The objective of this study was to evaluate the eugenol antioxidant efficiency on the oxidation stability of commercial biodiesel synthesized from soybean oil (SB) and waste frying oil (ORB) by determining induction period (IP) and acid number (AN) during storage at 10 °C or in accelerated oxidation test at 85 °C. For the oxidation stability evaluation, the Rancimat method (EN 14112) and AN (ASTM D664) analyses were used. The initial studies showed that the antioxidants eugenol and TBHQ when added separately increased IP values for the two types of biodiesel analyzed. During the storage at 10 °C, the IP was reduced for all samples. However, the lowest reduction percentages were for samples containing eugenol (-2.07 and -11.30% for SB and ORB, respectively). In relation to AN, the samples with the antioxidant eugenol led a greater decrease of this index, with the 10000 mg kg-1 concentration being the most efficient in the conservation of biodiesel. In the accelerated oxidation test in oven at 85 °C, pure BS presented higher susceptibility to degradation than the sample containing 10000 mg kg-1 of eugenol. In both storage studies, the eugenol natural antioxidant efficiency in the control of oxidative degradation of biodiesel becomes evident.
