When the fish oil presents high acidity, an acid esterification pre-treatment might be performed.
In a study by El-Mashad et al.an acid esterification using 1 wt.% H2SO4 and a reaction temperature around 52 C, during 1 h, was performed (molar ratio methanol:oil of 6:1, 600 rpm stirring) to reduce the acid value of a salmon oil (3.5 or 12 mg KOH g1) to values acceptable for alkaline transesterification (considering a maximum of 2 mg KOH g1).