The effects of agar addition and ultrasound treatment on thermomechanical and physical properties of smooth hound (Mustellus mustellus) skin gelatin film

This study investigates the effect of agar addition and ultrasound treatment on the properties of edible films made from smooth hound (Mustellus mustellus) skin gelatin. The FTIR spectra showed the classic profile of gelatin spectrum presenting three characteristic absorption peaks related to amide I (1620-1690 cm(-1)), amide II (1540-1580 cm(-1)) and amide III (1230-1280 cm(-1)). The agar addition or ultrasound treatment did not cause important influence on FTIR spectra. Agar addition resulted in an improvement of the film rigidity, since blended films of gelatin:agar (25:75) showed important increase of the film tensile strength (12.3 MPa) and a decrease in elongation at break (135%), as compared to the 100% gelatin-based films. All the films were colorless ( increment E < 6.8 and opacity < 2.5) and agar addition resulted in a slight increase of the L* values of the blended films that become more transparent. The ultrasound treatment had no significant effect on the mechanical and color properties. The water contact angle was increased for the blended films, which imply the increase of their hydrophobicity. At 120 s the increase in water contact angle was 11.5 and 36.6% for the gelatin:agar (50:50) and (25:75) films, respectively, as compared to the 100% gelatin-based film that suggested a decrease in surface hydrophilicity of the blended films. Agar addition (gelatin:agar; 25:75) decreased the film water solubility by 34%, which suggested that agar could improve their water resistance. Overall, agar addition improved barrier properties of smooth hound gelatin films encouraging their use as a food packaging material.