Civil Engineering Reference
In-Depth Information
for example tea infusions [21], wheat straw acid hydrolysate [22], grape bagasse and
crude glycerol [23], beet molasses [24], sugar-cane molasses and corn steep liquor [20],
Konjac powder [25], fruit juices, including orange, pineapple, apple, Japanese pear and
grape [26, 27], grape skins aqueous extract and suli te pulping liquor [28], and dry olive
mill residue [29]. h e development of culture media based on cheaper feedstocks will
simultaneously allow the production of BC at lower price and the valorization of the
residues themselves [29].
Enhancement of BC production has also been attempted through supplementation
of the culture medium with dif erent additives. Several chemicals including alcohols
[30], vitamin C [31], lignosulfonates [32], water-soluble polysaccharides [33-35],
thin stillage from rice wine distillery [36] have been investigated in this perspective.
For instance, Lu
et al.
[30] investigated the stimulatory ef ects of six dif erent alco-
hols, added at dif erent concentrations, during static fermentation of
G. xylinus
186.
All alcohols tested improved BC production and could be ranked as
n
-butanol > man-
nitol > glycerol > ethylene glycol > methanol >
n
-propanol. However, results showed
that
n
-butanol only improves BC production when added at concentrations lower than
1.5% v/v (maximum production of 132.6 mg/100 ml, 56.0% higher than the control),
while mannitol stimulates BC production at any concentration, with a maximum ef ect
at a concentration of 4% v/v (maximum production of 125.2 mg/100 ml, 47.3% above
the control).
Another purpose of adding chemicals to the culture medium is the chemical modi-
i cation of the structural and physical properties of bacterial cellulose, allowing the
preparation of composites directly during biosynthesis and broadening the applications
of cellulose [18], as will be discussed latter.
Another important point to be taken into account in BC production is the cultivation
method employed, once this af ects the structure, physical and mechanical properties
of the i nal material. h erefore, the selection must be made according to BC intended
applications [37]. BC has been synthesized through a number of dif erent routes, which
are broadly classii ed into static and agitated processes.
Static cultivation is the most common method, from which a highly hydrated BC
membrane (or pellicle) on the air-culture medium interface is obtained (Figure 2.2)
[19, 38]. As cellulose is synthesized, a membrane with increasing thickness is gener-
ated and, once oxygen is required for bacteria growth and cellulose production, it is
assumed that the mature BC membrane is constantly pushed down as new cellulose is
produced on the interface [8, 15].
Under static conditions, and using suitable molds, it is possible to obtain uniform and
smooth BC products with dei ned shapes, which can be employed for instance in the
biomedical i eld [12] as artii cial blood vessels [8] or artii cial skin [40]. h e moldability
of BC during biosynthesis and shape retention is a feature that may enable the develop-
ment of designed shape products directly in the culture media [8, 41], increasing the
application range of BC.
However, production under static conditions requires more working space and work-
load, turning the potential industrial scale production more expensive [7]. h erefore,
a major goal on BC research has been centered on the optimization of BC production
through the design of ei cient static culture reactors. Kralisch
et al.
[42] developed
a novel, ei cient bioreactor named Horizontal Lit Reactor (HoLiR) (Figure 2.3) that
