Chitin is the most abundant organic compound of nitrogen.

At least ten Gigatons (1.10¹³ Kg) of chitin are synthesized each year in the biosphere.

Chitin is therefore important for making nitrogen available to countless living organisms.

Nitrogen fixation from the atmosphere, a challenging task for our technology,

is made by Rhizobium in root nodules of leguminous plants, the major step being the formation of lipo-chitin.

Chitosan, a dietary supplement and a food technology commodity   Riccardo A.A. Muzzarelli and Corrado Muzzarelli

A chapter published in the book

Functional Food Carbohydrates

C.G. Biliaderis and M.S. Izydorczyk, eds.

Francis and Taylor, Orlando FL, USA, 2006

In the areas of fisheries, chitin is upgraded in order to exploit renewable resources and to alleviate waste problems. Today chitins and chitosans from different animals are commercially available, mainly from shrimps, but also from squids, lobsters and crabs. Chitin is obtained from the shells by removing calcium carbonate, pigments, proteins and lipids, immediately after peeling the shrimps to be deacetylated to chitosans; minor quantities are transformed into O-carboxymetyl chitin, glycol chitin and 6-oxychitin.

Chitin isolates differ from each other in many respects, among which: degree of acetylation, 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 the MegaDalton, but chitin isolates have lower molecular weights due to partial random depolymerization occurring during the chemical treatments and depigmentation steps. Polydispersity may vary depending on such treatments as powder milling and blending of various chitin batches.

Isolated chitin is a highly ordered copolymer of 2-acetamido-2-deoxy-beta-D-glucose, major component, and 2-amino-2-deoxy-beta-D-glucose. Chitobiose, O-(2-amino-2-deoxy-beta-D-glucopyranosyl)-(1-4)-2-amino-2-deoxy-D-glucose, is the structural unit of native chitin. Bound water is also a part of the structure.

Chitin is degraded in vivo either during digestion by many animals, or during the moulting: chitinases permit to resorb most of the N-acetylglucosamine to be reused within hours for a new larger exoskeleton built under the action of chitin synthase. In the environment chitin is degraded by bacteria present in the sea floor.

Chitosan is the only largely available cationic polysaccharide, whilst most polysaccharides are anionic, such as alginates and pectins. Chitosan indicates a family of deacetylated chitins. In general, chitosans have nitrogen content higher than 7 % and degree of acetylation lower than 0.40. The removal of the acetyl group is a harsh treatment usually performed with concentrated NaOH. Protection from oxygen, with a nitrogen purge or by addition of sodium borohydride to the alkali solution, is necessary in order to avoid undesirable reactions such as depolymerization and generation of reactive species. The acetyl groups in the acid-soluble fractions are randomly distributed, whilst the insoluble fractions contain relatively long sequences of acetylated units.

The presence of a prevailing number of 2-amino-2-deoxyglucose units in a chitosan permits to bring the polymer into solution by salt formation. Chitosan is a primary aliphatic amine that can be protonated by acids, the pK of the chitosan amine being 6.3. Certain salts are water soluble: for instance, hydrochloride, formate, acetate, lactate, malate, citrate, glyoxylate, pyruvate, glycolate and ascorbate, and offer a wide range of unique applications including formation of biodegradable films, immobilization of enzymes, preservation of foods from microbial deterioration, and clarification and de-acidification of fruits and beverages.