Physicochemical properties and bioactivity of whey protein isolate-inulin conjugates obtained by Maillard reaction

The functional properties and physiological functions of whey protein isolate (WPI) decreased near its isoelectric point (PI). The Maillard reaction covalently binding polysaccharides to proteins is an effective method to improve the functional activities of proteins. WPI-inulin conjugates were prepared by wet-heating method at 70 °C for 2 h, 4 h and 6 h, respectively. New bonds at higher molecular zone appearing at SDS-PAGE, decreased free amino acid content and new formed CN bonds in FT-IR of conjugates compared with WPI confirmed the formation of the covalent bonds between WPI and inulin. As the increase of the reaction time, both the brown intensity and fluorescence intensity of WPI-inulin conjugates became higher. Amino acid contents, Circular dichroism analysis and SEM analysis presented the primary structure, secondary structure and surface structure change of protein after covalent with inulin. Emulsion properties of emulsion activity (EAI) and emulsion stability (ES) of WPI-inulin conjugates were assessed and both showed significantly enhanced compared with WPI at range of pH 3 to pH 7. AAPH⁺ scavenging test and ORAC measurement also revealed that covalent binding with inulin enhanced the antioxidant activities of WPI. This work presented the conjugation with inulin successfully enhanced the functional properties of WPI.