Inhibition of the biodiesel oxidation by alcoholic extracts of green and black tea leaves and plum pulp: Application of the simplex-centroid design

The efficiency of extracts with antioxidant properties of plum pulp, black tea and green tea leaves was evaluated, analyzing the induction periods (IP) and rate constants (k) at 110 °C, in the 1st, 30th, 57th an 91st day of storage. Antioxidant activity was observed in all extracts, either by the rat constant decreased or by the induction period increase. The higher induction period obtained was 9.15 h for the black tea leaves extract while the lower was 6.97 h for the green tea leaves extract. The control had an average induction period of 6.95 h for the first day. The models obtained are significant and showed determination coefficients greater than 0.90 with no significance lack of fit at the level of 5%. The optimization with maximization of the induction period and minimization of the rate constant showed that the black tea extract was the most suitable to avoid oxidation. On the first day of test, the optimization indicates the mixture containing 50% of black tea and 50% of plum pulp extract as the best mixture. The optimizations provided a variation of 8.92 < IP (h) < 9.69 and 0.25 < k (h−1) < 0.38.