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Propolis is a natural source of bioactive compounds, but its conventional formulations are predominantly based on ethanol or other synthetic solvents, which limits their application in certain consumer and pharmaceutical products. This study aimed to develop and characterize a natural, alcohol-free oil formulation of propolis using orange peel essential oil as a functional terpene-rich solvent. Orange peel, an agro-industrial by-product, was subjected to hydrodistillation to obtain the essential oil, which was subsequently characterized by GC–MS. The obtained orange peel oil was dominated by D-limonene (97.09%), accompanied by smaller amounts of β-pinene (1.69%), α-pinene (0.45%), sabinene (0.43%), 3-carene (0.22%), and valencene (0.08%). The propolis was dissolved in orange peel oil and the formulation was further optimized by incorporating grape seed oil as a natural carrier and diluent. HPLC analysis confirmed the presence of citral in the propolis–orange peel oil formulation at 0.944 mg/mL, while p-coumaric and ferulic acids were detected at 0.109 mg/mL each. Thermal analysis demonstrated that incorporation of grape seed oil substantially improved the thermal stability of the final formulation compared with pure orange peel oil and the propolis–orange peel oil system. Antimicrobial testing further demonstrated selective activity of the formulations against Staphylococcus aureus and Candida albicans. The developed formulation provides a natural, alcohol-free approach for incorporating propolis bioactive compounds into an oil-based system while simultaneously valorizing orange peel agro-industrial waste. The proposed product has potential applications in food supplements, cosmetics, and other natural bioactive formulations.
Keywords: propolis, D-limonene, orange peel waste, essential oil-based formulation, bioactive compounds, HPLC analysis, thermal stability, antimicrobial activity.
Acknowledgements: This work was supported by the Ministry of Science, Technological Development and Innovation of the Republic of Serbia (Contracts No. 451-03-33/2026-03/200135 and 451-03-33/2026-03/200287) and the Western Balkans Innovation Voucher Programme (WBIV Application No. 09-2789; Contract No. 09-1007), within the Horizon Europe SPIN project (Grant Agreement No. 101058873).