2.2.4.2 Chitin Chitin, is the nitrogen containing polysaccharide which invariably occurs in certain fungi e.g., ergot. It is also commonly found in some invertebrate animals eg, crab, shrimp, lobsterspecifically located in the exoskeleton of the body. Besides, it is located in the appendages of insects and crustaceans.
Biological Source The mycelia of Penicillium species contain approximately 20% of chitin. The relatively hard crustacean shells of crab and lobster are reported to contain 15 to 20% chitin, whereas the rather soft crustacean shells of shrimp contain between 15 to 30% chitin. It is found in the spores of many fungi and yeasts.
Preparation The hard or soft crustacean shells are first ground to fine powder and then treated with dilute HCl (5%) for a duration of about 24 hours whereby most of the calcium* and other impurities are eliminated completely as soluble CaCl2. The above extract containing the proteins derived from the shells are eliminated by treating it with proteolytic enzymes like pepsin or trypsin.
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* Crustacean shell contain approx. 60-75% of calcium carbonate.
The resulting pink coloured liquid extract is bleached by H2O2 in an acidic medium for 5-6 hours at room temperature. The bleached product is subjected to deacetylation at 120oC with a mixture of 3 parts of KOH, 1 part EtOH and 1 part ethylene glycol. The deacetylation process is repeated several times till the ‘acetyl content’ reaches a minimum level. Chitin is obtained as an amorphous solid substance.
Description It is an amorphous solid. It is practically insoluble in water, dilute acids, dilute and concentrated alkalies, alcohol and other organic solvents. It is soluble in concentrated HCl, H2SO4, anhydrous HCOOH and H3PO4(78-97%). There exists a wide difference with regard to the solubility, molecular weight, acetyl value and specific rotation amongst chitins of different origin and obtained from different methods.
Chemical Constituents Chitin may be regarded as a derivative of cellulose, wherein the C-2 hydroxyl groups have been duly replaced by acetamido residues. In fact, it is more or less a cellulose like biopolymer mainly consisting of unbranched chains of β-(1→4) -2- acetamido -2-deoxy-Dglucose. It is also termed as N-acetyl-D-glucosamine. It contains about 6.5% of nitrogen.
Chemical Tests
1. Chitin affords a brown colour with Iodine solution which turns into red violet on acidification with sulphuric acid.
2. Chitin sulphate gives rise to a characteristic strain with acidic dyes, such as: picric acid and fuchsin.
3. When chitin is heated with strong KOH solution under pressure it fails to dissolve, but undergoes deacetylation to form acetic acid and other products collectively termed as chitosans.
4. Hydrolysis of chitin in the presence of strong mineral acids forms acetic acid and glucosamine.
5. When chitin is dissolved in dilute nitric acid (50%) and allowd to crystallise overnight it forms beautiful spherocrystals of chitosan. These crystals on being examined under polarised light, by making use of crossed nicols, a distinct cross is observed.
Uses
1. Chitosan i.e., deacetylated chitin, finds its application in water treatment operations.
2. It is used in pholographic emulsions.
3. It is used in improving the dyeability of synthetic fibers and fabrics.
4. Therapeutically it is used in wound healing preparations.
5. It shows considerable adhesivity to plastics and glass.
6. It is used as a sizing agent for cotton, wool, rayon and for synthetic fibers.
