# Chitosan Nanocomposite Coatings for Food, Paints, and Water Treatment Applications
## 2 Chitosan and its Properties
### Physico-Chemical Properties of Chitosan  
#### 2.2.1 Degree of Deacetylation (DD)  
The degree of deacetylation is one of the most significant chemical characteristics of chitosan that determines the content of free amino groups (-NH\({}_{2}\)) that is formed by the partial replacement of acetyl groups (-C\({}_{2}\)H\({}_{3}\)O) resulting in a copolymer of \(N\)-acetyl-glucosamine and D-glucosamine. Copolymers formed containing higher than 50% D-glucosamine units are typically considered as chitosan, whereas copolymers with more than 50% N-acetyl-glucosamine units are referred to as chitin. For chitosan, the percentage of D-glucosamine units are termed as the degree of deacetylation (DD), whereas for chitin, the percentage of N-acetyl-glucosamine units are known as the degree of acetylation (DA). The degree of deacetylation (DD) of chitosan (ratio of D-glucosamine to the sum of D-glucosamine and N-acetyl D-glucosamine) provides an indication of the number of amino groups in the polymer chains (e.g., D-glucosamine residues of 70% in deacetylated chitin corresponds to a deacetylation degree of 70% and acetylation degree of 30%).  
The deacetylation of chitin begins in the amorphous regions prior to the crystalline regions, which occur by hydrolysis through chemical and biological (enzymatic) processes. Chemical treatment methods generally involve acidic or alkaline treatments under a nitrogen environment, or by the addition of sodium borohydride to NaOH solutions, to avoid any undesirable side reaction. Enzyme hydrolysis is environmentally friendly but is comparatively more expensive and thus primarily limited to laboratory-scale experiments [27]. Researchers determine the DD of chitosan using acid-base titration [28], potentiometric titration [29], conductometric titration [30], \({}^{1}\)H-NMR spectroscopy [31; 32], elemental analysis [30], Fourier transform infrared (FTIR) spectroscopy [33], UV spectrophotometric analysis [34], capillary zone electrophoresis [35], and Raman spectroscopy [36].  
DD indicates the amount of amino groups in chitosan polymer, which will affect the properties of chitosan, such as charge, density, solubility, crystallinity, degradation behaviour, mechanical, barrier, and thermal properties [37; 38; 39]. An increased percentage of amino groups in chitosan polymer makes it soluble in weak acids, a characteristic difference from chitin. The amino groups in chitosan polymer, which is highly reactive, contribute towards its versatility for utilization in various industrial applications. Recently, [PERSON] et al. investigated the effect of deacetylation degree on mechanical and barrier properties of chitosan films with three different DD values, 81.0%, 88.1% and 95.2%, wherein CH films with higher DD values (88.1% and 95.2%) were found to have better water barrier property and tensile strength compared to films obtained with a chitosan value of 81.0% DD [37]. It has been reported that the antimicrobial efficiency also improves with the increase in the degree of deacetylation of chitosan that is generally attributed to the increase in the number of positive charges from the amine groups [39].