Biomedical Engineering Reference
In-Depth Information
3.4 Organic Acids
Because of their multifunctional groups, organic acids are extremely useful as food
additives, raw materials for the manufacture of biodegradable plastics, starting
materials for the chemical industry, and other applications [
106
,
107
]. Organic
acids constitute a key group of chemicals, and most of them can be produced by
microorganisms. Among the twelve sugar-derived building-block chemicals
identified by the US Department of Energy, nine are organic acids [
108
]. The
market for bio-based organic acids is huge. One example is fumaric acid;
at present, the annual production of fumaric acid is about 12,000 tonnes, but by
creating new economical production processes, the petroleum-derived commodity
chemical maleic anhydride could be replaced by fumaric acid, and the market for
fumaric acid will be greater than 200,000 tonnes annually [
109
]. Owing to price
limitations, most of the bio-based organic acids are still uncompetitive compared
to the corresponding petroleum-derived products. Low-cost availability is an
obvious prerequisite for the biological organic acid industry; one way to achieve
this goal is to utilize inexpensive substrates, such as hemicellulose hydrolyzates.
Lactic acid is one of the organic acids that could be produced from hemicel-
lulose. It is the most widely utilized organic acid in the food, pharmaceutical, and
chemical industries. Recently, more attention has been paid on lactic acid pro-
duction because of the development of polylactic acid (PLA), which is 100%
biodegradable, and has been approved to be used in food and drugs. Lactic acid
exists in two isomeric forms, D-lactic acid and L-lactic acid, and optically pure
lactic acid is important in the formation of PLA with desirable mechanical
properties.
L-Lactic acid can be naturally produced by bacteria and fungi; most lactic acid-
producing strains can convert glucose to lactic acid, and some are even capable of
utilizing hemicellulose-derived pentoses (Table
6
). Lactobacillus pentosus [
110
],
Lactobacillus bifermentans [
111
], and Rhizopus oryzae [
112
,
113
] have been
reported to convert hemicellulose-derived xylose and glucose to lactic acid.
However, compared with glucose, due to the difficulties in xylose metabolism, and
the inhibitors presented in the hemicellulose hydrolyzate, such as furfural and
acetic acid, which are generated during pretreatment, the productivity of lactic acid
is usually poor when using hemicellulose hydrolyzate as substrate. Reungruglikit
and Hang [
112
] described the conversion of xylose into lactic acid by R. oryzae,
and obtained a yield of 299.4 g/kg dry material after 48 h fermentation; such
productivity is equivalent to 0.31 g/(L h) compared with 2.5 g/(L h) achieved using
glucose. Most studies on methods to improve lactic acid production have been
concerned with the detoxification of the hydrolyzate, strain improvement,
or process optimization. Walton et al. [
114
] isolated a moderately thermophile
organism, named Bacillus coagulans MXL-9, which was found to have high tol-
erance for inhibitors such as acetic acid, and showed it to be an excellent lactic
acid producer using hardwood hemicellulose hydrolyzate as substrate. Wee et al.
[
115
]
reported
that
the
inhibition
of
fermentation
could
be
reduced
by
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