Chemical diversity and correlation analysis of volatiles from Citrus peels: Evaluation of toxicity, antioxidant and microbial inhibition efficacy

Abstract

Peel essential oils from selected Citrus species viz., C. aurantifolia, C. limon, C. maxima, C. medica and C. medica*C. limon (hybrid) were analysed by Gas Chromatography-Flame Ionisation Detector (GC/FID) and Gas Chromatography-Mass Spectrometric (GC/MS) methods. Volatile constituents of these oils were further assessed for their antioxidant and antimicrobial activities. Monoterpene hydrocarbons ($60.8\pm 0.20$%to $88.6\pm 0.09\%$) are the major volatile category present in the selected essential oils. D-limonene, a monoterpene hydrocarbon was the key component in the analysed Citrus peels $(31.9\pm 0.51$% to $85.6\pm 0.60\%)$. Correlation analysis revealed the relationship between various volatile constituents and interrelationship between the five Citrus peels. The selected essential oils were initially screened for their antioxidant potential by DPPH free radical scavenging and the results showed promising radical scavenging potential with an IC₅₀ magnitude of$41.8\pm 0.19$μg/mL. Antimicrobial assays showed that Citrus peel volatile oils shown significant activity against various pathogenic bacterial and fungal strains conducted. MIC values ranged from $7.5\pm 0.01$to $55.5\pm 0.24$mg/mL for gram-negative bacteria and$5.6\pm 0.02$ to $25.3\pm 0.03\mathrm{mg}/\mathrm{mL}$ for gram-positive bacteria. A significant antifungal activity of Citrus peel essential oils was observed against Aspergillus niger and Candida albicans, with a minimum inhibitory concentration from $5.3\pm 0.04$ to $30.4\pm 0.02$mg/mL. Significant antimicrobial and antioxidant effect of these peel essential oils was attributed mainly due to the synergistic potential of monoterpenoid volatiles present in the oils viz. D-limonene, γ-terpineol, neral, geranial, geraniol and citronellal. Results pointed out the low toxicity, high antioxidant and antimicrobial activities of essential oils from selected Citrus variants have the potential to treat oxidative damage and microbial infections.