Use of natural extracts with antioxidant properties in biodiesel: optimization of extract formulation applying the simplex centroid method

Natural extracts with antioxidant properties can be used as sustainable alternatives compared to synthetic products, as they can inhibit the biodiesel oxidation reaction, resulting in increased oxidative stability, measured by the Rancimat method, and reducing the reaction rate, which is a key advantage in this study. This research evaluated the efficiency of ethanolic extracts from cayenne pepper leaves (Capsicum annuum), etna pepper leaves (Capsicum frutescens), red pitaya peels (Hylocereus costaricensis) and purple heart leaves (Transdescantia pallida purpurea) through the optimization of the biodiesel oxidation reaction rate at 110 °C using an experimental mixture design. All extracts showed antioxidant activity when compared to the control sample, reducing the reaction rate. However, the mixture containing extract of etna pepper leaves (Capsicum frutescens) and red pitaya peels (Hylocereus costaricensis) was the one that presented the lowest experimental value of k (0.1358 h-1). The optimization of the mathematical model indicated that the most effective mixture of antioxidant compounds for reducing the oxidation reaction consisted of 57.14% etna pepper extract (Capsicum frutescens) and 42.86% red pitaya peel extract (Hylocereus costaricensis), resulting in a reaction rate of 0.1396 h-1. The application of the t-test for a simple sample showed that there was no significant difference (p = 0.8777) between the optimized value and the average value experimentally obtained under optimal conditions (k = 0.1402), validating the predictive equation obtained. All ethanolic extracts used showed antioxidant activity and could be sustainable alternatives when compared to synthetic products.