Geographical Variation in Ocimum basilicum Essential Oil Composition and Its Impact on Antioxidant Activity and DL-α-Tocopherol-Assisted Lipid Protection in Cosmetic Oil Formulations

Lipid oxidation is a major cause of instability in cosmetics containing vegetable oils, compromising their quality and durability. This study evaluates the in vitro antioxidant activity of two essential oils of Ocimum basilicum L., obtained from plants grown in Argentina (OBA) and Italy (OBI), and their contribution to the oxidative stability of sweet almond oil. The essential oils were characterized by GC-MS, revealing significant differences in chemical composition: linalool represents the major constituent in both samples, with a higher concentration in OBI (64.42%) compared to OBA (55.28%). A sharp contrast was observed in the levels of eugenol and 1,8-cineole: while OBA was characterized by a higher eugenol content (18.44% vs. 2.25%), OBI showed a more pronounced presence of 1,8-cineole (13.90% vs. 5.38%). Antioxidant activity was determined by DPPH and PCL assays, while the oxidative stability of the cosmetic oil was measured by OXITEST, evaluating the effect of DL-alpha-tocopherol combined with the two essential oils on oxidative stability. OBA, with its higher phenolic content, consistently outperformed OBI in antioxidant activity: the IC50 values from the DPPH assay were 43.3 ± 2.0 µg/mL (OBA) and 81.98 ± 1.98 µg/mL (OBI), while PCL values were 887.47 ± 6.01 and 363.82 ± 31.27 µmol Trolox equivalents (TE)/g, respectively. In the OXITEST model, the combination of 0.2% DL-alpha-tocopherol with 0.3% OBA yielded the highest oxidative stability (Induction Period = 1611 min), compared to 1114 min for the oil alone and 1508 min for the oil with tocopherol alone. This work provides new data on the functional use of basil essential oils in the oxidative stabilization of plant cosmetic oils, highlighting their potential use as natural antioxidants, in line with the increasing demand for clean label formulations and meeting the criteria of leading natural cosmetic standards.