A method of esterifying free fatty acid in natural oil comprises heating the natural oil (2) to a first temperature above a reaction temperature, feeding the heated natural oil into an acid-resistant pipe reactor (5), providing a mixture of acid catalyst (6) and short chain alcohol (8) at a second temperature below the reaction temperature, and feeding the mixture of acid catalyst and short chain alcohol into the natural oil in the pipe reactor (5). The short chain alcohol and free fatty acid react at the reaction temperature to form an ester.