Yield of biodiesel through lipase catalysis is effected by (1) feedstock quality, (2) choice of enzyme (extracel­lular or intracellular), (3) molar ratio (alcohol/oil), (4) temperature, (5) water content, (6) acyl acceptors, (7) sol­vent and (8) reactor system.

FEEDSTOCK

One of the main barriers for commercialization of bio­diesel production is choice and availability of feedstock, which comprise nearly 80% of the overall biodiesel pro­duction cost. Diverse kinds of feedstocks are available such as edible and nonedible vegetable oil, animal fats, waste oil, microbial oil and microalgae oil and they can be used for enzyme-catalyzed transesterification (Sevil et al., 2012).

Vegetable Oils

Vegetable oils are well-known for their high heat con­tent and they are alternative fuels for diesel engines. High viscosity restricts their consumption directly in diesel en­gines, which leads to many problems (Koh and Ghazi, 2011; Singh and Singh, 2010). Most widely used edible vegetable oils in enzymatic transesterification are soy­bean (Wenlei and Ning, 2010; Du et al., 2003), sunflower (Karout and Pierre, 2009), palm (Talukder et al., 2011; Matassoli et al., 2009), corn (Mata et al., 2012), cottonseed (Chattopadhyay et al., 2011), canola (Jang et al., 2012) and olive (Sanchez and Vasudevan, 2006). Higher quality of edible oil is good feedstock to produce biodiesel by enzy­matic transesterification. However, major concern is the economic viability of biodiesel since refined vegetable oils are expensive. Also, use of high-value edible vege­table oil as biodiesel feedstocks has caused food crisis. Furthermore, percentage of oil and yield per hectare are effective parameters in selecting potential renewable feedstock for biodiesel production (Nielsen et al., 2008). Hence, in order to make biodiesel production more economical, low-cost and nonedible oils need to be preferred. Babassu (Orbignya martiana), Jatropha curcas (Linnaeus), neem (Azadirachta indica), polanga (Calophyl- lum inophyllum), karanja (Pongamia pinnata), rubber seed tree (Hevea brasiliensis), mahua (Madhuca indica and Madhuca longifolia), tobacco (Nicotiana tabacum), etc. are most widely used nonedible oil sources for biodiesel production. Biodiesel produced from these nonedible oils meets key specifications of biodiesel as per the stan­dard organization requirements (Mohibbe et al., 2005). All these low-cost feedstocks contain large amount of FFA which leads to undesirable soap formation during traditional base-catalyzed transesterification. However, high free-acid content is not a problem in enzyme transesterification.

Animal Oils/Fats

Animal oils differ from vegetable oil in their fatty acid composition. Vegetable oils have high content of unsat­urated fatty acids (mainly oleic and linoleic acid), while animal fat has higher proportion of saturated fatty acids. Commonly used animal fats for biodiesel production via enzymatic route contains lard (Jike et al., 2007), lamb meet, beef tallow, chicken fat and animal fat mix (Vivian Feddern et al., 2011). Waste animal fats from animal pro­cessing industries and slaughter houses are also a good source for animal fats; however it is a decent alternative instead of their direct dispose in to environment. Their favorable features like noncorrosive nature, high cetane number, and renewable nature makes them a good source for biodiesel production. But their relatively high FFA (5—30%) and water content led to soap forma­tion in chemical transesterification process and their saturated fats prone them to oxidation and crystalliza­tion at high temperatures (Huynh et al., 2011). Removal of contaminants is another problem from animal fats, which generally contain phospholipids, or gums, and cause insoluble precipitates when they come into con­tact with water. Gums are removed by adding water and citric or phosphoric acid to the animal fats followed by centrifugal separation of precipitates. Phospholipids get separated with glycerin during processing, or by wa­ter washing/ion exchange separation. Removal of sulfur contents is also a serious issue. Beef tallow and some chicken fat contain around 100 ppm of sulfur. Vacuum distillation is the only reliable technique for reducing the sulfur level to permissible levels (15 ppm) (Farm energy, 2012).