temperatures (mostly 50–80 8 C). Several complications can cause more
complicated denaturation kinetics.
Solubility. The solubility of a protein is primarily determined by
the preponderance of charged groups and of hydrophobic groups that are
exposed to the solvent. Considering a protein as a macroion, the Debye–
Hu ̈ckel theory on the ion activity coefficient can be applied. It predicts that a
smaller net charge and a lower ionic strength in the solvent lead to a smaller
solubility. This is indeed observed, and most proteins are virtually insoluble
at their isoelectric pH in the absence of salt. Adding some salt then may
cause dissolution: i.e., ‘‘salting in.’’ High concentrations of salt may lead to
‘‘salting out,’’ but this is very salt specific. The salts, like many other solutes,
affect solvent quality. The greater the number of hydrophobic groups that
are in contact with the solvent, the smaller the solubility, and this effect is
enhanced by lowering solvent quality. Unfolding of a globular protein thus
greatly lowers its solubility, as more hydrophobic groups become exposed.
It often leads to aggregation and possibly gel formation.
The ‘‘solubility’’ as determined by practical tests on crude protein
preparations used in industry, is a different property. It may be fairly small
because the solubility of all the proteins present is fairly small, but also—
and, more generally—because some of them are virtually insoluble, whereas
others are well-soluble.
BIBLIOGRAPHY
Some textbooks on physical chemistry treat some of the aspects discussed here, e.g.,
Chang (see Chapter 1). Most texts on biochemistry give much information on
protein structure, function, reactivity, and other properties. For food scientists the
following introduction can be recommended:
S. Damodaran. Amino acids, peptides and proteins. In: O. R. Fennema, ed. Food
Chemistry, 3rded. Marcel Dekker, New York, 1996, Chapter 6.
Comprehensive and clear is the monograph
T. E. Creighton. Proteins: Structures and Molecular Properties, 2nded. Freeman,
New York, 1993.
Conformational stability is extensively discussed in two reviews by
P. L. Privalov. Stability of proteins. Adv. Protein Chem. 33 (1979) 167–241 and 35
(1982) 1–104.