52
Bacterial Cellulose
fiber.diameter..In.contrast.to.other.strains,.no.mutation.to.non-cellulose-producing.cells.of.this.bacterium.occurred.under.agitation,.thus.this.strain.is.
characterized.by.its.great.genetic.stability..This.fact,.and.the.good.production.yield,.might.have.a.great.impact.on.its.future.applicability.in.large-scale.
fermentation..Joseph.et.al..(2003).studied.shaken.flask.cultures.with.added.
polyacrylamide-co-acrylic.acid..Using.G. xylinus BPR2001.(ATCC.700178),.an.
increase.in.BNC.production.was.observed..Depending.on.the.rotation.speed,.
the.cellulose.production.increased.from.2.7.±.0.8.to.6.5.±.0.5.g/l.at.a.shaker.
speed.of.175.rpm.and.from.1.7.±.0.01.to.3.7.±.0.5.g/l.at.a.speed.of.375.rpm.
for. polyacrylamide-co-acrylic. acid. concentrations. up. to. 3. g/l. and. a. 7. day.
incubation.period..Also,.a.change.in.the.BNC.morphology.from.amorphous,.
stringy.forms.to.spheroidal.particles.with.diameters.≤2.mm.was.observed.
by.the.authors.
Agitated Cultivation by Stirring
In. contrast. to. shaking,. agitation. by. stirring. can. be. produced. by. impellers.
or.gas.transfer..Stirred-tank.fermenters.and.bubble.columns.use.this.effect;.
stirred-tank. fermenters. provide. superior. oxygen. transfer. by. manipulating.
the.aeration.and.agitation.speed.
Using. G. xylinus. NUST4.2,. Zhou. et. al.. (2007a). examined. the. structure. of.
BNC.produced.in.stationary.culture.in.comparison.with.that.generated.in.a.
stirred-tank.reactor..They.found.a.BNC.yield.of.7.5.g/l.in.stationary.culture.
after.6.days.and.3.13.g/l.in.the.stirred-tank.reactor.after.72.h..With.regard.to.
productivity,.a.decrease.from.0.052.g/l/h.to.0.043.g/l/h.could.be.estimated.
for. the. latter. cultivation.. Furthermore,. the. cellulose. ribbons. produced. in.
static.culture.were.more.entangled.and.built.a.denser.network.with.curved.
and.overlapping.fibers.compared.with.the.BNC.materials.produced.in.the.
stirred-tank. reactor.. Nevertheless,. BNC. synthesized. in. static. culture. was.
characterized.by.higher.crystallinity,.higher.I α .content,.and.larger.crystallite.
size.than.cellulose.produced.in.the.stirred-tank.reactor..The.authors.argued.
that.all.of.these.results.revealed.that.stirring.in.the.reactor.strongly.interferes.
with.the.process.of.nascent.microfibril.crystallization,.favoring.the.formation.of.smaller.microfibrils.and.increased.I β .
In.relation.to.this,.Watanabe.et.al..(1998).compared.BNCs.produced.in.two.
cultures. and. found. no. difference. between. reticulated. structures. of. bacterial.cellulose.fibrils.produced.in.agitated.and.static.cultures..Nevertheless,.
the. BNC. produced. in. agitated. culture. exhibited. microstructural. changes,.
namely,.a.lower.degree.of.polymerization,.with.a.DP w .of.10,900.(static.cultivation.DP w .14,400).and.a.crystallinity.index.of.63%,.in.contrast.to.71%.for.static.
cultivation,.as.well.as.a.decrease.of.cellulose.I α .content,.with.a.value.of.61%.
for. agitated. culture. (static. cultivation. 73°%).. Furthermore,. a. lower. Young’s.
modulus.of.the.sheet,.a.higher.water-holding.capacity,.and.a.higher.dispersion.viscosity.in.the.disintegrated.form.than.that.produced.in.static.culture.
have.been.observed.
Bacterial Cellulose
fiber.diameter..In.contrast.to.other.strains,.no.mutation.to.non-cellulose-producing.cells.of.this.bacterium.occurred.under.agitation,.thus.this.strain.is.
characterized.by.its.great.genetic.stability..This.fact,.and.the.good.production.yield,.might.have.a.great.impact.on.its.future.applicability.in.large-scale.
fermentation..Joseph.et.al..(2003).studied.shaken.flask.cultures.with.added.
polyacrylamide-co-acrylic.acid..Using.G. xylinus BPR2001.(ATCC.700178),.an.
increase.in.BNC.production.was.observed..Depending.on.the.rotation.speed,.
the.cellulose.production.increased.from.2.7.±.0.8.to.6.5.±.0.5.g/l.at.a.shaker.
speed.of.175.rpm.and.from.1.7.±.0.01.to.3.7.±.0.5.g/l.at.a.speed.of.375.rpm.
for. polyacrylamide-co-acrylic. acid. concentrations. up. to. 3. g/l. and. a. 7. day.
incubation.period..Also,.a.change.in.the.BNC.morphology.from.amorphous,.
stringy.forms.to.spheroidal.particles.with.diameters.≤2.mm.was.observed.
by.the.authors.
Agitated Cultivation by Stirring
In. contrast. to. shaking,. agitation. by. stirring. can. be. produced. by. impellers.
or.gas.transfer..Stirred-tank.fermenters.and.bubble.columns.use.this.effect;.
stirred-tank. fermenters. provide. superior. oxygen. transfer. by. manipulating.
the.aeration.and.agitation.speed.
Using. G. xylinus. NUST4.2,. Zhou. et. al.. (2007a). examined. the. structure. of.
BNC.produced.in.stationary.culture.in.comparison.with.that.generated.in.a.
stirred-tank.reactor..They.found.a.BNC.yield.of.7.5.g/l.in.stationary.culture.
after.6.days.and.3.13.g/l.in.the.stirred-tank.reactor.after.72.h..With.regard.to.
productivity,.a.decrease.from.0.052.g/l/h.to.0.043.g/l/h.could.be.estimated.
for. the. latter. cultivation.. Furthermore,. the. cellulose. ribbons. produced. in.
static.culture.were.more.entangled.and.built.a.denser.network.with.curved.
and.overlapping.fibers.compared.with.the.BNC.materials.produced.in.the.
stirred-tank. reactor.. Nevertheless,. BNC. synthesized. in. static. culture. was.
characterized.by.higher.crystallinity,.higher.I α .content,.and.larger.crystallite.
size.than.cellulose.produced.in.the.stirred-tank.reactor..The.authors.argued.
that.all.of.these.results.revealed.that.stirring.in.the.reactor.strongly.interferes.
with.the.process.of.nascent.microfibril.crystallization,.favoring.the.formation.of.smaller.microfibrils.and.increased.I β .
In.relation.to.this,.Watanabe.et.al..(1998).compared.BNCs.produced.in.two.
cultures. and. found. no. difference. between. reticulated. structures. of. bacterial.cellulose.fibrils.produced.in.agitated.and.static.cultures..Nevertheless,.
the. BNC. produced. in. agitated. culture. exhibited. microstructural. changes,.
namely,.a.lower.degree.of.polymerization,.with.a.DP w .of.10,900.(static.cultivation.DP w .14,400).and.a.crystallinity.index.of.63%,.in.contrast.to.71%.for.static.
cultivation,.as.well.as.a.decrease.of.cellulose.I α .content,.with.a.value.of.61%.
for. agitated. culture. (static. cultivation. 73°%).. Furthermore,. a. lower. Young’s.
modulus.of.the.sheet,.a.higher.water-holding.capacity,.and.a.higher.dispersion.viscosity.in.the.disintegrated.form.than.that.produced.in.static.culture.
have.been.observed.