Chitin Characteristics and Characterization

In industrial processes, chitin is extracted from crustaceans by acid treatment to dissolve calcium carbonate followed by alkaline extraction to solubilize proteins. In addition, a decolour-ization step is often applied to remove the residual pigments and to obtain a colourless product. These treatments need to be adapted to each chitin source due to differences in the ultrastructure of the initial materials. 

In the solid state, chitin mainly occurs as two allomorphs, the a- and (3-forms, which are differentiated by infrared and solid state NMR spectroscopies and X-ray diffraction [3,4]. a-chitin is the most abundant, occurring in krill, lobsters and crab tendons, shrimp and crab shells. It also results from recrystallization of (3-chitin in solution treated with strong aqueous HCl (over 7N) and washed with water because the a-form is thermodynamically more stable than (3-chitin [5]. (3-chitin is found in association with proteins in squid pens [3, 6] or in the tubes synthesized by pogonophoran and vestimetiferan worms [7,8]. In the solid state, the chains are parallel in (3-chitin and antiparallel in a-chitin. Their crystalline structures were reviewed in different papers [1, 4, 9-13]. Chitin is insoluble in all the usual solvents while taking into account their crystalline forms (a or (3) because the reactivity of p-chitin is larger than that of the a-isomorph, which is important for enzymatic and chemical transformations of chitin [14]. 

Only few data are available on the physical properties of chitin in solution due to the difficulties in dissolving this polymer. Solubilization would be necessary to characterize molar mass but also for processing because chitin cannot be processed in molten state. The cohesion of this polymer is too large due to stabilization by a dense hydrogen bond network and thermal decomposition instead of melting occurs upon heating. 

There is a good agreement between the values of DA obtained in the different experiments demonstrating the advantage of solid state NMR well adapted to all the range of acetylation degrees (see Table 4,1), Infrared spectroscopy was also often used but it is more delicate to interpret due to the difficulty in adopting a convenient base line. This was also discussed previously for samples with different DAs [37], 

The agreement between K and a parameters does not look very good but this may be related to the quality of the solution. The origin of chitin as well as the conditions of extraction and 

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