# Study of Physicochemical and Gelatin Properties of Fish Gelatin from Different Sources
## 1 Introduction  
Fish gelatin is an important source of gelatin in addition to mammalian gelatin [1]. Fish gelatin is a polypeptide polymer, extracted from fish scales, skin and bones. Due to its unique functional properties, good biocompatibility, good biodegradability and rich extraction sources, gelatin has been widely explored and applied in the fields of food, cosmetics, drug delivery and tissue engineering [1, 2, 3, 4]. Gelatin has a variety of properties [5, 6] and is widely used in the food industry. Gelatin forms thermoreversible gels with water and can be used as a gelling agent, stabilizer, emulsifier, crystallization modifier, thickener, foaming agent, glue, dispersant and food clarifying agent. Gelatin can be added to meat products as a gelling agent and used in the production of pork, aspic, canned ham, veal and other meat products to improve the yield and quality ofproducts [7; 8]. For example, gelatin with an optimal gel strength of 150-250 (g/cm) is primarily used for preservation of cold (4 \({}^{\circ}\)C) meat products and frozen foods (\(-\)20 \({}^{\circ}\)C) because of its gelatinous, adhesive, stable and protective gel properties. Its film forming, adhesion and protective colloid properties are used for the preservation of frozen foods, where the ideal gel strength is 200-250 (g/cm). Similarly, gelatin is mainly used for gelation, adhesion, thickening, emulsification, stability and protection of colloidal properties in food preservation at 10-20 \({}^{\circ}\)C, mainly including candy and dairy products, where the optimal gel strength is 100-250 (g/cm). Transglutaminase (TGase) is a naturally occurring enzyme that exists in almost all organisms. It is usually used to catalyze the formation of intermolecular and intramolecular covalent cross-linking of proteins, modify proteins and change their functional properties [9; 10]. The catalytic mechanism of TGase is to catalyze the formation of \(\varepsilon\)-gamma (-glutamyl) lysine isopeptide bonds within and between protein molecules by using the gamma-carboxamide group of glutamine residues in the polypeptide chain as the acyl group donor, and the amino group of lysine residues in the \(\varepsilon\)-polypeptide chain as the acyl group receptor. It catalyzes the formation of stable protein network structures by crosslinking the gamma-amido group of glutamine residues and \(\varepsilon\)-amino group of lysine residues in proteins to form covalent bonds, which in turn affect their functional properties [11]. The application of fish gelatin in the food sector has been greatly limited by its poor gelation and rheological properties. The gelation properties of fish gelatin, which is a macromolecular hydrocolloid produced by the partial hydrolysis of collagen, can be enhanced through crosslinking and generation of covalent bonds by transglutaminase (TG) enzymes, resulting in the formation of enzymatic gels. Salt-soluble proteins are those mainly affecting the gelation properties of meat proteins, directly affecting the tissue properties, adhesion and yield of meat products. Gelatin is a common meat product additive that can significantly improve the hardness, elasticity and water holding capacity of salt-soluble proteins. Among the three most common types of meat products, that is, chicken, beef and pork, chicken salt-soluble proteins are known to have better water-holding capacity leading to improved meat texture [12]. However, although China is a major chicken breeding country, the percentage of chicken meat converted into meat products is low. [PERSON] et al. selected chicken salt-soluble proteins as the experimental object and prepared composite salt-soluble proteins by only adding fish gelatin raw material to enhance the application characteristics of the obtained enzymatic gelatin salt-soluble protein composite [13]. In this study, compared with ([PERSON] et al.) [14], the gel strength of the M08 crosslinked TG enzyme of cold-water cod was increased by 480-500 g/cm and 2.3-2.6 times.