•The water resistance of gelatin films was improved by adding SPI and TGase.•The strength of SPI-gelatin films was increased with low concentration of TGase.•SPI could promote the cross-linking induced by TGase in the gelatin film.•Hydrophilic protein was blocked in strong film network induced by TGase and SPI.The effect of transglutaminase (TGase) on the properties of tilapia scale gelatin films in the presence of soy protein isolate (SPI) was investigated. When 3% TGase was added into gelatin films, the total soluble matter and protein solubility of films were decreased from 89.36% and 92.78% to 35.83% and 40.05%, respectively, and the decline was promoted by adding 5% SPI. The strength of the films was increased by adding 1% TGase irrespective of SPI addition, but decreased when the TGase concentration was further raised. No obvious colour change was observed in the films with or without TGase and SPI. Based on the results of SDS–PAGE, DSC and SEM, it was revealed that the movement of low molecular weight hydrophilic protein was depressed by the cross-linking network structure induced by TGase and SPI during film drying, indicating that adding SPI is essential to improve the thermal stability and water resistance properties of TGase-induced gelatin films.

•Alpha-chain subunits of tilapia skin gelatin could be separated by ultrafiltration.•The α1-subunit was separated from gelatin by regenerated cellulose membrane.•The α2-subunit was separated from gelatin by polyethersulfone membrane.•The Tg of α1-subunit was higher than that of α1-subunit during the first DSC scan.•Reduced viscosity of α1-subunit was higher than that of α2-subunit below 16 °C.Alpha-chain subunits were separated from tilapia skin gelatin using ultrafiltration, and the physicochemical properties of obtained subunits were investigated. As a result, α1-subunit and α2-subunit could be successfully separated by 100 kDa MWCO regenerated cellulose membranes and 150 kDa MWCO polyethersulfone membranes, respectively. Glycine was the most dominant amino acid in both α1-subunit and α2-subunit. However, the tyrosine content was higher in α2-subunit than in α1-subunit, resulting in strong absorption near 280 nm observed in the UV absorption spectrum. Based on the DSC analysis, it was found that the glass transition temperatures of gelatin, α1-subunit and α2-subunit were 136.48 °C, 126.77 °C and 119.43 °C, respectively. Moreover, the reduced viscosity and denaturation temperature of α1-subunit were higher than those of α2-subunit, and the reduced viscosity reached the highest when α-subunits were mixed with α1/α2 ratio of approximately 2, suggesting that α1-subunit plays a more important role in the thermostability of gelatin than α2-subunit.

The effect of dehydrated thermal treatment on the tilapia scale gelatin films was investigated to improve their water resistance and mechanical properties. The gelatin extracted from tilapia scales was mainly composed of β-chain, α-chain and their degraded products with imino acid content of 21.2 %. When the films prepared from tilapia scale gelatin were heated at 80 °C, no significant changes in the properties of films were observed. As heating temperature was increased to 100 or 120 °C, the tensile strength of films was increased gradually with increasing thermal treatment time, while film solubility and protein solubility were decreased. Based on the SDS-PAGE analysis and the protein solubility of the gelatin films in various protein denaturant solutions, it was found that the cross-linking in the gelatin film network between β-chain and α-chain could be induced by heating at 120 °C. It was revealed that the main interactions involved in the gelatin film formation were changed from ionic bonds and hydrogen bonds to hydrophobic interactions and covalent bonds, leading to improve water resistance properties of films.

•Antioxidant fractions were isolated from blue shark skin gelatin hydrolysates.•DPPH and OH▪ scavenging abilities of isolated fraction were close to those of GSH.•Two dipeptides and two tripeptides were identified by Q-TOF-MS/MS.•High antioxidant potency is associated with the presence of C-terminal Tyr and Arg.Antioxidative fractions were isolated from hydrolyzed blue shark skin gelatin with Protamex, and the DPPH and hydroxyl radical scavenging abilities were investigated. The IC50 values of hydrolysates to scavenge DPPH and hydroxyl radical signals were 13.30 and 4.30 mg/mL, respectively. Three fractions (I, II, III) were obtained after separated by Sephadex G-15 gel filtration. The IC50 values of fraction III to scavenge DPPH and hydroxyl radical signals were 0.57 and 2.04 mg/mL, respectively, which were close to glutathione. It was found that fraction III was mainly composed of four peptides and four amino acids by RP-HPLC and ESI–MS/MS analysis, including Glu-Gly-Pro, Gly-Pro-Arg, Gly-Tyr, Gly-Phe, Leu, Arg, Tyr and Phe. The activity assessments for these components indicated that the dipeptide of Gly-Tyr and amino acid of Tyr were important for the excellent antioxidant activities. It is concluded that fraction III has a great potential as natural functional materials for supplement.