Physicochemical and In Vitro Biological Characterization of Usnea barbata Extract in Karanja Oil for Potential Applications in Skincare

Plant extracts in vegetable oils are foundational and eco-responsible for skin care, combining their emollient properties with other additional benefits, derived from their antioxidant, antimicrobial and UV-absorbing activity. The present research conducted a complex investigation of Usnea barbata extract in Karanja oil (KO), aiming for its further incorporation into various cosmetic formulations. The lichen extract (UBKO) was obtained through cold maceration. Phytochemical screening was performed using the Folin–Ciocalteu method and Graphite Furnace Atomic Absorption Spectrophotometry (GFAAS). Physicochemical properties were evaluated via Fourier Transform Infrared Spectroscopy (FTIR) and Atomic Force Microscopy (AFM). The rheological behavior and oxidative stability of the oil samples, UBKO and KO, were also investigated. UBKO had a slightly lower density (0.827 vs. 0.955) and pH (4.22 vs. 4.86) than KO, and a slightly higher oxidative resistance, quantified as the induction period (IP) value (6.45 vs. 6.00). The total phenolic-equivalent content (TPC, µg GAE/mL oil sample) was significantly greater in UBKO than in KO (567.16 ± 14.96 vs. 433.26 ± 22.96, p = 0.001). The values of minimum inhibitory concentration (MIC, mg/mL) indicated significantly higher antibacterial effect against S. aureus and antifungal effect against C. albicans for UBKO than KO (9.62 ± 2.87 vs. 31.25 ± 18.75, p = 0.049, and, respectively, 5.06 ± 1.68 vs. 37.50 ± 12.50, p = 0.01). Finally, our results showed that UBKO had an estimated sun-protective factor (SPF) of 30.9, slightly higher than 29.8 for the base oil formulation, KO; these findings represent baseline in vitro UV-absorbing trends. All of these results suggest that U. barbata extract in Karanja oil may exhibit complementary bioactive properties with potential applications in skincare.