Effect of salts on the alkaline degradation of β-lactoglobulin gels and aggregates: Existence of a dissolution threshold

The effect of NaCl and CaCl₂ on the alkaline degradation of β-lactoglobulin gels and aggregates, and particularly on the onset of dissolution, is studied. For gels, measurements of solubility in 0.063-0.5 M NaOH at 20 °C show the existence of a practical dissolution threshold in NaCl concentration, lying between 0.24 and 0.47 M. For aggregates, destruction of soluble β-lactoglobulin in alkali, followed by size exclusion chromatography, yields similar results. Furthermore, during dissolution of a gel in alkali at high NaCl concentrations, the protein aggregates released are very large (e.g. ∼40% are larger than 200 kDa). CaCl₂ is found to cause similar inhibition of dissolution to NaCl, but at concentrations about 30× lower (∼10 mM). The threshold is hypothesised to arise from a combination of physical entanglements caused by the high protein concentration under conditions where little swelling occurs, and hydrophobic/electrostatic interactions between aggregates favoured by the high concentration of salts.