Impact of traditional cooking methods on total phenolics, flavonoids, and antioxidant capacities of dark green leafy vegetables

This study evaluated the effects of four domestic cooking methods-steaming, microwaving, low-water boiling and high-water boiling-on the phenolic composition and antioxidant capacities of five dark green leafy vegetables (sorrel, stinging nettle, spinach, Swiss chard and purslane). Total phenolics, phenolic acids, flavonoids and tannins were quantified spectrophotometrically, while antioxidant capacities were assessed by DPPH, ABTS and CUPRAC assays. Cooking induced species- and method-dependent changes in phenolic profiles. Total phenolic content significantly decreased in sorrel (p = 0.005), whereas other vegetables showed stable or slightly increased values. Phenolic acids increased markedly in stinging nettle and purslane, particularly after low-water boiling (+279.3% and +275.0%) and steaming (+153.2% and +159.5%). Flavonoid levels declined mainly under high-water boiling but remained stable in purslane. Tannin-associated phenolic compounds decreased in nettle and sorrel during boiling, whereas purslane exhibited increases across several treatments. Antioxidant capacities generally decreased with high-water boiling, while steaming and microwaving preserved or enhanced DPPH and CUPRAC activity in purslane. Strong correlations were observed between total phenolic content and antioxidant assays (r = 0.853-0.956). Overall, steaming and microwaving were identified as the most effective techniques for retaining phenolic compounds and antioxidant capacity, whereas high-water boiling caused the greatest losses. These findings highlight the importance of selecting appropriate cooking methods to preserve the nutritional quality of leafy vegetables.