Page 61 - Refining Biomass Residues for Sustainable Energy and Bioproducts

36 Refining Biomass Residues for Sustainable Energy and Bioproducts

produce FAEEs, which are having better fuel properties than FAME (Ro ¨ttig et al.,
2010). De novo microbiodiesel or FAEE production strategy was first time devel-
oped in 2006, from distinct carbon sources in strain E. coli (Kalscheuer et al.,
2006). After that various studies have been conducted to overexpress the WS or
DGAT gene and combined with the ethanol biosynthesis pathway of Zymomonas
mobilis, which encodes alcohol dehydrogenase B (adhB) and pyruvate decarboxyl-
ase (pdc). Similar approaches applied for the overproduction of FFA involve
increasing the fatty acid de novo biosynthesis and blocking the β-oxidation path-
way, which could promote the FAEE production in more cutting-edge researches
(Table 2.4). So far, a small amount of FAEE (1.5 g/L) is produced using glucose as
a carbon source at a small scale. E. coli inefficiently metabolizes lignocellulosic
biomass; to overcome such problems, the first step is to construct a robust strain,
which can use switchgrass hydrolysate as carbon source (Steen et al., 2010).
Another encouraging tactic is the biosynthesis of branched chain fatty acid alkyl
esters and short chain length alcohols such as isoamyl alcohol or isobutanol from
branched chain fatty acids, by manipulating the pathway of branched chain amino
acid biosynthesis. FAEE shows better fuel properties and performance at low tem-
peratures, as compared to traditional biodiesel (Tao et al., 2015). Nevertheless, the
production of branched FAEE still needs some improvements. Alternatively, bacte-
rial strain Streptomyces G25, which naturally accumulate branched chain fatty acids
into TAGs, could be a promising isolate for the production of biodiesel having both
residues of straight and branched chain fatty acid.

2.4 Transesterification reaction

Biodiesel is produced by transesterification reaction in which triglyceride and alco-
hol react in the presence of a catalyst (Kumar et al., 2017b). This comprises a series
of three sequential reversible reactions where at first triglycerides are transformed
into diglycerides that are further converted to monoglycerides, and finally the trans-
formation of monoglycerides into glycerol takes place. In each and every step, one
ester molecule is synthesized, and three FAME molecules are produced from single
molecule of a triglyceride (Sharma and Singh, 2008). The rate-determining step in
the overall transesterification reaction is the synthesis of alkyl esters from monogly-
cerides since they are the most stable intermediate compound (Ma and Hanna,
1999). The transesterification reaction involves a catalyst (acid or alkali), which
splits the fatty acid molecule and an alcohol either methanol or ethanol to react
with the separated esters (Kumar et al., 2018b). Transesterification is a well-known
viable and feasible method for the production of biodiesel as compared to others, as
it lowers down the viscosity of the end product (Demirbas, 2009). The end product
of this process also includes glycerol, which has high economic value and several
applications. Among all the existing processes, transesterification/this process is rel-
atively simple and preferable as the physiochemical characteristics of the produced
biodiesel are adjacent to conventional diesel fuel. The esterification of fatty acid