Stern et al. (1995)patented a method to make fatty acid esters from acid oil. The core of his method was to recover free fatty acids in the oil by transesterifing them with glycerol to form glycerides. After transesterification, a large portion of the glycerol was mixed with the ester wash water, then neutralized with acid. The salt was filtered and the alcohol evaporated. The separated free fatty acids reacted with the non-neutralized glycerol phase at about 200°C. The triglycerides (acidity of 3.2%) from the reaction were added to the next alcoholysis step. The ester obtained from the “starting oil plus glyceride” had a density of 880 kg/m3, a flash point of 185°C, a flow point of −12°C, a filterable limit temperature (FLT) of −18°C, a neutralization number of 0.5% mg KOH/g and a methyl ester content >98%. It was suggested that it could be used as a substitute for gas oil.