Comparative Study of Plant-Based Antioxidants on the Stability of Soybean and Beef Tallow Biodiesels

Abstract

Biodiesel is a renewable alternative fuel, but its oxidative instability reduces quality and performance over time. This study explicitly aimed to investigate the efficacy of natural antioxidants—acerola, soursop, mango, and pitanga leaves—and compare their performance to the synthetic antioxidant butylated hydroxytoluene (BHT) in enhancing the thermooxidative stability of soybean ethyl and bovine tallow methyl biodiesels. Biodiesels were synthesized and stored under controlled conditions (ambient and 60°C) for up to 1848 h. Analytical results showed that biodiesel treated with BHT maintained acidity index (AI) values below 0.5 mg KOH/g after 1176 h, while untreated biodiesel exceeded 1.0 mg KOH/g within 504 h. Kinematic viscosity for BHT-treated samples remained under the acceptable limit of 6.0 mm²/s for 1176 h, compared with untreated biodiesel, which surpassed the limit at 504 h. Natural antioxidants exhibited varying degrees of efficacy, with soursop and pitanga extracts achieving 35%–50% improvements in oxidative stability compared with untreated biodiesel. Thermogravimetric analysis indicated that natural additives delayed the onset of degradation temperatures by 10°C–15°C. These findings demonstrate that plant-based antioxidants significantly enhance biodiesel stability, offering up to 70% of the effectiveness of BHT, and highlight their potential as sustainable alternatives for improving biodiesel quality.