Effects of Salt Concentration and Traditional Fermentation on Physicochemical Properties, Bioactive Composition, Metabolite Diversity, and Antioxidant Activity of Fermented Cleome gynandra Linn

Cleome gynandra (CG), an indigenous leafy vegetable traditionally fermented across Africa and Southeast Asia, has a fermentation process that remains poorly characterised. This study evaluated, for the first time, the effects of salt concentration (2–5%) and fermentation medium (brine alone, cooked rice-supplemented brine, and washed rice water brine (WRW)) on naturally fermented CG, examining total phenolic and flavonoid content (Folin–Ciocalteu and aluminium chloride assays), antioxidant activity (DPPH, FRAP, and ORAC assays), and metabolite profiles (untargeted LC–MS metabolomics). Fermentation reduced pH (6.2 to 3.5–4.5) and increased total acidity (0.3 to 1.9–8.6 g/L) and lactic acid bacteria populations across all treatments. WRW with 3% salt yielded the highest phenolic and flavonoid contents and antioxidant activity among all treatments. Untargeted metabolomics identified 800 metabolites, including lipids, organic acids, and phenolic compounds, and partial least squares discriminant analysis (PLS-DA) confirmed that WRW with 3% salt was the treatment with the greatest metabolite diversity and upregulation. These findings establish washed rice water at a moderate salt concentration as an effective fermentation strategy to enhance the nutritional and functional quality of CG, offering a practical model for optimising fermentation in other indigenous leafy vegetables.