Reduction kinetics of the free stable radical 2,2-diphenyl-1-picrylhydrazyl (DPPH•) for determination of the antiradical activity of citrus juices

There is vast epidemiological evidence for the beneficial effects that the consumption of citrus juices exerts on human health. These effects are attributed to their content in substances exhibiting free radical scavenging and antioxidant activities, mainly ascorbic acid, flavonoids, carotenoids and polyphenolic derivatives. This work describes an approximation to the reduction kinetics of the stable free radical 2,2-diphenyl-1-picrylhydrazyl (DPPH center dot) by the free radical scavenging activity of juices from sweet orange (Citrus sinensis L.), clementine (Citrus reticulata Blanco) and satsuma (Citrus unshiu K. Marc), as well as the deduction of a kinetic equation to fit the experimental data. This equation contains two contributions. The first contribution accounts for the concentration of DPPH center dot reduced by the cumulative antiradical activity of those citrus juice components that are capable of fast hydrogen atom transfer (fast-kinetics), mainly ascorbic acid and some polyphenolic derivatives; the second accounts for the concentration of DPPH center dot reduced by the cumulative antiradical activity of those citrus juice components that are capable of slow hydrogen atom transfer (slow-kinetics), mainly flavanone-7-O-glycosides and some polyphenolic derivatives. This methodology thus permits the determination of the total antiradical activity of a citrus juice (expressed as molar equivalents of ascorbic acid), as well as the partial contributions of the cumulative antiradical activities (also expressed as molar equivalents of ascorbic acid) due to both fast-kinetics and slow-kinetics.