## 3 Transesterification for Producing Biodiesel from Animal Fats  
The major steps in the production of biodiesel from animal fat waste are shown in Figure 1. A pretreatment is needed because feedstocks like animal fats usually contain a high amount of free fatty acids (FFA) and water which reduces the yield of biodiesel [41] and increases production costs because of the difficulty of separation and purification [42; 43]. Biodiesel is produced through the transesterification reaction of a fat with a short-chain alcohol in the presence of a catalyst. Different catalysts are available to be used for biodiesel production. Those most typically used in transesterification reactions are alkalis (sodium hydroxide, sodium methoxide, potassium hydroxide, potassium methoxide, sodium amide, sodium hydride, potassium amide and potassium hydride), acids (sulfuric acid, phosphoric acid, hydrochloric acid or organic sulfonic acid), heterogeneous catalysts like enzymes (lipases) and complex catalysts like silicates, zirconias, nanocatalysts, etc. [44]. A faster reaction rate of animal fats transesterification is obtained using alkali catalysts in comparison to acid catalysts [45; 46] which are 4000 times faster [47] as well as less expensive and readily available [48]. Sodium and potassium hydroxides run quite well, and methoxides perform better but are more expensive [47]. Polyol-derived alkoxide base catalysts were prepared by heating aqueous sodium hydroxide solution and polyols under vacuum pressure [49] and potassium glyceroxide catalysts were produced from potassium hydroxide and non-volatile, non-toxic polyols like byproduct glycerol by heating and drying, making it cheap to produce. Furthermore, the rate of transesterification reactions in methanol was reported to be comparable to those observed for the conventional potassium methoxide catalyst under the same reaction conditions [50]. On the other hand, the use of acid catalysts for a transesterification reaction results in slower reaction rates and requires a higher alcohol to fat molar ratio and a larger reactor that may be subject to corrosion [51]. Acid catalysts are mainly used for reducing the free fatty acids content before its transesterification with alkaline catalysts [47]. The most commercially used short-chain alcohol for the transesterification reaction is methanol because of its cheap price, but other alcohols such as ethanol, propanol and butanol may also be used [52]. Transesterification consists of the conversion of triacylglycer overlools to diacylglycerols, releasing one fatty acid. Then, diacylglycerols are converted to monoacylglycers, releasing a second fatty acid and, finally, monoacylglycerols are converted to glycerol, releasing a third fatty acid. In general, transesterification has a high conversion efficiency and low cost [53]. However, once a pretreatment has been performed [54], the efficiency of the transesterification reaction depends on many variables like the time and temperature of the reaction, type and molar ratio of alcohol, type and amount of catalyst used, the amount of water present in the reaction media, the composition of fatty acids and the release of free fatty acids to the reaction media. In industrial processing plants, approximately 100 kg of fat react with 10 kg of a short-chain alcohol (usually methanol) in the presence of an alkaline