Phytochemical Profile and Antioxidant and Antifungal Activities of Mandarin (Citrus reticulata cv. “W. Murcott”) Peel Extract: A Sustainable Approach to Postharvest Green Mold Control

Mandarin peel (Citrus reticulata cv. “W. Murcott”) constitutes an agro-industrial byproduct rich in bioactive compounds with potential application in the postharvest control of fungal diseases. This study evaluated the effect of maceration time (24–96 h) and particle size (<500 and >500 µm) on total phenolic, flavonoid, and carotenoid content, as well as on antioxidant capacity (DPPH, ABTS, and FRAP) and antifungal activity against Penicillium digitatum. The 72 h/<500 µm treatment yielded the highest phenolic (15.17 ± 0.05 mg GAE/g dw) and flavonoid (17.87 ± 0.23 mg QE/g dw) content, together with the highest antioxidant capacity (DPPH: 9.73 ± 0.13; ABTS: 91.19 ± 0.49; and FRAP: 72.50 ± 0.90 µmol TE/g dw). HPLC-DAD identified hesperidin (35.89 mg/g dw) and naringin (12.42 mg/g dw) as the predominant phenolic compounds, whereas UHPLC-QTOF-HRMS/MS tentatively annotated 45 metabolites, including glycosylated flavonoids, phenolic acids, polymethoxyflavones, terpenoids, and limonoids. Against P. digitatum, the extract (12 mg/mL) produced an inhibition halo of 16.4 ± 1.1 mm and showed a minimum inhibitory concentration of 1.25 mg/mL. In “W. Murcott” fruit artificially inoculated with P. digitatum, the extract at 2.5 mg/mL reduced green mold incidence to 23.3 ± 8.5% and lesion diameter to 10.1 ± 1.7 mm by day 12, compared with 100% and 55.5 ± 3.9 mm in the untreated control. These findings support the potential of the extract for the postharvest management of P. digitatum and for the valorization of mandarin peel.