Chitosan is the most common chitin derivative. Obtained thought the deacetylation of the chitin macromolecule, by subjecting it to hot alkali (e.g., NaOH concentrated solutions) treatment, chitosan is characterized by a low acetylation degree, usually below 50% and molecular weight between 50 and 2000 kDa. In contrast to chitin, chitosan is soluble in dilute solutions (usually 0.1-2% v/v) of acetic acid, hydrochloric acid, or citric acid solubility at pH above its pKa, 5.5-6.5. 

Chemical structure and preparation of chitosan

Chemical structure and preparation of chitosan As the second most abundant natural biopolymer after cellulose, chitosan consists of β(1-4)-linked D-glucosamine with randomly located N-acetylglucosamine groups depending upon the degree of deacetylation of the polymer. Deacetylation is normally conducted by repetitions of alkaline hydrolysis due to the resistance of such groups owing to the trans arrangement of the C2-C3 substituents in the sugar ring. The conditions during deacetylation must be properly controlled so that the chitin may be deacetylated to chitosan resulting in a better yield.

Generally, two different methods are known for the preparation of chitosan from chitin with varying degrees of acetylation. One of them is heterogeneous deacetylation of solid chitin and the other is homogeneous deacetylation of preswollen chitin under vacuum in an aqueous medium. In both cases, concentrated alkali solutions and long processing times are required for the deacetylation reaction which may vary depending on the heterogeneous or homogeneous conditions from 1 to nearly 80 h. 

To reduce the long processing times and the requirement for large amounts of alkali to deacetylate chitin, several alternative-processing methods have been developed. Examples of these include the use of successive alkali treatments using thiophenol in DMSO; thermome chanical processes using a cascade reactor operated under low alkali concentration; flash treatment under saturated steam; the use of microwave dielectric heating; and intermittent water washing. There is evidence that in certain bacteria and fungi, enzymatic deacetylation can take place. Recently, a microwave technique for efficient deacetylation of chitin nanowhiskers to a chitosan nanoscaffold has also been reported. Both chitin and chitosan can be prepared using common processes, which are given in Fig. 1.