Biomedical Engineering Reference
In-Depth Information
(6) Inoculated volume and seed age
Increasing the inoculation amount by centrifugation collection can effectively
shorten the training time and improve the xylitol formation rate. Microaerobic
culturing of
D. hansenii
in 140 g
L
1
xylose medium for 72 h, when the inoculum
L
1
volume was 0.3 g
cells (equivalent to stem cells), the xylitol production
L
1
; when the inoculum volume was increased to 3.0 g
L
1
was 60 g
cells, the
L
1
. The high density of cells inoculated apparently
alleviated the toxic inhibitory effect in hemicellulose hydrolyzate to improve
fermentation performance. Using wood hydrolyzate containing 58-78 g
xylitol production was 105.8 g
L
1
xylose
L
1
cells (fresh cells),
the fermentation was almost impossible; when the inoculum volume was increased
to 50-80 g
to culture
D. hansenii
, when the inoculum volume was 16 g
L
1
L
1
.
cells, the xylitol yield was 39-41 g
(7) pH and temperature
Because only molecular acetic acid causes inhibition, thus inhibition of acetic acid
has a close relationship with pH value. In
Candida guilliermondii
fermentation
using bagasse hydrolyzate, when pH was 6.0 and the OTR was 10 or 22.5 h
1
,
acetic acid was not used up until the end of fermentation. Under the same pH, when
theOTRwas35h
1
, acetic acid was rapidly depleted, and its inhibition effect was
negligible. When the initial pH was 4.0 and ORT was 22.5 or 35 h
1
, acetic acid was
rapidly depleted; even the OTR was controlled at 10 h
1
during the whole process,
acetic acid would also rapidly depleted. Regarding the effect of the fermentation,
the concentration of the xylitol, and the yield of xylitol and xylose consumption rate
reached their highest when the pH was 6.0 and the highest xylitol yield was 79 %.
The pH and temperature could affect the vitality of two key enzymes: xylose
reductase and xylitol dehydrogenase. It was found that, for
C. guilliermondii
fermentation using bagasse hydrolyzate, the activity of xylose reductase was highest
from pH 4.0 to pH 6.0, the activity of xylitol oxidase increased with the rise of pH
and temperature, and the activity of xylitol dehydrogenase was highest at pH 6.5 and
35
ı
C. The xylose conversion rate and xylitol production rate reached a maximum
at pH 6.0 and 35
ı
C. The presence of xylitol had no effect on activity of xylitol
dehydrogenase of this strain, which showed that the optimum condition of xylitol
production and conditions at which the activity of xylitol dehydrogenase were not
consistent with the optimum condition of xylose activity.
The pH would change in a fermentation process with the decomposition of
nitrogen sources. Therefore, the optical pH is different when using different nitrogen
sources.
For example, in fermentation with
C. guilliermondii
utilizing straw hemicellulose
hydrolyzate, if ammonium sulfate is used as a nitrogen source, the optical pH
is 5.3 for xylitol production, and if urea is used as the nitrogen source, the
optical pH is 4.5. This is because, in the fermentation process, the urea will be
decomposed to generate ammonia and carbon dioxide, so the pH of the fermentation
broth increases. Therefore, when considering the optimum pH in fermentation, the
nitrogen source should also be considered.
