Biomedical Engineering Reference
In-Depth Information
7.4.3.2
Research Advances of Fermentation Process Control
An important role is played by pH in the regulation of the metabolism of bacteria,
especially in fermentation containing a large number of composite products. The
fermentation process of 2,3-butanediol contains many by-products, such as 3-
hydroxy-butanone, ethanol, acetic acid, and lactic acid. The optimum pH of the
related enzyme in the route of 2,3-butanediol generation and those of by-products is
not consistent; thus, the pH of the fermentation process will affect not only the
growth of bacteria but also the metabolic processes of bacteria [
75
]. Generally,
alkaline conditions are beneficial to the generation of organic acids, resulting in
low yields of 2,3-butanediol. Under acidic conditions, the yield of organic acids
is decreased to a tenth of that under basic conditions, while the yield of the
main product, 2,3-butanediol, is high [
56
]. Raspoet et al. [
76
] found that for
B. licheniformis
using glucose as the carbon source, the highest yield of 2,3-
butanediol could be obtained at pH 6.0. Jansen and Tsao [
68
] found that when
K.
oxytoca
used xylose as a carbon source, the cells grew fastest at pH 5.2, and 2, 3-
butanediol yield was also the highest under anaerobic conditions. Thus, the optimal
pH values in the fermentation process to produce 2,3-butanediol are closely related
to the fermentation strain types and characteristics and the type of substrates and the
concentration.
Dissolved oxygen (DO) is also important in the fermentation process. 2,3-
Butanediol is the typical product of anaerobic metabolism, but when
K. pneumoniae
is used as a starting strain, adequate ventilation can increase the yield of 2,3-
butanediol. The explanation for this phenomenon from Long and Patrick is that
the substrate and the product can be uniformly dispersed in the fermentation broth
during ventilation by stirring, which can improve the efficiency of the fermentation.
Laube and Martin [
77
] found that, compared with shake flask culture,
B. polymyxa
consumed less xylose substrate and produced less 2,3-butanediol in static culture.
Their initial explanation was that CO
2
produced during fermentation in shake flask
cultures can be released in a timely manner, resulting in speeding up the xylose
metabolic rate and increasing 2,3-butanediol production. However, further study
showed that 2,3-butanediol production was higher at a speed of 125 rpm compared
to 300 rpm.
Too much ventilation is unfavorable for 2,3-butanediol formation. Jansen et al.
[
68
] found that, in
K. oxytoca
fermentation of 2,3-butanediol with xylose as the
carbon source, the most important factor that affected 2,3-butanediol production
was the oxygen transfer rate (OTR), which was directly affected by the ventilation
parameters. The study found that DO has a significant effect on the conversion
of 3-hydroxybutanone, which is a precursor substance of 2,3-butanediol in the
fermentation process. Moes et al. [
78
] found that, in
B. subtilis
fermentation of 2,3-
butanediol, different DO levels in the process would affect the composition ratio of
the end products 3-hydroxyacetone and 2,3-butanediol. Further study showed that
when the DO concentration was more than 100
gkg
1
, the major product was
