Clean and sustainable extraction of gelatin: Effects of microwave and freeze-thaw on the crosslinking degree and hydrogen bond of fish skin collagen.

The traditional gelatin extraction methods (acid-base) may hinder to their green applications due to mass energy consumption and pollution. Herein, we constructed a clean and sustainable gelatin extraction method, investigated the molecular mechanism of microwave treatment (0-360 min) and freeze-thaw on the gelatin extraction from the perspective of the crosslinking degree and hydrogen bonds. Microwave (0-60 min) can improve the hydrolysis degree (DH) and expose more enzyme cleavage sites of collagen by destroying the intramolecular and intermolecular covalent crosslinking. Freeze-thaw loosened the collagen structure mainly by destroying hydrogen bonds, further increased degree of freedom, DH and enzyme cleavage sites, improved the gelatin yield and gel strength. However, the long-term microwave treatment (60-360 min) severely damaged the covalent crosslinking, increased the DH and enzyme cleavage sites. Freeze-thaw further loosened the collagen structure, caused the rapid dissolution and degradation of subunit, improved the gelatin yield and reduced the gels strength.