Tailoring subcritical water extraction conditions to enhance antioxidant selectivity from kale (Brassica oleracea var. acephala)

Subcritical water extraction (SWE) is an emerging technique for augmenting the recovery of phytochemicals in the plant-based matrices under optimized conditions. However, its use with leafy vegetables remains underexplored. This study optimized SWE conditions for antioxidant-rich compounds recovery from dried wilted kale (Brassica oleracea var. acephala) using response surface methodology (RSM). The effects of extraction temperature (120–150 °C), extraction time (10–30 min), and solid-to-solvent ratio (1,10–1,15) on total phenolic content (TPC), total flavonoids content (TFC), and antioxidant activity were investigated. Preliminary screening experiments conducted below 120 °C resulted in substantially lower TPC and antioxidant activity, with markedly longer extraction times required to achieve comparable yields. Therefore, 120 °C was selected as the practical lower boundary to ensure operation within the effective subcritical region while maintaining process efficiency. Within the investigated temperature range, extraction temperature was identified as the dominant factor affecting the antioxidant recovery. The optimal condition within the studied design space was 120 °C, for 10 min, and with solid-to-solvent ratio of 1:15, exhibited 66.31 mg gallic acid equivalents (GAE) g−1 dry weight (DW) (TPC), 12.58 mg quercetin equivalents (QE) g−1 DW) (TFC), 82.67 ± 0.01% 2,2-diphenyl-1-picrylhydrazyl radical scavenging assay (DPPH), and 799.53 ± 3.17 μmol Fe2+ g−1 DW ferric reducing antioxidant power (FRAP). Validation experiments showed relatives deviations of 11.24%, 6.07%, and 1.70% for TPC, TFC and DPPH activity, respectively, confirming satisfactory predictive accuracy of the developed models (R2 > 0.95). Fourier transform infrared spectroscopy (FTIR) analysis confirmed the presence of phenolic and sulphur-containing functional groups. Dietary fibre (0.8 g kg−1) and trace glucosinolates (0.38 μmol g−1) are retained showed limited matrix degradation in favourable conditions. The results indicate that moderate subcritical temperatures maximize antioxidant recovery by balancing enhanced solubility and minimized thermal degradation of phenolic compounds. These findings demonstrate the potential of SWE as an efficient and environmentally friendly approach for producing antioxidant-rich extracts from kale for functional food applications.

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