The efficiency of the reaction diminishes with the increase of
the acidity of the oil; basic transesterification is viable if the content of
free fatty acids (FFAs) is less than 2%.
In the case of highly acidic raw materials (animal fats from
cattle, poultry, pork; vegetable oils from cotton, coconut, most used
oils, etc.) an acid transesterification is necessary as a preliminary
stage, to reduce the level of FFAs to the above-mentioned value.