Chitosan deacetylation process

Conversion of chitin into chitosan involves the deacetylation process, which is a harsh treatment usually performed with concentrated sodium hydroxide solution. Chitin flakes are treated in suspension with aqueous 40-50% caustic solution at 80-120°C with constant stirring for 4-6 h and this treatment is repeated once or more than once for obtaining high-amino-content product. To avoid depolymerization due to oxidation, sodium borohydrate is added. Excess alkali is drained off and the mixture is washed with water several times until it is free from alkali. Most of the alkali is then used either in deproteinization or in deacetylation. Excess water is removed in screw press and the wet chitosan cake is either sun dries or in drier at 60°C. Chitosan thus obtained is in the form of flakes and can be pulverized to powder. The flowchart for the manufacture of chitosan from the starting material (crustacean shells) is shown in Fig. 19.5. 

The deacetylation process involves the removal of acetyl groups from chltln molecules. The DAC is defined as the average number of glucosamine units per 100 monomers expressed as a percentage. It determines the content of free amino groups [-NH2] in the chitosan and is one of the most important chemical characteristics that influence the physicochemical properties, biological properties, antibacterial activity, and applications of chitosan. In other words, DAC value determines the functionality, reactivity, polarity, and water solubility of the polymer. Chitin does not dissolve in dilute acetic acid. When chitin is deacetylated to a certain degree ( 60% deacetylation] where it becomes soluble in the acid, it is referred to as chitosan [18, 21]. 

Rhazi, M., Desbrieres, J., Tolaimate, A. et al. (2000) Investigation of different natural sources of chitin influence of the source and deacetylation process on the physicochemictd characteristics of chitosan. Polymer International, 49,337-344. 

Tolaimate, A, J. Desbriferes, M. Rhazi, A. Alagui, M. Vincendon, and P. Vottero. 2000. On the influence of deacetylation process on the physicochemical characteristics of chitosan from squid chitin. Polymer 41 2463-2469. 

Acetyl groups are then removed from chitin in order to obtain chitosan. This process of deacetylation can be performed in different ways such as thermal deacetylation or enzymatic bioconversion. 

Chitosan is most frequently produced from chitin by deacetylation process [31,34, 49, 52]. Figure 1.4 shows the scheme for the extraction of chitosan from chitin. 

In the conventional deacetylation process of chitin, concentrated NaOH solution and high temperature have been used. The obtained chitosan cannot have a large degree of polymerization than that of the original chitin. In order to avoid the degradation of the polysaccharide, the alkali treatment should be carried out at as low a temperature as possible. We have carried out a special heat treatment prior to deacetylation for increasing the reactivity of chitin and the chitin thus treated has been converted into chitosan at temperatures always below 100°C. [2]... 

Shell wastes from shrimp, crab, and lobster processing industries are the traditional source of chitin. However, commercial production of chitosan by deacetylation of crustacean chitin with a strong alkali appears to have limited potential for industrial acceptance because of seasonal and limited supply, difficulties in processing, particularly with the large amount of waste of concentrated alkaline solution causing environmental pollution, and inconsistent physico-chemical properties (Chatterjee et ah, 2005). 

Kurita et al. (2002) succeeded in introducing alkyl groups, such as methyl, ethyl, and pentyl groups, into chitin at the nitrogen of C2 acetamido moiety via an adjusted five-step modification process (Figure 5). Chitosan was completely deacetylated and treated with three types of aldehydes, namely formaldehyde (methanol), acetaldehyde (ethanol), and valeraldehyde (pen-tanol). The Schiff bases of chitosan were reduced to iV-alkylated chitosan using sodium cyanoborohydride (NaCNBH3). 

Chitosan is a linear polysaceharide that is composed of randomly distributed /3-(l-4)-linked o-glucosamine and N-acetyl-o-glucosamine (Kumar et al, 2004) (Fig. 4.9). It is commereially produced by deacetylation of chitin, which is the second abundant polysaccharide in nature and is usually found in the cell wall of fungi and exoskeletons of arthropods. Different processes produce chitosan with a deacetylation range of 60-100% (Kumar, 2000). The amino groups on chitosan allow it to react with varied types of reagents to introduce different functional groups to chitosan. 

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