee and its free radical concentration as a function of temperature. Based on data from Goodman et al (1994) and Pascual et al (2002). 8.…”