Chitin, (1-4)-linked 2-acetamido-2-deoxy--D-glucan, is widely distributed among invertebrates.

It is found as -chitin in the calyces of hydrozoa, the egg shells of nematodes and rotifers,

the radulae of chitons and limpets and the cuticles of arthropods; sponges possess chitin as a component of their skeletons.

Chitin is present in the shells of brachiopods and mollusks, the cuttlefish bone, the squid pen, and pogonophora tubes.

Chitin is found in exoskeletons, peritrophic membranes and cocoons of insects.

The chitin in the fungal walls varies in crystallinity, degree of covalent bonding to other wall components,

mainly glucans, and its degree of acetylation.

Chitin and chitosan hydrogels Riccardo A.A. Muzzarelli and Corrado Muzzarelli

A chapter of the book

Handbook of Hydrocolloids

Second edition

G.O. Philips and P.A. Williams, eds.

Woodhead Publishing Ltd., 2009.

Cambridge UK

Hydrogels, composed of water soluble or swelling macromolecules, exhibit high water content, biocompatibility and desirable mechanical properties; the xerogels resulting after dehydration exhibit large specific surface area, and a network suitable for the incorporation of biomolecules. Present and future microgel applications require control over properties that include novel functionality, controlled particle size and tunable biodegradability.

Major advances in the area of microgel / nanogel particles deal with drug delivery carriers for biological and biomedical applications, but the food applications since long testify a great deal of interest in natural and semi-synthetic hydrogels. This chapter provides information on some chitin- and chitosan-based hydrogels susceptible of applications in the food and welfare area. Among polysaccharides, chitin is known for a large variety of hydrogels that can be obtained via chemical or enzymatic modification.

Today, chitins and chitosans of various grades are commercially available in large quantities from producers all over the world. Chitin isolates differ from each other in many respects, among which: degree of acetylation, defined as the molar fraction of GlcNAc, typically close to 0.90; elemental analysis, with nitrogen content typically close to 7%, and N/C ratio 0.146 for fully acetylated chitin; molecular size and polydispersity. The average molecular weight of chitin in vivo is probably in the order of one million Da, but chitin isolates have lower values due to partial random depolymerisation occurring during the chemical treatments and de-pigmentation steps. The presence of a substantial quantity of nitrogen is a point of difference from other abundant polysaccharides: the fact that chitin contains 4 elements instead of 3 is sometimes ignored, unfortunately, when chitin is compared to cellulose.