Obtaining chitin
Due to its low solubility in popular solvents, chitin is obtained as a result of a multi-stage extraction process (chemical purification). The steps in this process are demineralisation , deproteinisation, and discolouration may also be performed (Figure 2).
The dried shells of crustaceans are demineralised by acid to remove minerals, especially calcium (hence in some sources this stage is called decalcination). The most common is an aqueous HCl solution (up to 10% concentration), but also HNO3, H2SO4, HCOOH or a more environmentally friendly acetic acid solution are commonly used.21,22The insoluble residue is centrifuged and then repeatedly washed with water until the pH is neutral. In the deproteinisation step, the residues are treated with alkalines in order to remove proteins from them. Most often, solutions of monohydroxy bases are used, e.g. NaOH, KOH, but also salt solutions such as Na2CO3, NaHCO3, K2CO3, NaHSO3.23 The fractions are separated again, washed with water until neutral pH and dried. Thus obtained chitin has a pinkish colour.13 Chitin discolouration takes place under the influence of an oxidant, e.g. KMnO4, H2O2, then the precipitate is washed with a solution of oxalic acid. In this way, colourless, pure chitin is obtained.13,22 It is also possible to purify chitin biologically with the use of enzymes – proteases24–26 or microbes in the fermentation process.27