Page 230 - Refining Biomass Residues for Sustainable Energy and Bioproducts

Assessment of crude glycerol utilization for sustainable development of biorefineries 201

The accumulation of docosahexaenoic acid (DHA) in glyceric acid production can
be controlled by encoding sld A gene of glycerol dehydrogenase (Habe et al.,
2010). In the study by , in a fed-batch fermentation process, Gluconobacter frateurii
NBRC103465 and Acetobacter tropicalis NBRC16470 are capable of producing
more than 100 g/L of glyceric acid. G. frateurii NBRC103465 produced 136.5 g/L
of glyceric acid with a 72% D-GA along with a coproduct of DHA of 0.58 and
0.12 mol/mol glycerol, respectively, under optimized glycerol and aeration condi-
tions. Ninety percent yield of glyceric acid was produced from glycerol by using an
immobilized enzyme laccase produced from Trametes versicolor under the
optimized condition at 25 C for 24 h using 30 mM 2,2,6,6-tetramethyl piperidine

N-oxyl as a mediator (Hong et al., 2015).

9.4.1.2 Lactic acid

Lactic acid is one among the industrially important organic acid produced by lactic
acid bacteria. Lactic acid generally exists in D (1), L (2) or in racemic mixture
(1/ 2 ) form. The conventional chemical synthesis is known to produce a racemic
mixture, whereas biological fermentation was witnessed to yield all the isomeric
forms either individually or in a mixture. As lactic acid became an important build-
ing block in the chemical industry, its microbial production has advanced signifi-
cantly (Almeida et al., 2012). Many naturally occurring wild type microorganisms,
such as Escherichia coli, Klebsiella, Clostridia, Bacillus, and the filamentous fungi
Rhizopus oryzae, are capable of producing lactic acid from glycerol (Harvey and
Meylemans, 2011; Cheng et al., 2006; Bai et al., 2003).
The E. coli AC-521 strain has an ability to grow quickly and produces lactic acid
on utilizing glycerol as substrate under aerobic condition. The maximum yield and
productivity of lactic acid achieved by this strain are 0.9 mol/mol and 0.49 g/g/h,
respectively (Hong et al., 2009). Chen et al. (2015) investigated that lactic acid can
be produced from glycerol using sodium hydroxide (NaOH) a homogeneous cata-
lyst in fed-batch process. In this process, 93 mol% of glycerol is converted to yield

of 82% of lactic acid under optimized reaction temperature at 300 C at reaction
time of 220 min with a feed inlet of 1.1 M glycerol. Ftouni et al. (2015) used an
inert catalyst Pt/ZrO 2 for conversion of glycerol into lactic acid. When compared
with other types of platinum catalyst, Pt/ZrO 2 provides high conversion and
stable lactic acid in all condition. The catalyst can able to convert both pure and
crude glycerol into 80% lactic acid at 180 C, under helium pressure maintained at

30 bar pressure after the reaction time of 8 h. Since the catalyst Pt/ZrO 2 is expen-
sive, this process is not economically viable for industrial-scale production process
of lactic acid.

9.4.1.3 Lipids
From the evidence of various studies, lipid is produced from crude glycerol from
numerous types of microorganisms, such as yeast, fungi, and microalgae (Yang
et al., 2012). The freshwater microalga, Chlorella protothecoides, was capable of