Abstract
Chitin is an abundant polysaccharide primarily produced as an industrial waste stream during the processing of crustaceans. Despite the limited applications of chitin, there is interest from the medical, agrochemical, food and cosmetic industries because it can be converted into chitosan and partially acetylated chitosan oligomers (COS). These molecules have various useful properties, including antimicrobial and anti-inflammatory activities. The chemical production of COS is environmentally hazardous and it is difficult to control the degree of polymerization and acetylation. These issues can be addressed by using specific enzymes, particularly chitinases, chitosanases and chitin deacetylases, which yield better-defined chitosan and COS mixtures. In this review, we summarize recent chemical and enzymatic approaches for the production of chitosan and COS. We also discuss a design-of-experiments approach for process optimization that could help to enhance enzymatic processes in terms of product yield and product characteristics. This may allow the production of novel COS structures with unique functional properties to further expand the applications of these diverse bioactive molecules.
Highlights
Chitin, Chitosan and Chitosan OligomersChitin is a natural polysaccharide predominantly found in the cell walls of fungi and the exoskeletons of crustaceans and insects
The optimization of chemical chitin degradation can be achieved by combining different catalysts and by modifying the physicochemical parameters to produce chitosan oligomers (COS) in a controllable manner, but it is not yet possible to produce individual species of COS with the DP, degree of acetylation (DA) and pattern of acetylation (PA) fully defined
A synthetic biology approach has recently been applied in E. coli to achieve the biosynthesis of defined COS starting from the substrate glucose by expressing the nodC and nodB genes encoding chitin synthase and chitin deacetylase, respectively [34]
Summary
Chitin is mainly found in fungal cell walls and in the exoskeletons of crustaceans and insects, and in some algae and micro-algae. NaOH at temperatures up to 160 ◦ C, but other alkalis such as KOH, Na2 CO3 , NaHCO3 , K2 CO3 , Ca(OH) , Na2 S, CaHSO3 and Na3 PO4 have been used successfully [61,65,66,67] These treatments cause the random cleavage of the chitin backbone and random deacetylation, resulting in chitosans and COS that are undefined in terms of DP and DA [61,65,67,68]. Biological methods can reduce the environmental burden of the alkali/acid steps and avoid unwanted changes to the chitin structure [69]. In such methods, naturally produced lactic acid is used for demineralization and proteases are used for deproteination [70,71,72]. Whole bacterial extraction procedures have been developed involving the co-fermentation of proteolytic and lactic acid bacteria on shrimp waste, resulting in milder treatment conditions and a better-defined chitin product [69,73,74,75] (Figure 3)
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