Enzymatic transesterification for biodiesel production from used cooking oil, a review

This paper reviews various aspects of the enzymatic transesterification method to convert used cooking oil to biodiesel. The goal of this paper is to provide a thorough overview from general biodiesel production processes, reaction conditions, challenges, and solutions for higher biodiesel production yield through introducing various microorganisms that are capable of producing the enzymes required to convert used cooking oil into biodiesel. The characteristics, composition, and advantages of the used cooking oil, as feedstock for biodiesel, is also discussed. In addition, the existing transesterification methods including homogeneous alkali-catalyzed, homogeneous acid-catalyzed, non-catalytic reaction under super-critical conditions, and enzyme-catalyzed reactions are explained. Furthermore, the advantages of the enzymatic method over other methods, and the enzymes, which are the key elements of such reactions, are discussed. Lipases are the most promising enzymes currently known for biodiesel conversion. The physiological and physical properties of microbial lipases, the catalytic mechanisms of the enzymes, various methods of enzyme immobilization such as adsorption, covalent and affinity binding, entrapment, and the whole-cell immobilization are also reviewed. At the end, three case studies demonstrating unique and efficient enzymatic transesterification approaches are presented. (C) 2019 Elsevier Ltd. All rights reserved.