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Bacterial production of fatty acid and biodiesel: opportunity and challenges 39

content, and FFA content (Kumar et al., 2017b; Bharti et al., 2014a). It was
reported that increasing the reaction temperature and the oil-to-alcohol molar ratio
increased the yield of FAMEs (Ma and Hanna, 1999; Bharti et al., 2014a; Kumar
et al., 2016c).

2.4.1 Catalytic transesterification methods
Lipids and oils are converted into biodiesel at a certain temperature through the
transesterification reaction using excess methanol and acids or alkalis as catalyst.
As mentioned earlier, the used catalyst can be an alkali (Bharti et al., 2014a,b), acid
(Kumar et al., 2016c; Mondala et al., 2009), or enzyme (Shieh et al., 2003; Khosla
et al., 2017). Several researches have been performed for transesterification utiliz-
ing various lipids or oils as raw material, various alcohols along with several cata-
lysts, homogeneous (NaOH, KOH, and H 2 SO 4 ) and heterogeneous (lipases) in
nature (Kumar et al., 2016c, 2017a; Khosla et al., 2017).

2.4.1.1 Acid-catalyzed transesterification methods
In acid-catalyzed transesterification reactions, mostly methanolic sulfuric acid
(H 2 SO 4 )(Demirbas, 2005; Kumar et al., 2016c; Ma and Hanna, 1999), ferric sulfate
[Fe 2 (SO 4 ) 3 ](Wang et al., 2007), sulfonic acid (H 2 O 3 S) (Guerreiro et al., 2006),
methanolic hydrogen chloride (HCl) (Darnoko and Cheryan, 2000), and methanolic
boron trifluoride (BF 3 )(Rule, 1997) are used. In such reactions, sulfuric acid,
hydrochloric acid, and sulfonic acid are commonly used as catalysts. First, catalyst
is mixed properly with methanol by vigorous shaking in a small vessel. The acidi-
fied methanol is transferred to the transesterification vessel along with raw material
(lipids or oils) for transesterification. The acid-catalyzed reaction gives higher yield
of FAMEs, but the rate of reaction is slow. The alcohol/vegetable oil molar ratio is
one of the main factors that influence transesterification. The use of excess alcohol
increases the yield of FAMEs but simultaneously hampers the recovery of glycerol,
which makes the process economically challenging, so a proper oil-to-methanol
ratio is needed to optimize the process. The primary mechanism involves the forma-
tion of oxonium ion by the acid protonation, which produces an intermediate by the
exchange reaction with an alcohol, and forms ester after losing a proton. The over-
all process is reversible at each and every step, so the use of excess methanol will
shift the equilibrium toward the product side, and the reaction is completed.

2.4.1.2 Alkali-catalyzed transesterification methods
Generally in alkali-catalyzed transesterification, preferable catalysts are KOH or
NaOH. Initially catalyst is dissolved into methanol by vigorous shaking in a small
vessel, then an already prepared alkaline methanol is used in transesterification ves-
sel having oils. Following this, the reaction mixture is shaken vigorously for 2 h at
340K in ambient pressure (Demirbas, 2009). After the completion of reaction the