Abstract: Luteolin, quercetin and myricetin are natural plant flavonoids with similar structure and different numbers of hydroxyl groups. In this work, the mechanical and optical properties of fish scale gelatin edible films incorporated with luteolin, quercetin or myricetin were measured and compared with the aim of exploring the role of interaction between gelatin and flavonoids with different numbers of hydroxyl groups inside the composite films in improving the properties of the films. Differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy showed that steric hindrance between gelatin molecules was formed by the addition of the flavonoids. In the meantime, hydrogen bonds and dipole-dipole force were formed between the hydroxyl groups of flavonoids and gelatin, and they were strengthened with the increasing number of hydroxyl groups. The interactions between the hydrophobic rings and hydroxyl groups of flavonoids and gelatin molecules led to the secondary structural transition from β-turn to β-sheet, random coil and α-helix. As a result of the strengthened hydrogen bonds and dipole-dipole force between flavonoids and gelatin molecules with the increasing number of hydroxyl groups, the molecular mobility of gelatin was weakened, making the film structure more compact and elevating the tensile extensibility. Although incorporation of flavonoids improved the ultraviolet absorbance, the number of hydroxyl groups did not affect the light transmittance of gelatin films. These results can provide a theoretical guideline for the improvement of gelatin edible films with the addition of flavonoids.

Key words: interactions, flavonoids, gelatin, improvement of film properties