Abstract: Red bean protein isolate (RBPI) was treated by ultrasound (US) combined with transglutaminase (TG), and analyzed for functional and structural characteristics. The relationship between structural modification and functional properties was explored. The results indicated that 5-min US treatment alone could improve the emulsifying activity and foaming ability of RBPI, but reduce the foam stability without affecting the emulsion stability, and it could also increase the surface hydrophobicity (H0) and free sulfhydryl content. TG alone was able to improve the emulsifying activity, emulsion stability and foam stability, but reduce foaming ability, H0 and free sulfhydryl content of RBPI. RBPI treated by US-TG had higher emulsifying activity and foam stability, and lower H0 and free sulfhydryl content. The gel induced by TG after US treatment for 5 min had a more uniform and compact microstructure with lower syneresis, and possessed increased hardness and adhesiveness. The absorption peak strength in the amide I band of RBPI treated by US-TG was enhanced, and more random coil structure was transformed into an ordered β-sheet structure, which may contribute to the improvement of functional properties. Treatment with US for 5 min combined with TG could significantly increase the peak temperature (Tp) and enthalpy (ΔH) (P < 0.05), thereby improving the thermal stability or tertiary structure stability of RBPI, which indicated that TG-induced cross-linking of RBPI was promoted by treatment with US and the combined treatment could facilitate the development of functional properties of proteins.

Key words: ultrasound, transglutaminase, red bean protein, functional properties, structure