Fractional distillation of guayule essential oils: Volatile characterization and antioxidant activity

The essential oils (EOs) of guayule (Parthenium argentatum A. Gray) contain up to 50 different volatile compounds, making them a potentially valuable resource. While it is challenging to attribute specific bioactive effects to individual components, fractionation can enhance targeted activities. In this study, a guayule EO (G_EO1) was fractionated at the laboratory-scale into three groups with distinct chemical profiles: G_EO2, primarily composed of monoterpenes (97 %); G_EO3, enriched in sesquiterpenes (76 %); and G_EO4, consisting almost entirely of sesquiterpenes (97 %). Fractions G_EO3 and G_EO4 exhibited the strongest antioxidant activity against both test (DPPH and ABTS), as indicated by their low IC₅₀ values, 55.5 and 78.9 mg mL⁻¹ from DPPH and 4.66 and 4.16 mg mL⁻¹ for ABTS respectively. Comparing these fractions with four commercially available EOs with known antioxidant activity revealed that G_EO4 and Amyris balsamifera EO possess the greatest antioxidant capacity, being four sesquiterpenes (humulene, D-germacrene, bicyclogermacrene and δ-cadinene) and an oxygenated sesquiterpene (γ-muurolene), the volatiles with higher contribution. To explore industrial-scale EO fractionation, four guayule batches was steam-distilled up to 2 h, using two steam supplies (150 kg h⁻¹ and 200 kg h⁻¹) resulting in 0.24 and 0.27 % extraction yields. Chemical composition extraction yields of monoterpenes and sesquiterpenes were similar, the oxygenated sesquiterpene content was significantly higher at the end of the 120-min distillation process when using a greater steam supply. These findings suggest that fractional distillation of guayule EO, both at laboratory and industrial scales, offers a future potential to produce fractions with targeted bioactivities to be considered in various industries, including food, agricultural (phytosanitary), cosmetics, pharmaceuticals, and others. Such new applications will enhance the development of its cultivation and its industrial exploitation.