Postharvest preservation of Kinnow mandarin (Citrus reticulata L.) using oregano essential oil nanoemulsions with enhanced biochemical stability and antifungal efficacy against Penicillium digitatum

Biochemical degradation occurs in fruits stored under poor conditions. It negatively affects physical properties, such as color and firmness, while also reducing key nutrients, including antioxidants, phenols, vitamins, and flavonoids. This study explores the use of edible coatings as a postharvest treatment to prevent this degradation and minimize the color change in citrus. Specifically, it focuses on nanoemulsions of oregano essential oil (OEO NE), as a novel coating agent with a controlled-release delivery system that was stored for a 30-day duration (5 °C and 95% humidity), protecting citrus fruits from Penicillium digitatum contamination and quality degradation. Antimicrobial testing demonstrated that the minimum inhibitory concentration (MIC) and minimum fungicidal concentration (MFC) of OEO NEs were observed as 170 µg/mL and 350 µg/mL, respectively. Results showed that OEO NE-coated citrus had significantly lower physiological losses in terms of weight (PLW) (3.61%), total soluble solids (TSS) (10.94 °Brix), pH (4.34), titratable acidity (TA) (1.61%), and chilling injury (1 score) as compared to the control treatment. The key enzymes involved in fruit ripening and senescence, such as pectin methylesterase (PME) (1.25 µmol min−1 g−1 FW), lipoxygenase (LOX) (2.67 µmol min−1 g−1 FW), and catalase (CAT) (4.9 µmol min−1 g−1 FW), were significantly reduced in OEO NE-coated fruits. In addition, OEO NE-coated fruits retained higher firmness (5.67 N), ascorbic acid (28.94 mg L−1), total phenols (82.61 mg L−1), total flavonoids (12.94 mg L−1), total antioxidant activity (TAA) (25.99 µmol TE g−1 FW) and β-carotene (14.28 µg/g), as well as higher sensory scores compared to control and other coating groups. When combined with chitosan (CH) and carboxymethylcellulose (CMC) coatings, OEO NE exhibits better results than individual CH and CMC coatings. In conclusion, OEO NE treatment alone was significantly superior to CH, CMC, and combined coatings in preserving fruit quality and reducing fungal decay.