# A Walkway from Crayfish to Oligochitosan
## 1 Introduction  
Chitosan, poly[\(\beta\)-(1\(\rightarrow\)4)-2-amino-2-deoxy-D-glucose], is the by-product of the crustacean food processing industry and is mainly produced from crab, shrimp, or krill shells [1]. Low toxicity, biodegradability, and biocompatibility, together with a nonspecific antimicrobial activity has made chitosan a product of worldwide application in pharmaceutical, food, and cosmetic compositions [2, 3, 4, 5, 6, 7, 8]. The growing market for chitosan [9] requires the application of pure material free of harmful residual impurities (proteins and heavy metals) as chitosan is used, in particular, as an ingredient in biopharmaceuticals, skincare, and other cosmetic compositions [5, 10]. Unfortunately, both chitin that is separated from aquatic crustaceans and chitosan that is industrially manufactured from chitin can contain excessive amount of proteins and residual heavy metals, which come from polluted land-locked seas and rivers [11]. The narrow-toed crayfish _Actacus leptodactylus_ is an object of local fishing and artificial breeding in many countries [12]. As with other crustaceans, chitin is the second main component of crayfish shells [13]. Unfortunately, the growing application of chitosan produced from crayfish is far from being at the level it could be at [14, 15]. The solution to this could be the use of crayfishing in not only in food processing and improvement of soil structure but also for the extraction of chitin and production of chitosan and its derivativesas additional products of crayfishing, as this would increase the economic efficiency of crayfish plants by widening the application of crayfish waste and chitosan [16; 17; 18]. For several decades, the terms chitin, chitosan, water soluble chitosan, low molecular weight chitosan, oligochitosan, chitosan oligomers, chitosan oligosaccharides, heterochitosans, heterochitosan oligosaccharidesacharides, chitin oligomers, and chitol oligosaccharidesacharides have been widely used in scientific literature. Unfortunately, all these terms are used voluntary since the terminology in chitin/chitosan sciences has not been commonly accepted so far. The terms \"chitin\" and \"chitosan\" are the most used terms in chitin and chitosan sciences (\"chitinology\"). The term \"chitin\" describes a homopolymer of N-acetyl-\(\beta\)-D-glucosamine linked via 1,4-bonds. Chitin extraction from natural sources usually has molecular weight of a million Daltons and more. Chitin is insoluble in organic (acetic and lactic) acids but is highly swelling and hydrolyzing in concentrated inorganic (hydrochloric and phosphoric) acids. Deacetylation of chitin by one or another method leads to the preparation of a completely or partially deacetylated chitin. When the degree of deacetylation reaches the value of 60% or more, chitin becomes soluble in diluted acetic or hydrochloric acids. This soluble form of chitin is named \"chitosan\". Thus, the solubility in aqueous acidic media is the criterion for distinguishing between chitin and chitosan. Here, we should take into account that the term \"chitosan\" is not a chemical but a conditional one. As shown and discussed in several articles, chitosan could be distinguished from oligochitosans (short chain chitosan) by molecular weight (MW). Still, the boundary between these groups are fluid, although the term \"oligochitosan\" can be used for chitosan molecules with MW \(\leq\) 16 kDa (degree of polymerization \(\leq\)100) [19; 20; 21; 22]. So far, a number of articles published have been devoted to the elaboration of varied methods for the separation of chitin from different chitin containing sources [23]. In spite of the multiplicity of the methods, all of them include the main ones, such as deproteination, demineralization (decalcification), and decolorization (if required). Deproteination is required since crayfish, like other crustacean animals, contains unbound edible proteins and polysaccharide conjugated proteins [13].