**1. Introduction**

The discovery of chitosan dates back to 1811 when Professor Henri Braconnot, director of the botanical garden in Nancy, France, isolated what he called "fungine" from fungal cell walls. About 30 years before the isolation of cellulose, in 1823, Odier conducted a study on insects and found that the same structure was present in insects as well as plants. Odier later named the fungine "chitin" a word derived from Greek that means membrane or envelope. The concept of chitin became more understandable when Lassaigne showed the presence of nitrogen in the structure of chitin in 1843. The term "chitosan" emerged following a discovery by Rouget in 1859. When heating chitin in a concentrated potassium hydroxide solution Rouget observed that the chitin became soluble with the chemical and heat treatment. Ledderhose described in 1878 that chitin consists of glucosamine. Hoppe-Seyler adapted the term chitosan from chitin in 1894. At the beginning of the 20th century, many studies on chitosan from sources of chitin were conducted. Rammelberg proved that chitosan was found in crab shells and fungi through his work in 1930. In addition, chitin was hydrolyzed in many ways and found to be a glucosamine polysaccharide. Studies on the formation of chitin and chitosan in mushrooms were performed with x-ray analyses in the 1950s. The first book on chitosan was published in 1951, 140 years after Braconnot's first observations. In the early

1960s, studies were conducted on the ability of chitosan to bind red blood cells. In the same year, chitosan was also considered as a hemostatic agent. In the next 30 years, chitosan was used in treatment plants to provide asepsis water. In the last 20 years, research on chitosan has intensified due to its many important properties [1]. Today, chitosan has many industrial applications and after cellulose, it is the most common polysaccharide chitin in the world. As one of the most important derivatives of chitin, chitosan is a polycationic biopolymer obtained by partial or complete deacetylation (removal of an acetyl functional group from an organic compound) of chitin in an alkaline environment [2]. The only difference between cellulose and chitosan biopolymer is the presence of the acetyl (-NH2) functional group instead of the hydroxyl (-OH) functional group in the cellulose structure. This difference ensures that the chain structure of the chitosan biopolymer is polycationic. Many superior properties of chitosan arise from this polycationic structure. In addition to this advantage, the presence of both –OH and –NH2 groups in the chain structure of chitosan and the fact that these groups can be modified in different ways is a situation that highlights its uses [3]. Chitosan, which can be obtained in large quantities from many natural sources containing chitin, such as the exoskeleton of mushrooms, crayfish, shrimp, and crabs, is more advantageous than other biopolymers including chitin in terms of non-toxicity to organisms, easy biodegradability, and biocompatibility. For these reasons, chitosan is a natural, safe, cheap, raw material biopolymer used in many industrial areas such as food, medicine, pharmaceuticals, cosmetics, agriculture, wastewater treatment, and textiles. Besides having antiviral, antibacterial, and antifungal properties, chitosan is also an effective agent in controlling and reducing the spread of diseases by promoting the defense system of plants. In addition, chitosan is being used for improvement in agriculture because it chelates metal ions in the environment (water, soil, etc.) and prevents the uptake of toxic metals in plants [4].

Chitosan is a natural and biodegradable biopolymer used in different industrial applications as an agent for flocculation and chelating, permeability control, and as an antimicrobial, among other processes. Predominantly produced today by the deacetylation of chitin on an industrial scale, chitosan is found in the exoskeleton of crustaceans and insects, and the cell walls of many fungi and some algae. Although the main source of chitin is crab, shrimp, crayfish, and shrimp residues, the importance of insect chitosan depends on the role insects play as a sustainable protein source. Insects are seen as an alternative to traditionally consumed proteins derived predominantly from traditional livestock (mainly cows, chickens, and pigs) and fish. In addition, using the insect as a protein source produces two by-products of interest to the industry, lipids that can be used as biofuels (30–40% total dry weight) as well as a residual material made of chitin with some bioactive properties from which chitosan can be produced [5].
