Acid oil is a by-product in vegetable oil refining, and contains free fatty acids (FFA) and acylglycerols mainly. An enzymatic method for conversion of acid oil to fatty acid methyl esters (FAME) was established; a two-step reaction system comprising methyl esterification of FFA and methanolysis of acylglycerols using immobilized Candida antarctica lipase. An acid oil model, which is a mixture of equal amount of refined FFA and vegetable oil, required 2 molar amounts of methanol (MeOH) to FFA and 0.5% lipase for the first-step reaction. After removing generated water and MeOH from the first-step product, the second-step reaction was conducted using equimol of MeOH to fatty acids in triacylglycerols (TAG) and 6% lipase. The two-step reaction achieved >98% conversion and the lipase could be used for >100 days when each step of reaction was repeated at 30^oC every 24 h.
  Acid oil (78% FFA, 11% AG) was converted to FAME under the modified condition; the first-step reaction required 5 molar MeOH to FFA, whereas the second-step required addition of vegetable oil, glycerol, and equimolar MeOH to fatty acids in acylglycerols. The content of FFA and TAG in acid oil reduced to <1% and that of FAME reached 91%. The composition was maintained for >100 cycles.
  The two-step system, established in a lab-scale, was conducted in a 30-L scale pilot plant. The reaction time courses were similar in both scales. The two-step system using immobilized lipase was thus considered to be applicable for industrial production of biodiesel fuel from acid oil.