Kinetic and Spectroscopic Evaluation of β-Carotene and α-Tocopherol Degradation in Fatty Acid Methyl Esters

The limited oxidative stability of fatty acid methyl esters (FAME) constrains their use as biofuels. This study presents a direct, side-by-side comparison of β-carotene and α-tocopherol as natural antioxidant additives in FAME produced from refined rapeseed oil, evaluated under identical thermo-oxidative conditions (100–140 °C). We combine kinetic modelling (first-order and zero-order fits, Arrhenius analysis) with spectroscopic monitoring (UV–Vis for β-carotene; FT-IR for α-tocopherol) and standard oxidation indices (PV, AnV) to link antioxidant depletion to fuel oxidation. Key findings are: (1) β-carotene effectively delays hydroperoxide formation at lower temperatures but degrades rapidly above 120 °C (Ea 6–23 kJ·mol−1), producing secondary products that increase AnV; (2) α-tocopherol shows greater thermal resistance and predictable, dose-dependent protection across the tested range (optimal at 556 µg·mL−1), with higher doses exhibiting potential pro-oxidant effects; (3) activation energies and kinetic orders differ between antioxidants, indicating distinct degradation pathways in the FAME matrix. These results demonstrate that reintroducing natural antioxidants removed during refining can improve biodiesel durability, and that antioxidant selection and dosing must be tailored to expected thermal exposure. The combined kinetic–spectroscopic approach provides a practical framework for optimizing natural additive strategies in biodiesel formulations.