Comparison of Total Antioxidant Capacities and Phenolic Constituents of Grapes Cultivated in Turkey for Wine Production

Abstract

Grapes are renowned for their health-promoting properties due to polyphenols, particularly anthocyanins in red grapes and flavonols in white grapes. This study evaluated the total antioxidant capacities (TAC) of specific grape varieties (Sultaniye, Chardonnay, Shiraz, Carignan, Alicante Bouschet, and Merlot) using the CUPRAC (CUPric Reducing Antioxidant Capacity) and ABTS/HRP (2,2’-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)/horseradish peroxidase) methods. Additionally, total phenolic, flavonoid, and anthocyanin contents were determined via Folin Ciocalteu, AlCl3/NaNO2, and pH differential methods. Primary phenolic compounds were identified with High-Performance Liquid Chromatography-Photodiode Array (HPLC-PDA). TAC values of the grape juices (mmol Trolox L−1) ranked as Alicante > Shiraz > Sultaniye > Carignan > Merlot > Chardonnay by CUPRAC, and Shiraz > Alicante > Merlot > Carignan ≥ Sultaniye > Chardonnay by ABTS/HRP and for skin extracts (mmol Trolox g−1), Carignan > Alicante > Shiraz > Merlot > Chardonnay > Sultaniye by CUPRAC, and Carignan > Shiraz > Alicante > Merlot > Chardonnay > Sultaniye by ABTS/HRP. The study demonstrated a strong correlation between phenolic composition and TAC values in grape samples, with anthocyanins as the predominant family contributing significantly to TAC. Varietal and intra-fruit differences in specific phenolic compounds were found to account for variations in observed TAC levels. Specifically, grape skin samples showed a more robust correlation between antioxidant capacity and total phenolic content (r = 0.986) than grape juices (r = 0.777). These findings underscore the considerable variation in antioxidant potential among grape varieties and emphasize the critical role of phenolic composition in the health-related benefits of grape consumption.