Page 41 - Refining Biomass Residues for Sustainable Energy and Bioproducts

22 Refining Biomass Residues for Sustainable Energy and Bioproducts

renewable raw materials and waste in a cost-effective and sustainable manner (Li
et al., 2010; Aresta et al., 2013; Gnansounou and Pandey, 2017). The main stum-
bling block in the path of achieving the goal of sustainable development and
resource management is the extensive use of fossil fuels such as oil, coal, and natu-
ral gas (Venkata Mohan et al., 2016). The emerging concept of biorefineries can
overcome such problems as it involves the simultaneous production of biofuels and
bio-based products along with curtailing the environmental damage by managing
and utilizing the waste (Venkata Mohan et al., 2014; Abraham et al., 2015).
The basic biomolecules for the production of biofuels include fatty acids, alcohols,
and alkanes. The term “biodiesel” is generally used for the alternate energy source pro-
duced from renewable biomass and waste substances, and it is considered a substitute
or an ancillary for the conventional petro-diesel. Biodiesel is composed of monoalkyl
esters of higher chain of fatty acids, produced by the transesterification reaction of alco-
hol in the presence of catalyst (Kumar et al., 2016c). In general, oils or fats extracted
from various sources, such as cyanobacteria, algae, jatropha, palm trees, and soybeans,
have been used as precursors for the production of biodiesel (Schenk et al., 2008).
Virtually biodiesel can be produced from any organic oil source, which includes
restaurant waste oil, domestic waste oil, animal fats, and different seed oils. Owing
to the scarce supply of waste oil, the production of biodiesel from waste oil is very
limited, whereas at small scale for independent producers, it is very effective
(Demirbas, 2009; Schenk et al., 2008). Large-scale commercial producers conven-
tionally invest in oil extracted from seeds, such as soybean, rapeseed, palm, and
corn (Demirbas, 2009; Schenk et al., 2008; Demirbas, 2008). Unluckily, biodiesel
produced from seed oil is a cause of food versus feed debate concluding biodiesel
as a commercially more expensive energy source. The higher yield of bacterial bio-
mass and subsequently fatty acids (lipids, oil) could be a possible alternative to
reduce the raw material cost of biodiesel production.
Hydrocarbons (alkanes/alkenes) are the metabolic by-products of many bacterial
species and generally produced from fatty acids and triacylglycerol (TAG) (Bharti
et al., 2014a). Fatty acid trimesters of glycerol are called TAG, which shows differ-
ent properties depending on the fatty-acid composition. TAG as an energy-reserve
material is well known among eukaryotic organisms, such as yeast, fungi, plants,
and animals, whereas in bacteria, it is not much explored (Bharti et al., 2014b).
Only a few group of bacteria belonging to the Actinomycetes group, such as
Streptomyces, Nocardia, Rhodococcus, Mycobacterium, have been studied and
shown the enzymatic biosynthesis and accumulation of TAG (Alvarez et al., 2002)
(Fig. 2.1). The biosynthesis of TAG in such groups is achieved by using various
carbon sources, such as sugars, organic acids, alcohols, n-alkanes, branched alkanes,
phenylalkanes, oils, and coal (Alvarez, 2003). The excess availability of carbon and
limiting nitrogen condition in growth media trigger the biosynthesis of TAG inside
the microbes. Due to an impaired cellular growth in this situation, microbial cells
consume carbon source essentially for building up neutral lipids (Alvarez et al.,
2002; Kumar et al., 2016e). The biosynthesis of lipids takes place inside the bacte-
ria during its exponential growth phase using fatty acids as precursor molecules. In
the similar context the TAG synthesized by bacteria can be used as a starting mate-
rial for microbial production of biodiesel (Bharti et al., 2014b).