Bioactive Recovery from Watermelon Rind Waste Using Ultrasound-Assisted Extraction

In response to the imperative to address food and agricultural waste and promote environmental sustainability, this study investigates the transformation of watermelon rind waste into a valuable resource. We extract polyphenols and antioxidants from freeze-dried watermelon rind across four cultivars (Crimson Sweet, Black Prince, Black Chairman, and Sugar Baby) using ultrasound-assisted extraction. Solvent selection, based on a preliminary experiment, was optimized through the response surface methodology (RSM), fine-tuning the solvent concentration and sonication time. The RSM effectively modeled our responses (R 2 > 0.90), confirming its suitability for optimization. Applying the optimized parameters (80% solvent concentration and 20 min of sonication) to the rinds yielded extracts rich in flavanols (10–30 mg/g) and phenolics (1–2.5 mg/g), with high DPPH (22–40 mg Trolox/g) and FRAP (26–42 mg ferrous sulfate/g) antioxidant activities. Furthermore, fatty acid and phytosterol profiles of watermelon rinds were reported, demonstrating their potential applications in food, pharmaceuticals, nutraceuticals, and industry.