From Chitin to Chitosan - Chitosan Based Hydrogels: Functions and Applications - page 3

1.1.2 existence
As a natural organic compound, cellulose content is the largest on Earth, followed by chitin
content. Cellulose is produced from plants and chitin is produced from animals. It is esti-
mated that approximately 10 billion tons of chitin can be biosynthesized in nature each
year. Chitin is also a natural nitrogen-containing organic compound with the largest
content on Earth, except for protein. Sources of chitin are the following.
1. Arthropods: the primary source is crustacea such as shrimp and crab whose chitin
contents are 58-85%, and the secondary source is insects (e.g., locust, butterfly,
mosquito, fly, and silkworm chrysalis) whose chitin contents are 20-60%, and
myriapods and arachnids.
2. Molluscs include amphineura, gasteropods, scaphopoda, lamellibranch, and
cephalopods whose chitin contents are 3-26%.
3. Annelids include archiannelida, chaetopoda, and hirudinea, some of which con-
tain little chitin whereas others contain 20-38% chitin.
4. Protozoans, namely unicellular animals, include mastigophora, sarcodina, sporozoa,
and ciliatea, which contain a little chitin.
5. Coelenterates include hydrozoa, scyphozoa, and actinozoa, some of which contain
little chitin while others contain 3-30% chitin.
6. Seaweed: the primary source is green algae, which contains just a little chitin.
7. Fungi include ascomycetes, basidiomycetes, and phycomycetes whose chitin con-
tents are from trace to 45% chitin. Only a few fungi do not contain chitin.
8. Others include rigid parts of animal joints, hoofs, and feet and joints of muscles
and bones.
All the chitins found in nature exist as complex compounds rather than as separated or
alone. In the shells of insects and the exoskeletons of molluscs, chitins combine with
proteins to harden the cuticles of insects by cross-linking with polyhydric phenol. In fungi,
chitins combine with other polysaccharides such as cellulose [3].
Chitins can be divided into α-, β-, and γ-chitins due to hydrogen bonds. α-Chitin com-
prises two antiparallel polysaccharide chains. β-Chitin comprises two parallel polysaccha-
ride chains. γ-Chitin comprises three parallel polysaccharide chains, two of which are in
the same direction. α-Chitin is the most stable one; thus it has the largest content, and the
other two types can transform into α-chitin if conditions permit. Different configurations
lead to different functions. α-Chitin can be found in parts with high hardness (e.g., the
cuticle of arthropods) and usually combines with shell protein or inorganic compounds.
γ-Chitin and β-chitin exist in soft and firm parts. All the three chitins can be found in
i in k i s h : α-chitin forming a thin esophageal epithelium in the stomach, β-chitin forming the
skeleton, and γ-chitin forming a thick esophageal epithelium in the stomach [4].
1.1.3 general Situation of research and Production
Between 1811 (when chitin was first identified) and 1910, there were only 20 research papers
on chitin and chitosan in the literature. In the 1930s, the very first patent for industrial
preparation of chitosan and a patent for the preparation of chitosan film and chitosan fiber
were issued in the United States, which had promoted research on chitin and chitosan. But
chitin and chitosan did not attract research attention until the 1970s. In 1977, scientists