Effects of ionic strength on the solubility of whey protein products. A colloid chemical approach

Solubility and native structure of whey proteins are important determinants for the functional properties of whey protein products. Whey protein denaturation is related to protein solubility at pH 4.6; a value which is similar to 0.5 pH unit below the iso-ionic points of the main whey proteins. Part of the native globulin-like whey proteins (mainly beta-lactoglobulin) may also precipitate at pH 4.6, depending on ionic strength (salting-in effect) and salt composition. In order to distinguish protein denaturation from salting-in effects at pH 4.6, recent information on the three-dimensional structure of beta-lactoglobulin-A has been combined with a quantitative description of the electrostatic interactions of beta-lactoglobulin-A as a dipolar ion in media of low ionic strength. Comparison of these results with the experimentally determined solubility data at pH 4.6 revealed significant differences in the pH of minimum solubility between whey protein concentrates (from membrane processes) and whey protein isolates (from ionic exchange processes). A whey protein isolate obtained by ionic exchange processing showed minimum solubility near the iso-electric point of beta-lactoglobulin-A (pH 5.2), which differs sufficiently from denaturation and salting-in effects at pH 4.6. Whey protein concentrates having greater than or equal to 80% protein on total solids show minimal solubility in the pH range 4.6-5.0, which coincides with insolubility induced by whey protein denaturation. This feature seems to be related to specific adsorption of multivalent anions. Adjusting the ionic strength to 0.1, using e.g. NaCl, may prevent insolubilisation of globuline-like proteins between pH 4.5 and 5.5. When protein solubility tests at pH 4.6 are used for the characterization of whey protein denaturation, it is recommended to use media having an ionic strength of 0.1, for all whey protein products.