Alkaline transesterification: the same apparatus as for the esterification reaction was used. the following reaction conditions were selected: 1wt.% NaOH, methanol amount corresponding to a 6:1 methanol to oil molar ratio,temperature of 65C and 1.5h of reaction. Two steps of 45 min each were performed. the amount of methanolic solution used in the first step varied from 60 to 90vol.% (corresponding to the addition of 40% to 10% of solution in the second step. respectively) according to the experimental planning previously described (section 2.1). After the first reaction, the glycerol was separated and the rest of the methanolic solution was added to perform the second stage of reaction for another 45 min.