Upcycling lemon peel waste into sustainable bioactive ingredients: chemical characterization and efficacy assessment

The upcycling of agro-industrial by-products represents a sustainable strategy to reduce waste, generating high-value bioactive compounds. In this study, exhausted Citrus limon peels, which are residues generated as a by-product of limoncello production, a traditional liqueur of Southern Italy, were investigated as a potential source of bioactive compounds using supercritical CO2 extraction. The resulting extracts were chemically characterized by gas chromatography coupled with mass spectrometry, revealing a complex phytochemical profile mainly composed of monoterpenes and oxygenated monoterpenes. Two representative extracts (EO-A and EO-B) were selected based on the extraction yield, chemical profile, and safety considerations. Evaluation in human keratinocytes (HaCaT cell line) demonstrated their high biocompatibility, with cell viability exceeding 90% at all tested concentrations. Antioxidant properties were assessed through ABTS and DPPH radical scavenging assays, showing significant activity, particularly for EO-B. Both extracts also exhibited antimicrobial activity against selected Gram-positive and Gram-negative bacterial strains, including Salmonella typhi and Escherichia coli. The essential oils exhibited inhibitory effects on collagenase and elastase, suppression of melanogenesis, and activity against Cutibacterium acnes, highlighting their potential for anti-aging, skin-brightening, and antimicrobial applications. Preliminary clinical evaluations indicated improvements in skin hydration, softness, and elasticity. Overall, these findings support the use of supercritical CO2 extraction as a green, sustainable, and highly efficient technology, coupled with a well-established and widely recognized analytical technique, for converting lemon peel waste into multifunctional and eco-sustainable cosmetic ingredients.