• Requires high reaction temperatures.

• Soap formation: in the base-catalyzed transesterification process, free fatty acid (FFAs) level of feedstock should be less, otherwise it will result in too much soaps formation.

• Recovery of by-product: purification of glycerol is very difficult.

• Pretreatment step needed: FFAs level of the feedstocks should not exceed 3 wt%, beyond which it has to undergo pretreatment steps before transesterification (Leung et al., 2010).

• Yield of methyl esters: yields of the methyl esters are lower compared to enzymatic transesterification.

• Purification of methyl esters: purification of methyl esters requires repeated washing which increases process operational cost.

• Less active: since alkali catalysts (NaOH and KOH) are inexpensive, they are preferred but activity is less (Demirbas, 2008).

• Energy consumption: alkali-catalyzed transesterification needed large energy consumption during downstream biodiesel refining process (Madras and Kolluru, 2004).

• Corrosion: when H2SO4 is used as catalyst, it leads to corrosion of the reactor and huge wastewater generated during neutralization of mineral acid (Atadashi et al., 2013).

• Use of homogenous catalysts makes biodiesel product separation difficulty and recovery of catalyst cumbersome (Atadashi et al., 2013).

• Acid-catalyzed transesterification reaction needs higher alcohol-to-oil molar ratios (Atadashi et al., 2013).

• In base-catalyzed transesterification reaction, large amount of catalyst is needed.

Difficulties arise during chemical catalysis can be overcome by enzyme-mediated (biocatalysts) transester­ification and they are becoming increasingly important in biodiesel preparation due to their ability to beat chem­ical catalysts. Lipases (E. C.3.1.1.3) are widely considered as biocatalysts to catalyze transesterification and esteri­fication reactions.