Excess adsorption of β-lactoglobulin on the soft surface of casein powder

Extents (Γ 2 1 ) of adsorption of globular milk protein β-lactoglobulin (βlg) from salt solution onto the soft surface of casein powder have been measured as functions of βlg concentration at pH 6.5 keeping ionic strength and temperature constants. Ionic strengths were maintained by using neutral salts LiCl, NaCl, KCl, RbCl, CsCl, CaCl 2 and Na 2 SO 4 . Six molar concentration of denaturant urea was also used for the study of adsorption. Except KCl and KCNS, Γ 2 1 in the presence of other salts and urea respectively are positive. Values of Γ 2 1 in all cases increases with increase of C 2 for βlg concentration in the bulk phase. Γ 2 1 also increases with increase of C 2 until at a critical value C 2 m , it attains the maximum value Γ 2 m at the state of surface saturation. Values of Γ 2 m stands in the order: Na 2 SO 4 > NaCl > RbCl > LiCl > CsCl > CaCl 2 This order is partly agreeing with lyotropic or Hoffmeister series effect. Γ 2 1 for KCl and KCNS are negative in sign at all values of C 2 due to the excess positive hydration of βlg in the presence of these salts. The maximum affinities of adsorption at the state of surface saturation represented by the free energy decrease -ΔG 0 calculated from the use of integrated from of the Gibbs adsorption equation in kJ per kg of casein varies linearly with increase of Γ 2 m and average values of the free energy change -ΔG 0 B calculated from the slope of this plot is 38 kg per mole of adsorbed βlg in all cases.