Effect of protein concentration on the film formation of tilapia skin gelatin at high temperature

With the aim to clarify the film-forming characteristics of fish skin gelatin at high temperature, the effect of protein concentration of film-forming solution on the properties of edible films based on the tilapia skin gelatin was investigated. The gelatin was extracted from tilapia skins which were obtained from fresh fish during fillet processing. The obtained gelatin were swollen in distilled water for 30 min and then dissolved at 60°C to obtain the film-forming solutions (FFS). Glycerol as a plasticizer was added at the concentration of 20% of gelatin, and the concentration of gelatin dry basis in FFS was adjusted to 5%, 10%, 20%, 30% and 40%. Air bubbles in FFS were removed by a hybrid mixer. Subsequently, the FFS was cast onto a rimmed silicone resin plate (50 mm ×50 mm) and dried at 100°C for 30 min, and the obtained films were then conditioned at (25±0.5)°C and (50±5)% RH for 48 h. After having been peeled off, the resulting gelatin films were used for the determination of mechanical properties, water resistance properties and heat shrinkage (HS). Furthermore, the formation mechanism of skin gelatin films was elucidated by electrophoretic analysis, differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FT/IR). As a result, the tensile strength (TS) and HS of gelatin films prepared at 25°C (Control) could reach up to 67.72 MPa and 48.75%, with the matter solubility (MS) and protein solubility (PS) of films was 22.97% and 15.59%, respectively. When film-forming solution with protein concentration of 5% was dried at 100°C, the TS and HS of the resulting films was decreased to 8.99 MPa and 8.13%, while the MS and PS was increased to 33.36% and 37.47%, respectively. However, the values above were all gradually close to those of the control with increasing the protein concentration to 40% in the film-forming solution. On the other hand, based on the analyses with DSC and FT/IR for the gelatin films, it was found that the triple-helical structure of tilapia skin gelatin was destroyed when the film-forming solution with lower protein concentration was dried at high temperature, resulting in the gelatin films obtained with poor mechanical properties and water resistance ability.