Effects of high pressure and heat processing on the structure and rheological properties of food proteins.

This study evaluated the effect of heat and high pressure processing of whey protein isolate, beta-lactoglobulin, egg albumen, ovalbumin and their binary mixtures. Rheological studies of whey proteins (15% w/w) in distilled water indicated that gels made by heating at 90°C for 30 min were stronger and more elastic compared with pressure-treated gels (400-600 MPa for 20 min). Gel strengths at higher pressure (650-800 MPa) were similar to heated samples, hi contrast, egg albumen proteins (15% w/w) in distilled water showed no gelation below 500 MPa but increased in strength with increasing pressure, although values remained below those of heat- induced gels. Heat and pressure treatment of whey/egg albumen protein mixtures (10:5) produced gels stronger than expected indicating synergistic interactions. Sucrose influenced gelation and interactions, with 20% sucrose being the optimal for egg albumen and whey proteins. Sucrose addition produced weaker pressure-treated gels compared with heated gels. Addition of 1% NaCl produced stronger whey protein gels compared with egg albumen but weaker mixed whey/egg albumen gels. Combined heat (50°C or 60°C) and pressure (600 MPa) produced weaker gels compared with heat treatment alone, but stronger gels than pressure treatment alone for both whey and egg albumen proteins and mixtures. Values increased with increasing temperature due to greater protein unfolding, as shown by Differential Scanning Calorimetry. FT Raman spectroscopy indicated that both heat and high pressure affected alpha-helix, beta-sheet structure, hydrophobic interactions and disulphide bonds. Heat caused greater changes in disulphide bonds and beta-sheet structures but pressure produced greater changes in hydrophobic interactions. Self deconvolution of the Amide I band showed quantitative changes in secondary structures. Random coil increased in high pressure treated (600 MPa, 30 min) beta-lactoglobulin, whereas, for ovalbumin, beta-turns doubled. The different mechanisms of gelation observed for heating and high pressure treated egg albumen, whey proteins and their mixtures can provide novel textures