Chemistry Reference
In-Depth Information
12
Chitin
Hans Merzendorfer
Among Nature 's many biopolymers the sugar polymers take on a special position
as they possess distinctive biological and physicochemical properties that make
them highly attractive for industrial applications. One well-known example is cel-
lulose, which is actually considered the most abundant organic compound on
Earth. It is the major constituent of paper and textiles, but is also converted in
coatings, laminates, pharmaceuticals and food. However, cellulose is not the only
prevalent biopolymer on Earth. With an estimated annual production of at least
10 gigatons (1
1 0
13
kg) chitin is presumably the second most abundant organic
compound [1]. In contrast to cellulose, which is obtained from terrestrial sources,
such as wood or cotton, chitin is regarded a marine biopolymer, as oceanic crus-
taceans, in particular those of the pelagic zooplankton, produce most of its biomass.
Krill has an exceptional position as a chitin-producing organism, because these
small crustaceans appear in gigantic swarms in every ocean of the world. Neverthe-
less, chitin has remained an unused biomass resource for a long time, although
it was in fact discovered earlier than cellulose (see Info Box 1). In more recent
times, however, attention has increased dramatically, because chitin and its deriva-
tives have unique physicochemical properties allowing a broad spectrum of techni-
cal applications. Meanwhile, more than 10,000 tons are extracted every year from
shellfi sh waste accumulating in the seafood processing industry. Most of the
worldwide chitin production is used to obtain glucosamine and various oligosac-
charides by acidic hydrolysis (see also Chapter 3). Accelerated alkaline hydrolysis
leads to the deacetylation of chitin yielding chitosan - a more soluble polymer
consisting mainly of glucosamine units (Figure 12.1). Chitosan is a cationic
polymer at acidic or neutral pH, and has antimicrobial properties and a relatively
low toxicity profi le. Therefore, it can be employed in the food industry for preserva-
tion and as a dietary supplement, in water treatment plants for fl occulation and
adsorption of heavy metal ions, and in the medical/pharmaceutical industry as an
excipient and for wound dressings. Furthermore, chitosan is used as base material
in biopolymer research, because it is comparatively easy to chemically modify
the primary amino and the secondary hydroxyl groups. A multitude of chitosan
derivatives have therefore been synthesized allowing the design of polysaccharide-
based intelligent materials with specifi c functions.
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