Effect of High-Pressure Treatment of Protein on the Rheology of Flocculated Emulsions Containing Protein and Polysaccharide

We report on the effect of high-pressure treatment (up to 7 kbar) on the rheology of concentrated emulsions containing the globular protein, bovine serum albumin (BSA), and the anionic polysaccharide dextran sulfate. Small-deformation rheological properties have been determined for oil-in-water emulsions (40 vol % n -tetradecane, 2.7 wt % BSA, pH 7) containing polysaccharide added after emulsion formation. In the absence of high-pressure treatment, a plot of complex shear modulus G * at 1 Hz and 30 °C against polysaccharide concentration C P shows a maximum in G * at C P ≈ 0.1 wt % which is consistent with bridging flocculation caused by a net attractive electrostatic protein−polysaccharide interaction at the emulsion droplet surface. High-pressure treatment of BSA before emulsification, for 30 min at a pressure in the range of 400−700 MPa, leads to substantial changes in the flocculation and rheological behavior of the emulsion after polysaccharide addition but no discernible change in the emulsion droplet-size distribution prior to the addition. In contrast, heat treatment of BSA at 70−80 °C before emulsification leads to an increase in average droplet size and to changes in emulsion rheology (following addition of polysaccharide) that are qualitatively different from those found with the pressure-treated systems. These results are discussed in relation to current knowledge about effects of high-pressure processing on protein structure and functionality.