Investigation of the effects of different cooking techniques on the antioxidant capacity and total phenolic content of some dark green leafy vegetables

This study aimed to comparatively evaluate the effects of different cooking techniques on total phenolic content, phenolic acids, tannins, flavonoids, and antioxidant capacity in selected dark green leafy vegetables. Five vegetables—stinging nettle (Urtica dioica), spinach (Spinacia oleracea), sorrel (Rumex acetosa), Swiss chard (Beta vulgaris subsp. cicla), and purslane (Portulaca oleracea)—were subjected to steaming, microwave cooking, low-water boiling, and high-water boiling. Total phenolics, flavonoids, phenolic acids, and tannins were measured using the Folin–Ciocalteu, AlCl₃/CH₃COOK, Arnov, and Folin–Ciocalteu–based methods, respectively. Antioxidant capacity was assessed by DPPH, ABTS, and CUPRAC assays. Data were analyzed using SPSS 29.0. The findings showed that cooking techniques had vegetable-specific effects. Swiss chard was the most sensitive, exhibiting marked reductions in phenolic content across all methods (p=0.002). Phenolic acids increased in stinging nettle and purslane under all cooking conditions but decreased in spinach and Swiss chard. Flavonoid levels declined in stinging nettle, spinach, and Swiss chard (p<0.05), whereas purslane showed minimal change. Tannin levels decreased in stinging nettle across all methods (p<0.001) but increased in purslane. Overall antioxidant capacity tended to decrease after cooking, with purslane being the only vegetable that showed increases under some methods. Strong positive correlations were observed between phenolic compounds and antioxidant capacity (r=0.853–0.956). Among the cooking techniques, high-water boiling caused the greatest losses of phenolic compounds and antioxidant capacity. In contrast, steaming and microwave cooking better preserved bioactive compounds, particularly in sorrel and stinging nettle. These results indicate that cooking technique is a key determinant of bioactive compound retention and that no single method is optimal for all vegetables. Future studies should systematically vary cooking time, temperature, and water volume to establish optimal cooking protocols for each vegetable. The use of in vitro digestion models is also recommended to more comprehensively evaluate the post-cooking bioavailability of phenolic compounds.