Phenolic compounds and antioxidant activities of three grape cultivars at different ripening stages

In this study, phenolic compounds present in three grape cultivars ('Heukbosuk', 'Heukgoosul' and 'Kyoho') at different ripening stages were determined, and their antioxidant activities were compared. Twelve phenolic compounds were used as standards, and eight phenolic compounds (flavonols, e.g., kaempferol, myricetin, quercetin and rutin-trihydrate, and flavan-3-ols, e.g., (+)-catechin and (−)-epicatechin) were detected in all grape cultivars by HPLC-MS. The total polyphenol content (TPC) of all grape cultivars was determined by the Folin-Ciocalteu method, and their antioxidant activities were assayed by the DPPH radical scavenging activity assay. The TPC values for skin extracts of 'Heukbosuk', 'Heukgoosul' and 'Kyoho' ranged from 35.93 to 199.26 μg GAE g-1 FW, 32.20 to 57.32 μg GAE g-1 FW, and 13.85 to 41.33 μg GAE g-1 FW, respectively, at different ripening stages. The TPC values for the seed extracts of 'Heukbosuk', 'Heukgoosul' and 'Kyoho' ranged from 33.02 to 36.81 μg GAE g-1 FW, 42.59 to 48.64 μg GAE g-1 FW, and 33.77 to 41.22 μg GAE g-1 FW, respectively. The DPPH radical scavenging activity is expressed as the IC50 value, representing the concentration at which 50% inhibition was observed. Similar IC50 values were observed for the skin extracts during the ripening of all grape cultivars. In particular, the highest IC50 value (11.87 mg AEAC g-1, FW) for the 'Kyoho' seed extract was recorded on August 20 during ripening.