β-Lactoglobulin adsorption layers at the water/air surface: 1. Adsorption kinetics and surface pressure isotherm: Effect of pH and ionic strength

The dynamic surface pressure II of air bubbles aging in buffered beta-lactoglobulin (BLG) solutions containing various protein concentrations C-BLG (10(-9)-10(-4)M), pH (3-7) and buffer concentrations C-buff (1, 10 and 100 mM) was measured as a function of time t by bubble profile analysis tensiometry. Adsorption kinetics was studied by recording II(t) data for 80 000 s and the final II-values were used to construct the surface pressure isotherm II(C-BLG) for the pH values of 3, 5, 7 (C-buff = 10 mM) and 6.3 (in pure water). On the basis of obtained kinetic and concentration dependencies the effect of pH on the protein surface activity was qualitatively analyzed. At constant Cbuff =10 mM and relatively low protein concentrations (C-BLG<10(-8)M), BLG exhibits the shortest induction time T-ind and highest II-values at pH 7 (negative net charge), in comparison to pH 5 and 3, whereas at sufficiently high protein concentrations (ca. C-BLG >10(-6) M) BLG is the most surface active at pH 5 (negligible net charge in the vicinity of the isoelectric point, pI approximate to 5.1). At constant C-buff, BLG solutions with pH 3 show the lowest surface activity. The influence of the ionic strength (buffer concentration) of the solution on the dynamic II was studied at a selected protein concentration of C-BLG =10(-5) M. The kinetics of adsorption is weakly affected by C-buff for solutions with pH 5 and significantly enhanced by increasing C-buff for solutions with pH not equal pI. The effect of the chemical nature of the electrolyte was obtained for non-buffered aqueous BLG solutions (natural pH similar to 6.3) at different concentrations of NaCI or CaCI2. It is demonstrated that the adsorption kinetics and the surface pressure isotherm of BLG at the water/air surface depend strongly on the protein effective charge, which is dictated by the solution pH and ionic strength. (C) 2016 Elsevier B.V. All rights reserved.