Effects of Natural Antioxidants on the Oxidative Stability of Winterized Biodiesel

This study investigated the oxidative stability of palm oil biodiesel following winterization via progressive freezing and subsequently evaluated the effectiveness and synergistic behaviour of natural antioxidants in stabilizing the winterized fuel. Progressive freezing at 6 °C, 1500 mL min–¹ and 20 min reduced the saturated fatty acid content from 41.08 to 31.31 wt%, lowering cloud point and pour point from 16.04 to 9.12°C and from 10.00 to 5.40 °C, respectively. However, the reduction in saturated species increased susceptibility to autoxidation, evidenced by the decrease in induction period (IP) from 2.40 to 0.83 h. The incorporation of natural antioxidants was therefore evaluated, where α-tocopherol and quercetin were added individually and in binary combinations. Individually, quercetin exhibited stronger radical-termination capacity than α-tocopherol due to its lower bond dissociation energy and multiple phenolic OH groups. Binary mixtures demonstrated non-linear behaviour, where synergism was only observed at ≥400 ppm total concentration with ≥5:1 quercetin:α-tocopherol ratio. Under these conditions, IP exceeded both ASTM D6751 (≥3 h) and EN14214 (≥8 h), implying industrial feasibility at realistic additive loadings. Overall, quercetin-dominant natural antioxidant blends offer an effective oxidative stabilization strategy for winterized palm biodiesel.