## 3 Production of Chitosan  
The major source of chitosan production is chitin found abundantly in nature. chitin extraction on an industrial scale is done from remnants of crustaceans, especially shrimp shells. Based on a country-level analysis, Japan is the highest producer of chitosan. On average, the waste generated from the crustacean shell consists of 30-40% chitin and the rest comprises proteins and calcium carbonate [2]. The first process is to isolate chitin from the crustacean shells, obtained from varied marine sources, by use of chemical extraction. The shells are washed, dried and crushed into a fine powder. This is followed by the process of demineralization that involves acidic treatment of shells, which leads to the dissolution of calcium carbonate. It is carried out at room temperature with the aid of a dilute HCl solution. An increase in temperature can lead to the degradation of polymeric units, thus a proper control as well as temperature must be maintained throughout treatment depending upon the source of extraction [11; 12]. Modification to the treatment consists of demineralization with milder acids, ethylenediaminetetraacetic acid (EDTA), and lactic acid [13; 14]. This is followed by the deproteination step, which involves treatment with an alkaline solution (1M NaOH) for a duration of 2 h at 95 \({}^{\circ}\)C temperature conditions. The solid obtained is washed continuously to achieve a neutral pH and decoloration is carried out by treatment with an organic solvent, or H\({}_{2}\)O\({}_{2}\) which breaks down various pigments like carotenoids. The chitin is oven-dried at a temperature of around 70-80 \({}^{\circ}\)C [15]. In a study conducted to determine the correlation between the seasonal changes and its effect on the quality and chemical composition of chitin, it was found that no significant seasonal variations occur. Thus, chitosan producers can rely on chitin as a raw material for synthesis [16].  
The second process for extraction is dependent on the use of specific enzymes for extraction. The bacterial protease enzymes from _Pseudomonas aeruginosa_ can be used to remove 72% of the proteins from shrimp, but the efficiency as compared to the chemical extraction process is 5-10% lower [17; 18]. All these processes require high energy consumption and are not cost-effective. Thus, a greener approach for the extraction of chitosan from fungal sources is being investigated. Chitosan can be extracted in an eco-friendly and cleaner method from the hyphal walls of fungus _Mucor rouxii_ which, on analysis, was revealed to contain 35-40% of glucosamine content [19]. The extracted chitin can be converted into chitosan by the process of deacetylation (Figure 2). The glycosidic bonds in the chitin are more susceptible to the treatment of alkali. The process of deacetylation can be conducted either homogeneously or heterogeneously. In homogenous deacetylation, alkali chitin is obtained, which is then suspended on crushed ice. This leads to the formation of soluble chitosan with an average 48-55% degree of deacetylation. In contrast, in the heterogenous process chitin is exposed to hot and concentrated NaOH, which forms insoluble chitosan with a higher 85-99% degree of deacetylation [15]. During the process of deacetylation of chitin, the acetyl functional group located on the C\({}_{2}\)- position of D-glucosamine is removed and an amine is formed [5; 20].