Time-Dependent Effects of Cooking on Antioxidant Capacity and Phytochemical Retention in Green Leafy Vegetables

• Esraa fadhil mohamed1

  Author

Green leafy vegetables (GLVs) are major dietary sources of phenolic compounds, flavonoids, vitamin C and pigments, yet these constituents respond very differently to domestic heat treatment. Most previous work contrasts only raw versus cooked material and relies on a single antioxidant assay, leaving the time course of change poorly resolved. This study quantified the time-dependent effect of atmospheric boiling (0, 2, 5, 10, 15 and 20 min; 1:10 w/v in distilled water, timed from the return to a rolling boil) on five commonly consumed GLVs spinach, kale, Swiss chard, molokhia and amaranth using three complementary in vitro antioxidant assays (DPPH, ABTS, FRAP), total phenolic (TPC) and flavonoid content, vitamin C, chlorophylls, carotenoids and HPLC-DAD quantification of six identified phenolic markers, with mass-corrected true-retention (TR) analysis and multivariate statistics. Short boiling (2–5 min) increased Folin–Ciocalteu-measured TPC by 12–16% in the more robust species and was accompanied by higher measured antioxidant capacity; this apparent gain is interpreted as increased extractability and release of bound phenolics rather than net phenolic synthesis. Prolonged boiling (≥15 min) reduced every measure. Vitamin C was the most labile constituent (TR 11–22% after 20 min), while carotenoids were the most stable. A significant vegetable×time interaction confirmed species-specific thermal behaviour, with kale most resilient and Swiss chard least. Among the cooked levels tested, an equal-weight integrated retention index was highest at 2 min for all five species, and the ranking was unchanged on a mass-corrected TR basis; however, once mass loss was accounted for no cooked level retained the full raw complement of the measured constituents, and 2 and 5 min did not differ statistically for phenolics or antioxidant capacity in four of the five species. The data therefore support brief boiling (2–5 min, toward the shorter end where vitamin C is the priority) as a retention-favourable window for the species and boiling conditions studied. The assays quantify chemical antioxidant capacity of the drained edible portion only; bioaccessibility, constituents leached into the discarded cooking water, sensory quality and other cooking methods were not assessed, and the recommendations are bounded accordingly.

• pdf