Abstract: In this paper, fluorescence, Fourier transform infrared and circular dichroism spectroscopies were used in conjunction to study the interaction between bovine casein (Cs) and xylitol (XY) and its effect on the structural and functional properties of casein. The results of fluorescence spectroscopy indicated that xylitol strongly quenched the fluorescent intensity of bovine casein. The quenching mechanism was static quenching that generates a new complex. The binding constants for the interaction at 300, 310 and 320 K were 5.326 × 106, 2.600 × 106 and 2.160 × 106 L/mol, respectively, and the binding sites were 1.513, 1.452 and 1.422, respectively. The main binding forces were electrostatic attraction and hydrophobic force with a binding distance of 3.564 nm. The results of synchronous fluorescence and threedimensional fluorescence spectroscopies revealed that the interaction site was closer to the tryptophan residues; this interaction enhanced the hydrophobicity of the surrounding microenvironment and changed the molecular conformation of bovine casein. Fourier transform infrared and circular dichroism spectroscopies showed that xylitol changed the secondary structure of bovine casein by increasing the relative content of α-helix and reducing the relative content of random coil structure and as a result, the structure of bovine casein became more compact. Furthermore, this structural change led to a decrease in the emulsifying activity, an increase in the emulsion stability, and a decrease in the surface hydrophobicity of casein. These results provide a theoretical basis for the development of functional dairy-based ingredients.

Key words: xylitol, bovine casein, fluorescence spectroscopy, Fourier transform infrared spectroscopy, circular dichroism spectroscopy