Sodium hydroxide anhydrous is used as a catalyst for the transesterification reactions of methanol and triglycerides.

Methanol is used as: biodiesel transesterification,fuel Cell Hydrogen Carrier,wastewater Denitrification and Transportation Fuel.

Potassium methylate is 98% methanol with elemental potassium used in transesterification of vegetable oils into methyl ester (biodiesel).

esterification ----->

ROH + R'-COOH -------> R'COOR acid with alcohol gives ester

transesterification

ROH + R'COOR'' -------- > R'COOR change of alkyl group present with R'COO by using a alcohol

if you want to http://en.wikipedia.org/wiki/Transesterification

Esterification is the general name for a chemical reaction in which two reactants (typically an alcohol and an acid) form an ester as the reaction product.

H3C-COOH + HO-CH2-CH3 → H3C-COO-CH2-CH3 + H2O

Transesterification is the process of exchanging the alkoxy group of an ester compound with another alcohol. These reactions are often catalyzed by the addition of an acid or base.

May be because it gives more volatile esters, easy to separate or concentrate or strip off.