# A Walkway from Crayfish to Oligochitosan
## 4 Conclusions  
In this study, a walkway from the aqua cultured crayfish _A. leptodactylis_ to chitosan and oligochitosan hydrochloride was shown on a laboratory scale. Molecular characteristics of oligochitosan hydrochloride determined by HP-SEC showed acceptable values for oligochitosan characteristics. The content of heavy metals and proteins, as well as the physicochemical properties of chitosan and oligochitosan hydrochloride, were compared with the requirements of the USA and European Union pharmacopoeias. As shown, chitosan and oligochitosan hydrochloride could be prepared as products purified from residual crayfish proteins and heavy metals. It was demonstrated that the use of polytetrafluoroethylene-made reactors at the chitin deacetylation stage produced chitosan pure of Fe, Cr, and Ni  
Figure 5: \({}^{1}\)H-NMR spectrum of crayfish chitosan (red) and oligochitosan (green).  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Characteristics**} & **USP (2013) Acceptable** & **Factual Data for** & **EP (2004) Acceptable** & **Factual Data for** \\  & **Criteria for Chitosan** & **Chitosan** & **Criteria for Chitosan** & **Oligochitosan** \\  & & **Hydrochloride** & **Hydrochloride** \\ \hline Appearance & & & & \\ (in solid state) & absent & white & white or almost white & white \\ \hline Matter insoluble & & & & \\ in water & absent & 0.15\% & \(\leq\)0.5\% & 0.05\% \\ \hline Degree of deacetylation & & & & \\ (\%) & 70-95\% & 96\% & unstandardized value & 98\% \\ \hline Proteins (\%) & \(\leq\)0.2\% & 0.11 & not applied & \textless{}0.01 \\ \hline Chlorides (\%) & not applied & not found & 10–20\% & 16.3\% \\ \hline Humidity (\%) & \(\leq\)5\% & 3.1\% & \(\leq\)10\% & 9.1\% \\ \hline Ash (\%) & \(\leq\)1\% & 0.05\% & \(\leq\)1\% & \textless{}0.01 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Physicochemical characteristics of chitosan and chitosan hydrochloride.  
residues, which have been usually present in commercial chitosan. In addition, it was shown that the application of an alumosilicate-made vessel used for deproteination of crayfish waste in sodium hydroxide at elevated temperatures should be avoided, since this application led to contamination of chitin by Al and Si residues. The results showed that chitosan extracted from crayfish exoskeletons and oligochitosan hydrochloride could be prepared to qualities meeting the requirements for the permitted presence of residual proteins, heavy metals, solubility, contents of insoluble matter, and other properties. The laboratory protocol might be used for the elaboration of an industrial one that could improve the commercialization of crayfish by widening the utilization of crayfish waste, and for the production of pharmaceutical-grade chitosan and oligochitosan hydrochloride.  
E.A.B.: chemical methodology, samples preparation, assisting experiments; O.V.V.: analytical methodology, samples analysis; B.B.B.: samples analysis, software, resources & original graft supporting; I.V.B.: reviewing, supervision & validation; V.E.T.: writing, editing. All authors have read and agreed to the published version of the manuscript.  
This work was supported by Russian Science Foundation (grant No. 23-23-00198). NMR, ICP-MS and EDX studies were performed with the financial support from Ministry of Science and Higher Education of the Russian Federation (Contract No. 075-03-2023-642) employing the equipment of the Center for molecular composition studies of INESO RAS. Elemental analysis and determination ash and humidity were carried out using facilities of the Laboratory for element microanalysis of INEOS RAS.  
Not applicable.  
Data Availability StatementData is contained within the article.  
The authors declare no conflict of interests.