Civil Engineering Reference
In-Depth Information
further gather into microi brils of approximately 3-15 nm thick and 70-80 nm wide [1,
4, 15, 16]. Microi brils, in turn, entangle into a ribbon of crystalline cellulose whose
interwoven produces the BC i brous network (Figure 2.1) [5, 8, 17, 18]. h e reason
why these bacteria generate cellulose is still unclear, but it has been suggested that it
is a mechanism that bacteria use to maintain their position close to the surface of the
culture medium, where there is a high oxygen content; and also serves as a protective
coating against ultraviolet radiation, prevents the entrance of enemies and heavy-metal
ions whereas nutrients dif use easily throughout the pellicle [1, 19].
BC has many applications which have triggered high interest in its production at a
commercial scale. However, the main problems that hamper this process are the low
yield and production costs, especially for low added value applications. h erefore, some
attempts have been made in terms of optimization of culture conditions and medium
composition, as well as the scaling-up process [14]. Bacterial cellulose is commonly
produced using the Hestrin-Schramm (HS) medium, that uses glucose as the car-
bon source and a combination of peptone and yeast extract as nitrogen sources [14].
However, the use of glucose as carbon source for BC production is quite expensive
and causes the formation of by-products such as gluconic acid that decreases the pH
of the culture medium and ultimately declines the production of BC [4, 8]. h erefore,
researchers have investigated the capability of
G. xylinus
to grow and produce BC using
dif erent carbon sources. Besides glucose and sucrose (the most commonly used), other
carbohydrates such as fructose, maltose, xylose and starch, and polyols as glycerol have
also been successfully tested [4].
In addition, other ef orts have been also devoted to the identii cation of cheap feed-
stocks as alternatives to the expensive conventional culture media, with pure com-
pounds, for the economically viable production of BC [20]. In this context, several
industrial wastes have already been ef ectively explored for the production of BC, as
Single microiber
Ribbon
Glucan chain
aggregation
HO
HO
OH
O
O
O
O
OH
n
OH
HO
OH
Figure 2.1
Scanning electron microscopy (SEM) images of
Gluconacetobacter xylinus
and BC network
of micro and nano i brils; and schematic description of the formation of bacterial cellulose. Reproduced
with permission from [7].
