Dark green leafy vegetables supply vitamins, minerals, dietary fibre and diverse bioactive phytochemicals, including polyphenols, carotenoids and glucosinolates, which have been linked to cardiometabolic and anticancer benefits. Because most vegetables are consumed cooked, domestic heat treatments can alter both concentrations and bioavailability of these compounds. Literature reports on cooking effects are inconsistent across species and methods, and few studies quantify phenolic subclasses while also applying multiple antioxidant assays in a single, standardized protocol. This study evaluated the impacts of four common household cooking methods—steaming, microwaving, low-water boiling and high-water boiling—on total phenolics, phenolic acids, flavonoids, tannins and antioxidant capacities (DPPH, ABTS, CUPRAC) in five dark green leafy vegetables commonly consumed in Türkiye: sorrel, stinging nettle, spinach, Swiss chard and purslane.
Fresh samples (approximately 3 kg per species) of nettle (Urtica dioica), spinach (Spinacia oleracea), sorrel (Rumex acetosa), Swiss chard (Beta vulgaris subsp. cicla) and purslane (Portulaca oleracea) were purchased from local farmers’ markets in Istanbul. Samples were washed, dried and processed immediately to avoid storage-related changes. Vegetables were chopped to uniform pieces, weighed into 100 g portions and divided into three independent biological replicates per treatment. Chemical and antioxidant analyses were performed in analytical triplicate for each biological replicate to ensure measurement precision.
Four cooking methods were standardized and applied to each vegetable: steaming, microwaving, low-water boiling and high-water boiling. Low-water boiling was implemented using 50 mL water per 100 g sample to reflect minimal-water domestic practice, whereas high-water boiling used 250 mL per 100 g to represent excess-water boiling commonly used in households. The microwave oven and stainless-steel cooking pot described in the methods were used to perform the respective thermal treatments under controlled, repeatable conditions adapted from published protocols.
Total phenolics, phenolic acids, flavonoids and tannins were quantified spectrophotometrically using established colorimetric methods and reference standards. Antioxidant capacity was measured using three complementary assays: DPPH and ABTS (which assess radical scavenging by hydrogen atom transfer and single electron transfer mechanisms) and CUPRAC (a cupric ion reduction assay that evaluates electron transfer capacity). The combined use of these assays aimed to provide a mechanistically diverse profile of antioxidant activity for each sample and treatment.
Cooking produced variable effects on total phenolic content depending on vegetable species and method. Sorrel experienced a statistically significant decrease in total phenolics (p = 0.005). Other species did not show the same magnitude of loss; some vegetables retained similar total phenolic levels after cooking or exhibited modest increases under particular methods. Thus, overall change in total phenolics was not uniform across the five vegetables and depended on the interaction between species-specific phytochemical composition and applied thermal treatment.
Phenolic acids increased markedly in stinging nettle and purslane following certain treatments. The largest reported increases occurred after low-water boiling—+279.3% in nettle and +275.0% in purslane—and after steaming—+153.2% in nettle and +159.5% in purslane. These rises suggest that some cooking procedures can enhance extractable phenolic acid fractions, possibly by disrupting the plant matrix and releasing bound compounds.
Flavonoid concentrations tended to decline primarily with high-water boiling, although purslane’s flavonoid levels remained relatively stable across the tested cooking methods. Tannin-associated phenolic compounds decreased in nettle and sorrel when boiled, while purslane exhibited increases in tannin-associated measures under several treatments. These subclass-specific patterns underscore that different phenolic groups respond differently to heat and water exposure.
Antioxidant activities measured by the three assays were generally sensitive to cooking method. High-water boiling most consistently produced reductions in antioxidant capacity across vegetables. In contrast, steaming and microwaving better preserved antioxidant activity; in particular, DPPH and CUPRAC activities were preserved or enhanced in purslane after these methods. The three-assay approach provided complementary information: DPPH and ABTS probe radical scavenging through HAT/SET mechanisms, while CUPRAC evaluates cupric ion reduction, and together they indicated method-dependent shifts in overall antioxidant potential.
Strong positive correlations were observed between total phenolic content and the antioxidant assay results, with correlation coefficients reported in the range r = 0.853–0.956. These findings indicate that variation in total phenolics across species and treatments largely explained changes in measured antioxidant capacity, supporting the role of phenolic compounds as principal contributors to the antioxidant properties of these leafy vegetables.
Within this standardized comparison of four domestic cooking methods applied to five dark green leafy vegetables, steaming and microwaving were identified as the most effective techniques for retaining phenolic compounds and antioxidant capacity. High-water boiling caused the greatest losses in phenolics and antioxidant activity. Species-specific responses were evident: sorrel showed a significant total phenolic decrease, whereas nettle and purslane showed marked increases in phenolic acids under low-water boiling and steaming. Flavonoids and tannin-associated compounds displayed variable sensitivity to boiling. The authors highlight the importance of selecting appropriate cooking practices to preserve the nutritional quality of leafy vegetables. Details on experimental protocols, numerical tables and figures, and supporting data are provided in the manuscript and its supporting information.