Biomedical Engineering Reference
In-Depth Information
Cellulose
Low molecular sugars (mainly glucose and pentose)
Hemicellulose
EMP path
Glucose
2 Lactic acid
2 Pyruvate
Acetyl phosphate
Acetic acid (or ethanol)
Pentose
Pyruvate
Lactic acid
Fig. 7.2
Mechanism of lactic acid production from lignocellulosic materials
In lactic acid fermentation, the sugar content of the medium is closely related to
the production of lactic acid; the best sugar content should be determined based on
the characteristics of the selected strains. Zhuang [
34
] used treated straw as the raw
material to ferment lactic acid with
Lactobacillus
. The results showed that, as the
sugar content of the straw cellulose liquid increased from 100 to 140 g L
1
, lactic
acid production was in an upward trend, but the changes of the conversion rate were
small. When the sugar content was increased to 150 g
L
1
, the lactic acid yield
L
1
and the conversion rate were highest. They were 132 g
and 88 %, respectively.
L
1
, although lactic acid production
had a small increase, the conversion rate was in a downward trend. It actually caused
the waste of sugars.
As lactic acid fermentation is not aerobic fermentation, the microbial cells do not
proliferate in the fermentation process, so the amount of seed inoculation should be
large to ensure that there are always enough microbial cells in the fermentation
process. Chen et al. [
35
] used screened lactobacillus, ZJU-1 (
Lactobacillus
sp.), to
ferment on the enzymatic hydrolyzate of steam-exploded fiber materials. The result
showed that 10 % of inoculum was appropriate, which was consistent with the report
of Zhuang et al. [
34
].
In addition, the temperature and time of fermentation have an important impact
on the lactic acid fermentation process, so in the experiment it is necessary to
explore the optimum fermentation conditions based on the selected strains as well
as the raw material.
When the sugar content was greater than 150 g
7.3.1.3
Synthesis and Production of Polylactic Acid
Preparing monomers of lactic acid by biological fermentation is the first step
of PLA synthesis, and then the PLA with high molecular weight is synthesized
by chemical methods. Currently, there are two synthesis methods; one is direct
polycondensation. Another is the indirect synthesis method, in which lactide is
synthesized from lactic acid first and further synthesized into PLA with catalyst
[
36
,
37
]. The former is simple and an important way to reduce the cost of PLA
