An Overview of Biological Basics - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 63

2.3.3. Chitin and Chitosan
Chitin is a polysaccharide constituted of N-acetylglucosamine, which forms a hard, semi-
transparent biomaterial found throughout the natural world. Chitin is the main component
of the exoskeletons of crabs, lobsters, and shrimps. Chitin is also found in insects (e.g. ants,
beetles, and butterflies), cephalopods (e.g. squids and octopuses), and even in fungi. Never-
theless, the industrial source of chitin is mainly crustaceans.
Because of its similarity to the cellulose structure, chitin may be described as cellulose with
one hydroxyl group on each monomer replaced by an acetylamine group. This allows for
increased hydrogen bonding between adjacent polymers, giving the polymer increased
strength.
Chitin's properties as a tough and strong material make it favorable as surgical thread.
Additionally, its biodegradability means it wears away with time as the wound heals. More-
over, chitin has some unusual properties that accelerate healing of wounds in humans. Chitin
has even been used as a stand-alone wound-healing agent.
Industrial separation membranes and ion-exchange resins can be made from chitin, espe-
cially for water purification. Chitin is also used industrially as an additive to thicken and
stabilize foods and pharmaceuticals. Since it can be shaped into fibers, the textile industry
has made extensive use of chitin, especially for socks, as it is claimed that chitin fabrics are
naturally antibacterial and antiodor ( www.solstitch.net ). Chitin also acts as a binding agent
in dyes, fabrics, and adhesives. Some processes to size and strengthen paper employ chitin.
Chitosan is produced commercially by deacetylation of chitin. It is a linear polysaccharide
composed of randomly distributed, b -(l,4)-linked D -glucosamine units (deacetylated) and
N -acetyl- D -glucosamine units (acetylated). The degree of deacetylation in commercial chito-
sans is in the range of 60 e 100% ( Fig. 2.26 ).
The amino group in chitosan has a pKa value of about 6.5. Therefore, chitosan is positively
charged and soluble in acidic to neutral solutions with a charge density dependent on pH
and the deacetylation extent. In other words, chitosan readily binds to negatively charged
surfaces such as mucosal membranes. Chitosan enhances the transport of polar drugs across
epithelial surfaces and is biocompatible and biodegradable. Purified qualities of chitosan are
available for biomedical applications.
Chitosan possesses flocculating properties, which are used in water processing engi-
neering as a part of the filtration process. It may remove phosphorus, heavy minerals, and
oils from the water. In the same manner, it is used to clarify wine and beer (as a substitute
for egg albumin).

OH

NH 2

HO

O

O

O

O

HO

O

NH

OH

x

O

FIGURE 2.26 General formula for chitosan (x 1.5) and chitin (x ¼ 0).