Characterisation of whey protein-polyphenol conjugates prepared by the noncovalent and covalent methods for their effect on the functional properties of whey proteins

Whey proteins were conjugated with polyphenols (caffeic and ferulic acid) using different methods viz. noncovalent (pH 3, 5 and 7) and covalent (alkaline, chemical coupling and free radical grafting method). The binding of polyphenols was higher in the covalent conjugates (maximum in free radical grafting) than the noncovalent conjugates (maximum seen at pH 7). The conjugation was confirmed by decreased intrinsic fluorescence intensity (>30%), increased UV-visible absorbance and changed intensity of peaks in amide I region of Fourier transform infrared spectroscopy (FTIR). The conjugates that were prepared by the covalent (free radical grafting) and noncovalent (pH 7) methods exhibited much better emulsifying activity and emulsion stability than the alkaline conjugates for both caffeic and ferulic acid.