2. Mangosteen Peel Antioxidant Extraction and Its Use to Improve the Stability of Biodiesel B20 Oxidation

Antioxidants can be extracted from mangosteen peel with ethanol as solvent using microwave assisted extraction (MAE) efficiently and economically. The mangosteen peel antioxidant can be used to inhibit the biodiesel B20 oxidation. The microwave power gives a great factor of antioxidant conversion in mangosteen peel extraction. At 35 min and 300, 450, 600 W, the antioxidant conversions obtained were 15.45, 17.00, 18.33%, respectively. The total phenolic concentration was about 156–202 mg GAE/g. In addition, the extraction kinetic can be quantitatively described by antioxidant diffusivity from inside the solid to the solid’s surface and antioxidant mass transfer from the solid’s surface into solution with diffusion coefficient (D _e) of 2.81 × 10⁻¹¹, 3.42 × 10⁻¹¹, 3.8 × 10⁻¹¹ cm²/s, mass transfer coefficient (k _c) of 6.36 × 10⁻⁸, 8.97 × 10⁻⁸, 1.05 × 10⁻⁷ cm/s for 300, 450, 600 W, respectively, and Henry equilibrium constant (H) of 0.032. In the oxidation, the mangosteen extract antioxidant can improve 26.32% of the oxidative stability of biodiesel B20. Theoretically, the performance of mangosteen peel extract antioxidants in biodiesel B20 oxidation can be evaluated from its oxidation kinetic which can be approached using the pseudo-homogeneous first-order model. The reaction rate constant follows the Arrhenius equation with activation energy (E _a) of 54.34 and 56.27 kJ/mol as well as collision factors (A) of 348,711 1/min, for the oxidation of biodiesel B20 and the mixture of biodiesel B20 and mangosteen peel extract antioxidant, respectively. The activation energy of the mixture of biodiesel B20 and mangosteen peel antioxidant was higher, so that the mixture of biodiesel B20 and antioxidant is more difficult to oxidize.