indeed what is observed for fairly hydrophilic globular proteins; an example
is in Figure 7.12. The increase in solubility by increasing ionic strength is
called salting in.
A complication is that it may take a long time before equilibrium is
reached. For instance,b-lactoglobulin is quite soluble at neutral pH, even at
low ionic strength, as indicated by Figure 7.12. If a concentrated solution of
pH 7 is brought to pH 5.2, however, it may take some days before visible
protein aggregation occurs.
Equation (7.8) does not hold for proteins of a more or less unfolded
conformation: the protein may then form a coacervate rather than a
precipitate (Section 6.5.1). However, the trends would be the same. Another
point is that extreme pH values may lead to unfolding, even at room
temperature. The molecules then generally have such a high charge as to be
well soluble, despite the exposure of apolar groups. Most proteins used in
the food industry are soluble at pH>9. An additional cause would be
disruption of 22 S 22 S 22 bridges at such a pH.
FIGURE7.12 Salting in. Solubility (csat)ofb-lactoglobulin as a function of pH for
various NaCl concentrations (indicated, millimolar).