Civil Engineering Reference
In-Depth Information
cellulose, resulting in calculations of Young's modulus and tensile strength. h ese are
ot en undertaken in order to compare the cellulose used in a composite to a matrix
material alone.
4.2.2.3
Growth Conditions
h ere are a number of dif erent methods that can be used to grow bacterial cellulose,
however, the low productivity and high cost of production of this form of cellulose
has been problematic, especially if the production of this cellulose is to be upscaled
for commercial applications. Several groups have examined media composition and
cultivation conditions on dif erent species and strains of
Gluconacetobacter
, ot en with
dif ering results, to determine the optimal growing conditions for high yields of bacte-
rial cellulose. h ese studies, and the optimal yields of cellulose achieved as part of the
testing conditions, are summarized below. Note that maximizing the yield of cellulose
is the predominant factor considered here, while the ef ects on the structure of cellulose
are described in Section 4.3.2. h e concentrations described in media are given in wt%
or vol%, where appropriate.
4.2.2.3.1 Base Media
Growth of
Gluconacetobacter
has traditionally been in a complex (and expensive)
medium dei ned by Hestrin and Schramm in 1954 [40]. h is HS medium, composed of
2% glucose, 0.5% yeast extract, 0.5% peptone, 0.27% Na
2
HPO
4
and 0.115% citric acid
monohydrate, with a pH between 4.0 and 6.0, has been used to grow
Gluconacetobacter
under static conditions at temperatures between 25°C and 30°C. As previously stated,
the pellicle produced from these conditions is believed to draw the bacterial cells to
the surface, as
Gluconacetobacter
is an obligate aerobe [41, 42]. As cellulose is formed
at the upper-most air-layer, the older cellulose is pushed down into the media as newly
formed cellulose is produced [43]. Spherical pellets can be achieved when grown under
agitated conditions [32].
Another culture medium that has been used for the cultivation of
Gluconacetobacter
was developed by Yamanaka
et al.
[44]. Yamanaka medium consists of 5% sucrose, 0.5%
yeast extract, 0.5% (NH
4
)
2
SO
4
, 0.3% KH
2
PO
4
and 0.005% MgSO
4
.7H
2
O with a pH of
5.0. Corn steep liquor (CSL)-fructose medium has also been used in cellulose produc-
tion. CSL-fructose medium consists of 20 ml CSL, 40 g fructose, 3.3 g (NH
4
)
2
SO
4
, 14.7
mg CaCl
2
.2H
2
O, 1.0 g KH
2
PO
4
, 3.6 mg FeSO
4
.7H
2
O, 2.42 mg Na
2
MoO
4
.2H
2
O, 250 mg
MgSO
4
.7H
2
O, 1.73 mg ZnSO
4
.7H
2
O, 1.39 mg MnSO
4
.5H
2
O, 0.05 mg CuSO
4
.5H
2
O, 2
mg inositol, 0.4 mg niacin (nicotinic acid), 0.4 mg pyridoxine HCl, 0.4 mg thiamine
HCl, 0.2 mg Ca pantothenate, 0.2 mg ribol avin, 0.2 mg
p-
aminobenzoic acid, 0.002 mg
folic acid, 0.002 mg biotin in 1 litre of distilled water with a pH of 5.0 [45].
4.2.2.3.2 Carbon Sources
Using glucose as the carbon source in media for growing
Gluconacetobacter
is not
only expensive, but is likely not optimal for cellulose production. Glucose is oxidized
to gluconic acid and the formation of gluconic acid causes a decrease in pH, which
can inhibit cellulose production [46, 47]. h ere are other carbon sources that do not
produce gluconic acid and thus do not lead to the unfavorable decrease in pH. In light
