β-Lactoglobulin adsorption layers at the water/air surface: 2. Dilational rheology: Effect of pH and ionic strength

The dynamic surface pressure II and the two components E' (real part) and E" (imaginary part) of the dynamic surface dilational modulus of adsorbed beta-lactoglobulin (BLG) layers at the water/air interface were measured by oscillating drop/bubble profile analysis tensiometry with the aim to enlarge information on the surface rheology of solutions of this practically important protein. The effects of the solution pH and ionic strength (represented by the buffer concentration C-buff) on the kinetic dependences of Pi, E' and E" were monitored for various protein concentrations in the range 5x10(-9)-2x10(-4) M. Combining the E'(t) and E"(t) data with the Pi(t) data allowed for obtaining the dependences E'(Pi) and E"(Pi). The real part E' was found to increase monotonically with increasing Pi, while the E"(Pi) data exhibit a maximum in the surface pressure range 14-18 mhl/m. The height of the maximum in the E"(Pi) dependence was found to increase with increasing protein concentration. Apparently, in these interfacial layers, relaxation processes take place and their relaxation strengths depend on the history of the layer formation, i.e. the rate of adsorption. After the maximum in the E"(Pi) dependence the E"-values progressively diminish, which can be assumed to occur due to solidification of the protein gel-like network accompanied by less energy dissipation and high elasticity of the interfacial layer. BLG layers at pH 5 (close to the isoelectric point pI approximate to 5.1) exhibit the highest E'-values and the lowest E"-values measured in this study, which suggest the formation of a strong protein network at these (isoelectric) conditions. Variations of the buffer concentration strongly influence the kinetic dependences of E' and E" for BLG layers at pH not equal pI, while at pH -> pI no effect was detected. The viscoelastic characteristics of BLG layers adsorbed from buffer-free solutions containing NaCI or CaCl2 were also studied. For the first time, hysteresis in the frequency dependence of the surface dilational modulus of protein layers measured during repeating cycles of oscillations is reported. Such hysteresis is typical for BLG layers adsorbed at any solution conditions used in this study. (C) 2016 Elsevier B.V. All rights reserved.