Studies on egg albumen and whey protein interactions by FT-Raman spectroscopy and rheology

Large deformation rheological studies of either egg albumen or whey protein isolate (15% protein w/w) gels made by heating at 90 °C for 30 min indicated a higher gel strength and Youngʹs modulus values for egg albumen compared to whey protein isolate. The proteins mixed in the ratio 10:5 whey/egg albumen indicated synergistic interactions producing greater than expected gel strength values. Small deformation rheological studies of egg albumen 15% (w/w) and whey protein isolate 15% (w/w) showed very low values of the storage modulus (G′) at 20 °C suggesting that both proteins do not easily aggregate at room temperature. Whey proteins gelled at 80 °C, whereas egg albumen proteins were already cross-linked at 20 °C (G′>G″). A mixture of 7.5% whey/7.5% egg albumen showed the highest G′ value at the end of both heating and cooling cycles but this value was lower than that of the individual proteins. Changes in the conformation, molecular structure and protein–protein interactions in whey and egg albumen (15% w/w in D2O pD7.2) and their mixtures (7.5:7.5 protein w/w) were investigated by FT-Raman spectroscopy. Upon heating to 90 °C for 30 min, individual whey and egg albumen proteins showed changes in the CH (1350 cm−1), CH2 (1450 cm−1) bending vibrations and Trp residues at 760 cm−1, reflecting involvement of hydrophobic interactions. Changes in the disulphide stretching vibrations were also observed. Peak intensity increased for β-sheet structures in the Amide 111′ region 980–990 cm−1 and decreased for helix structures (900–960 cm−1) for both egg albumen and whey proteins. Differences in the experimental and theoretical average spectra indicated changes in β-sheet structures, CH2 bending, carboxyl and hydrophobic groups in heated binary mixtures of egg albumen and whey proteins, providing evidence of protein–protein interactions, observed by large deformation rheology.