Fruit and vegetable processing wastes as natural sources of antioxidant-rich extracts: Evaluation of advanced extraction technologies by surface response methodology

This study is focused on the recovery of polyphenols from vegetable and fruit residues and further evaluation of antioxidant features of extracts. Spinach and orange have been selected as representative matrices for a more comprehensive study since they contain significant polyphenols amount that could be used in food, pharmaceutical and/or cosmetic industries. Extraction of polyphenols from spinach and orange waste was performed using three extraction techniques: ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), and pressurized liquid extraction (PLE). Tested factors include percentage of organic solvent (ethanol 0–80%), acidity (0–0.5% HCl), extraction time (0–30 min) and temperature (25–120 °C). Optimal extraction conditions for spinach and orange waste have been stablished by design of experiments (DoE). The performance of the extraction process has been preliminarily assessed from the overall polyphenolic content given by high-performance liquid chromatography (HPLC) and Folin–Ciocalteu index. In addition, reducing power and antiradical capacity of vegetable and fruit extracts have also been determined. For spinach, the best conditions corresponded to UAE with a mixture of ethanol/water/HCl in ratio 80/19.9/0.1 (v/v/v) as the solvent at 25 °C for an extraction time of 30 min, providing 0.82 mg gallic acid equivalent (GAE) per g fresh weight (fw). For the orange matrix, PLE has been chosen using 60/39.9/0.1 ethanol/water/HCl (v/v/v) solvent at 80 °C for 15 min, providing 3 mg GAE g−1 fw. However, UAE is proposed for extraction of polyphenols from spinach and orange waste at industrial scale, due to its simplicity and low cost, among other reasons. [Display omitted] •Orange peel and spinach leaf residues are a source of polyphenols.•Polyphenols from agro-wastes were extracted by UAE, MAE and PLE techniques.•Optimal extractions conditions (EtOH, HCl, time, Tª) depended on the waste matrix.•Compound degradations were found under stronger conditions.