Chitin (C8H13O5N)n (/ˈkaɪtɪn/ KY-tin), a long-chain polymer of N-acetylglucosamine, is a derivative of glucose. It is a primary component of cell walls in fungi, the exoskeletons of arthropods, such as crustaceans and insects, the radulae of molluscs, cephalopod beaks, and the scales of fish and lissamphibians. The structure of chitin is comparable to another polysaccharide—cellulose, forming crystalline nanofibrils or whiskers. In terms of function, it may be compared to the protein keratin. Chitin has proved useful for several medicinal, industrial and biotechnological purposes.
Chitin is a natural polysaccharide and the second most abundant biopolymer on Earth after cellulose. This biopolymer consists of units of β-(۱–۴)-N-acetyl-D-glucosamine and is the main component of the exoskeleton of arthropods and crustaceans but also can be found in relevant amount in the cell walls of fungi. Chitin is a structural biopolymer whose role is analogous to that of collagen in the higher animals and cellulose in terrestrial plants In a similar way, plants produce cellulose in their cell walls, and insects and crustaceans synthetize chitin and accumulate it in their shells. Chitin may be regarded as cellulose with hydroxyl at position C2 replaced by an acetamide group (-CONH2). This similarity partly explains some analogies occurring in chitin and cellulose, such as low solubility and low chemical reactivity . Chitin is found in three polymeric forms, α-, β- and γ-chitin, usually found in shrimp and crab shells, squid pen and stomach cuticles of cephalopod, respectively. From the three aforementioned forms of the biopolymer, α-chitin is the most abundant and stable form. α-, β- and γ-chitin correspond to antiparallel, parallel and alternated arrangements of polymer chains, respectively. A hydrogen bond between the acetamide group on the C2 carbons and the secondary alcoholic hydroxyl groups on the C3 carbon is linked through a water molecule with the primary alcoholic hydroxyl groups on a C6 carbon. As a result of this configuration, chitin possesses a strong crystalline structure, which explains the high chemical and solvent stability of the biopolymer. Due to its crystalline structure, chitin exhibits remarkable differences from cellulose in the solubility and reactivity despite of the relatively similar chemical structure
The production of chitin uses basic raw materials of the cuticles of various crustaceans, principally crabs and shrimps. In regular fishery wastes, the biopolymer chitin is associated with proteins, minerals, lipids and pigments .All these substances are considered impurities, and they all have to be quantitatively removed to achieve the required purity of the chitin. The chitin is normally extracted from the carapaces from crustaceans treating the crushed material with acid to achieve complete dissolution of the calcium carbonate structure. After this process, the material is submitted to an alkaline extraction to achieve the solubilization of the proteins. In a later purification step, the material obtained from the deproteinization process follows a decolorization step to remove residues of pigments to yield an almost colorless product . Partial deacetylation of chitin leads to the formation of the polymer chitosan, consisting of units poly(D-glucosamine).