Recent advances in extraction and processing of chitin using deep eutectic solvents

Chitin, as the most abundant non-wood biopolymer on earth, exists widely in the shells of crustaceans, the cell walls of fungi, and the cuticles of insects. It consists of N-acetyl-D-glucosamine and D-glucosamine units with various molar ratios linked by beta-(1 -> 4)-glycoside bonds. The inherently recalcitrant structure of chitin-rich raw materials and the high density of hydrogen bonding between chitin's molecules result in difficulties in the extraction and further processing. Currently, the mineral acids and bases are dominant in the extraction and processing of chitin; however, they are harmful to our human health as well as the environment. In recent years, the use of deep eutectic solvents (DESs) to extract and process chitin has attracted considerable interest due to its superior sustainability, low toxicity, cost-effectiveness, facile preparation, biodegradability and recyclability. The present review provides a critical overview of recent advances in the utilization of DESs for the extraction and processing of chitin, including isolation, dissolution, surface modification, and nanomaterial production. Particular emphases are placed on the mechanism, characterization method, and governing factors. We also outline the crucial challenges and limitations in this field, and then propose perspectives and future directions. It is anticipated that this review will provide some insights into the structure-function relationship of the extraction and processing of chitin using DESs.