Thermal Processing Techniques Differentially Modulate Phytochemicals, Antioxidant Potential, and Genoprotective Effects of Kale (Brassica oleracea var. acephala) and Chard (Beta vulgaris L. var. cycla)

Thermal processing alters the nutritional and functional properties of vegetable food. In this study, using electrophoretic, spectrophotometric, and statistical analyses, we analyzed the effects of boiling, blanching, steaming, and blanching followed by pan-frying and air-frying on the concentration of bioactive compounds in kale and chard, and the biological effects of their extracts. In addition to analyzing the vegetable tissues, the residual water remaining after thermal processing was also examined to assess the nutritional potential of this often overlooked and typically discarded by-product. The residual cooking water had the highest antioxidant capacity, according to ABTS, DPPH, and FRAP assays (57.83% ± 18.16%, 33.58% ± 16.55%, and 81.58% ± 0.78% for kale and 74.53% ± 4.56%, 13.62% ± 7.34%, and 82.97% ± 0.44% for chard, respectively). Air-frying and cooking water contained the highest total phenolics (0.48 ± 0.17 mg GAE/g fw and 0.35 ± 0.06 mg GAE/g fw for kale and 0.88 ± 0.21 mg GAE/g fw and 0.80 ± 0.06 mg GAE/g fw for chard, respectively). Thermally processed chard had a higher concentration of oxidative marker H₂O₂ than kale. An inverse relationship between soluble sugars and H₂O₂ levels was observed. In kale, cooking processes caused the greatest reduction in soluble sugars, whereas in chard, this effect was most pronounced during blanching. Chard had more photosynthetic pigments than kale. Heat treatments caused more differences between kale samples than chard samples. Pan-frying best preserved chlorophylls, porphyrins, and carotenoids. According to both PC and HC analysis, the tissues of kale were clearly distinguishable from the water remaining after boiling/blanching/steaming kale. These results may help to optimize industrial processing conditions to better preserve bioactive compounds and create opportunities for the valorization of cooking by-products.