Biomedical Engineering Reference
In-Depth Information
In hydrolyzate fermentation, with the increase of the ventilation volume, the cell
amount increases. But, the excessively large amount of bacterial cells would con-
sume a large amount of xylose. On the other hand, excess ventilation is conducive to
the consumption of the inhibitory substances (e.g., glucose and acetic acid). Acetic
acid not only will interfere with the cell membrane of xylose absorption but also will
inhibit the activity or synthesis of the xylose reductase. The anaerobic fermentation
of glucose also impedes xylose fermentation to xylitol. Vandeska et al. [
85
] studied
the effect of ventilation on the activity of
Candida boidinii
dehydrogenase and found
that as OTR increased from 10 to 30 mmol
h
1
, the cell biomass increased
sixfold, XR/XDH was reduced from 2.8 to 0.98, and the yield of xylitol was reduced
from 0.38 to 0.23 g
L
1
g
1
.
Horitsu et al. [
86
] pointed out that, for efficient production of xylitol, a high
level of DO should be maintained in the early period of culture, then reduce the
DO level to inhibit microbial respiration. The two-stage aeration is effective for
improving the rate of xylitol formation. In the later stage of low levels of DO, the
redox potential of the medium is a sensitive indicator for monitoring the DO level.
Kim et al. [
87
] reduced ventilation to reduce the redox potential of the medium to
the optimum value of 140 mV in the second stage when they cultured
C. tropicalis
.
After fermentation for 66 h, 300 g
L
1
L
1
xylose were changed into 250 g
xylitol,
L
1
h
1
.
with a generation rate of 3.79 g
(2) Nitrogen source
Xylitol accumulation is significantly affected by nitrogen source. The suitable
nitrogen source and concentration are different between the yeast strains in the
different media at different pH conditions. An organic nitrogen source can reduce
the level of
Candida shehatate
XDH. Yeast extract is the best organic nitrogen
source. Using straw hemicellulose hydrolyzate to culture
Candida guilliermoneii
,
urea used as a nitrogen source was better than ammonium sulfate at pH 4.5, and
xylitol yield increased by 25 %; there was no significant difference when the pH
was 5.3 or 6.0. Some scholars mixed organic and inorganic nitrogen sources and
achieved good results.
(3) Xylose concentration
Enhancing the initial xylose concentration can improve both the concentration of
xylitol and the yield and formation rate of xylitol. However, the sugar tolerance of
different strains is significantly different.
When the xylose concentration was increased from 50 to 100 g
L
1
, there was
no significant change in the maximum growth speed (
max
)of
C. parasilosis,
C. guilliermondii,
and
C. tropicalis
,butthe
max
of
C. mogii
fell by almost half.
It was found that the XR activity of two kinds of yeast,
C. guilliermondii
and
C. parasilosis,
was significantly affected by xylose concentration. The optimal
xylose concentration for production is located near the concentration when XR
activity reaches the maximum. In shake flask culture of
C. tropicalis
KFCC-10960,
as the xylose concentration was increased from 50 to 250 g
L
1
, xylitol yield
g
1
. When the xylose concentration was 150 g
L
1
,
increased from 0.76 to 0.90 g
L
1
h
1
). Then, it
the fermentation rate of xylitol reached the maximum (2.9l g
