A comparative study of heat and high pressure induced gels of whey and egg albumen proteins and their binary mixtures

Large deformation rheological studies of either egg albumen or whey protein isolate (15% protein w/w) gels induced by heating at 90 °C for 30 min were compared to those induced by a range high pressures (400-800 MPa for 20 min). Gels made by heating indicated higher gel strength and Young's modulus values for whey protein from pressures of 400-600 MPa for 20 min but similar values at 650-800 MPa. In contrast, egg albumen showed no gelation below 500 MPa for 20 min, but there was an increase in both gel strength and Young's modulus with increasing pressure, although values remained lower than those of the heat-induced gels. A mixture of 10:5 whey/egg albumen showed the highest gel strength and Young's modulus for both heated and high pressure-treated (400-600 MPa) gels, although, the heated mixture had the highest values. Electron micrographs indicated that high pressure-treated gels had a porous aggregated network for egg albumen while whey proteins showed a continuous fine stranded network. The heated mixtures of whey:egg albumen (7.5:7.5) showed large dense aggregates whereas high pressure-treated mixtures produced smaller aggregates. Raman spectroscopy of both heated and high pressure-treated whey and egg albumen (15% w/w in D2O pD7) and their binary mixtures (7.5:7.5, protein w/w) indicated changes in β-sheet structures in the Amide 111′ region (980-990 cm−1); however, peak intensity was reduced for high pressure-treated samples. β-Sheet structure (1238-1240 cm−1) present in heated whey was absent in high pressure-treated whey and in egg albumen. Involvement of hydrophobic regions was reflected by changes in the CH (1350 cm−1) and CH2 (1450 cm−1) bending vibrations. In addition to the Trp residues at 760 cm−1, there were broad peaks at 874-880 cm−1 and tyrosine 1207 cm−1 in the high pressure-treated samples. Disulphide bands (500-540 cm−1) in heated whey and egg albumen proteins showed higher peak intensities compared to high pressure-treated samples. Differences in the experimental and theoretical spectra indicated changes in the hydrophobic regions, tyrosine (1207 cm−1) and tryptophan (880 cm−1) and CH2 bending in high pressure-treated samples, whereas heated samples indicated marked changes in β-sheet structures (987 and 1238 cm−1) as well as hydrophobic regions CH (1350 cm−1) and CH2 (1450 cm−1) bending vibrations.