Preparation of co-immobilized yeasts: S. cerevisiae and C. tropicalis
NSW-NW were initially revived on PDA medium for several
times. The revived seeds were grown in 250 mL Erlenmeyer flasks
containing 100 mL seed medium at 30 C on a rotary shaker incubator
at 120 rpm for 48 h. The culture was centrifuged at 4000 rpm
for 25 min, the precipitate was collected and suspended in sterile
0.1 g/mL physiological saline solution. The cell number of
S. cerevisiae suspension was approximately 4.84 108 cells/mL
calculated by blood cell counting method. The cell number of
C. tropicalis NSW-NW suspension was approximately 3.43 108
cells/mL. Next, a 0.5 mL C. tropicalis NSW-NW suspension and a
0.5 mL S. cerevisiae suspension were mixed with 20 mL embedding
solution (2% SA, 6% PVA, and 1% SiO2). This mixture was dispensed
through a 20 mL syringe into the fixing solution (3% CaCl2
and 5% H3BO3) with mild agitation, resulting in spherical beads
approximately 3–5 mm in diameter. The two yeasts were
co-immobilized in the beads, and the mixed culture system II
was successfully constructed. After overnight at 4 C, the
SA–PVA–SiO2 beads were collected and washed with physiological
saline solution. Then, the beads were cultured in PDA medium at
30 C for 24 h to regenerate.
Preparation of immobilized S. cerevisiae: 1mL S. cerevisiae suspension
was mixed with 20 mL embedding solution (2% SA, 6%PVA, and 1% SiO2). This mixture was dispensed through a 20 mL
syringe into the fixing solution (3% CaCl2 and 5% H3BO3) with mild
agitation, resulting in spherical beads approximately 3–5 mm in
diameter. The beads were carried out the above regeneration
treatment.
Ethanol fermentation in hydrolysate by co-immobilized yeasts
and immobilized S. cerevisiae: 20 mL co-immobilized yeasts beads
and immobilized S. cerevisiae were transferred separately into
250 mL Erlenmeyer flasks containing 30 mL fermented liquors
and covered with rubber stopper. The flasks were placed on a
rotary shaker incubator at 30 C at 90 rpm for 12 h. After fermentation,
the fermented products were collected and distilled in a still.
The ethanol content was analyzed using a spectrophotometric
method for the determination of chromic ion according to Caputi
et al. [36]. The residual sugars content was analyzed by the DNS
method.
Ethanol batch fermentation in hydrolysate by co-immobilized
yeasts: 20 mL SA–PVA–SiO2 beads were transferred into 250 mL
Erlenmeyer flasks containing 30 mL fermented liquors and covered
with rubber stopper. The flasks were placed on a rotary shaker
incubator at 28 C at 90 rpm for 4 h. After fermentation, the fermented
products were collected, and beads were washed by sterile
physiological saline solution for the next batch of fermentation.
Batch fermentation carried out for two more times. The ethanol
and residual sugars content were analyzed. The structures of SA–
PVA–SiO2 beads were observed by SEM technology.