Variation in the phenolic content and in vitro antioxidant activity of Sorbus aucuparia leaf extracts during vegetation.

Seasonal variation in the antioxidant activity and content of phenolic compounds was studied for the 70% methanol extracts of Sorbus aucuparia leaves harvested monthly over the full course of the growing season. The antioxidant potential of the extracts was evaluated using two complementary in vitro tests: the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical-scavenging assay and the AAPH [2,2'-azobis-(2-amidinopropane)dihydrochloride]-induced [corrected] linoleic acid (LA) peroxidation test. The radical-scavenging capacities of the extracts towards the DPPH radical were in the range of 0.40 to 0.57 millimolar Trolox equivalents/g dry weight of the leaves. They were significantly correlated (r = -0.8480, p < 0.05) with the results of the LA-peroxidation test, indicating the S. aucuparia leaf extracts to be universal antioxidants. Significant linear correlations were also found between the different antioxidant potentials and total phenolic contents as estimated by the Folin-Ciocalteu method and further verified by serial determinations of proanthocyanidins, chlorogenic acid isomers and flavonoids ([r] in the range of 0.81-0.97, p < 0.05). As the best antioxidant capacities and the highest phenolic contents were found for the leaf samples harvested during the three summer months (June, July and August), this period could be considered to be optimal for cost-effective production of natural health products. For the leaf samples collected in July, the values of EC50 and IC50 for the two antioxidant tests were 2.02 and 93.45 μg [corrected] phenolics/mL, respectively. These antioxidant capacities were found to be higher or comparable to those of synthetic and natural phenolic antioxidants, such as BHT (butylated hydroxytoluene), BHA (butylated hydroxyanisole), TBHQ (tert-butylhydroquinone), quercetin and Trolox.