Biomedical Engineering Reference
In-Depth Information
(8) Fermentation type
The previous study of the author's lab showed that among batch; fed-batch
(hydrolyzate containing 79 g
h
1
after culture for 65 h); semicontinuous culture (two thirds of the culture were
removed when cultured to 63 h, then supplied with hydrolyzate), the maximum
xylitol concentration and production rate of semicontinuous culture (34 g
L
1
xylose was added at a speed of 28 mL
L
1
and
79 %, respectively) were significantly higher than for batch culture and fed-batch
culture. This is because in semicontinuous culture, the cells are further adapted to
the hydrolyzate, thereby improving the conversion rate and production efficiency. If
the xylose concentration exceeded a certain value, it would limit the growth of yeast
cells, resulting in extension of the fermentation time and a great decrease of the
product formation rate. Many scholars immobilized yeast cells to produce xylitol.
Silva et al. used porous glass to culture
C. tropicalis
in a fluidized bed reactor;
155 g
L
1
L
1
xylose was converted into 90-95 g
xylitol, and the formation rate
h
1
. According to the research of Yahashi et al. [
88
], the effect of
using nonwoven fiber to immobilize
C. tropicalis
was best.
Oh and Kim [
89
] reported that if the xylose concentrations exceeded 150 g
L
1
was 1.35 g
L
1
,
an apparent lag phase would appear in the growth of
C. tropicalis
KFCC-10960
cells. They controlled the initial xylose concentration of 150 g
L
1
and added a high
concentration of glucose/xylose at a constant speed when the cell density was up to
15 g
L
1
and the total xylose addition was 270 g
L
1
. After 55 h of fermentation, the
L
1
, the product formation rate was 4.56 g
L
1
h
1
,
xylitol concentration was 251 g
g
1
.
Enzymatic synthesis of xylitol is a delicate and promising process route, but
it does not yet have the competitiveness with the current fermentation process.
According to current data,
C. guillietmiodii
,
C. tropicalis
,
C. parapsilosis
, and
D. hansenii
microorganisms have the potential for xylitol fermentation because
they can exhibit a high production rate or high conversion rate under high xylose
concentrations.
Saccharomyces cerevisiae
strains expressing the XYL1 gene must
be imported to the xylose permease system and utilize both xylose and hexose, then
xylitol formation rate may be improved. Batch constant-speed feeding culture is
an effective method of eliminating the inhibitory effect of a high concentration of
xylose and increasing the rate of product formation. This batch culture technology
may become another feasible way to handle the toxic effect of hemicellulose
hydrolyzate.
Glucose addition can effectively improve xylitol yield. However, excess glucose
in media will generate a higher concentration of ethanol, which would seriously
affect the formation rate of xylitol. This effect prompts that an excessively high
proportion of glucose or other hexoses should not be contained in the cellulose
material hydrolyzate. Hemicellulose hydrolyzate contains many components toxic
to microbial metabolism and may also have some substances that inhibit cells from
secreting xylitol. Research on more cost-effective detoxification technology is an
urgent issue for a large-scale fermentation process using hemicellulose hydrolyzate
to produce xylitol.
and the conversion rate was 0.93 g
