Optimization of Ultrasound-Assisted Extraction of Phenolic Compounds and Characterization of Krokos Kozanis (Crocus sativus L.) Byproducts Using Spectrophotometric Assays, ATR-FTIR and Liquid Chromatography–Tandem Mass Spectrometry Analysis

The global demand for saffron (Crocus sativus L.) generates a large number of floral bio-residues, presenting an environmental challenge but also an opportunity for value-added byproduct recovery within a circular economy framework. This study aimed to optimize the ultrasound-assisted extraction (UAE) of phenolic compounds from Krokos Kozanis (Greek saffron) tepals and its post-hydro-distillation solid residues. A 27-run Box–Behnken design (BBD) was employed to evaluate the effects of ethanol content (%, v/v) in the extracting solvent, extraction time (min), solvent-to-material ratio (mL/g), and ultrasound power (%) on the total phenolic content (TPC). Optimal conditions were determined as follows: 20% (v/v) ethanol, 10 min extraction time, 20 mL/g solvent-to-material ratio, and 80% ultrasound power. Applying these parameters, fresh tepals and post-distillation residues yielded maximum TPC values of 3370.86 and 799.17 mg GAE/100 g dw, respectively. Antioxidant and radical scavenging activities (FRAP and ABTS assays) correlated strongly with TPC (r > 0.95). Several bioactive substances were revealed by the ATR-FTIR spectroscopy, including carotenoids, terpenes, phenolic compounds and carbohydrates. Furthermore, LC-MS/MS identified 24 phenolic compounds, predominantly flavonol glycosides like quercetin 3-O-sophoroside. Notably, the application of hydro-distillation as a pre-treatment promoted the recovery of distinct metabolites in saffron byproducts compared to the intact tepals. This optimized UAE process offers a rapid, green strategy for valorizing saffron waste into natural antioxidants.